text: /*
  ** 2001 September 15
  **
  ** The author disclaims copyright to this source code.  In place of
  ** a legal notice, here is a blessing:
  **
  **    May you do good and not evil.
  **    May you find forgiveness for yourself and forgive others.
  **    May you share freely, never taking more than you give.
  **
  *************************************************************************
  ** This is the implementation of the page cache subsystem or "pager".
  ** 
  ** The pager is used to access a database disk file.  It implements
  ** atomic commit and rollback through the use of a journal file that
  ** is separate from the database file.  The pager also implements file
  ** locking to prevent two processes from writing the same database
  ** file simultaneously, or one process from reading the database while
  ** another is writing.
  */
  #ifndef SQLITE_OMIT_DISKIO
  #include "sqliteInt.h"
  #include "wal.h"
  
  
  /******************* NOTES ON THE DESIGN OF THE PAGER ************************
  **
  ** This comment block describes invariants that hold when using a rollback
  ** journal.  These invariants do not apply for journal_mode=WAL,
  ** journal_mode=MEMORY, or journal_mode=OFF.
  **
  ** Within this comment block, a page is deemed to have been synced
  ** automatically as soon as it is written when PRAGMA synchronous=OFF.
  ** Otherwise, the page is not synced until the xSync method of the VFS
  ** is called successfully on the file containing the page.
  **
  ** Definition:  A page of the database file is said to be "overwriteable" if
  ** one or more of the following are true about the page:
  ** 
  **     (a)  The original content of the page as it was at the beginning of
  **          the transaction has been written into the rollback journal and
  **          synced.
  ** 
  **     (b)  The page was a freelist leaf page at the start of the transaction.
  ** 
  **     (c)  The page number is greater than the largest page that existed in
  **          the database file at the start of the transaction.
  ** 
  ** (1) A page of the database file is never overwritten unless one of the
  **     following are true:
  ** 
  **     (a) The page and all other pages on the same sector are overwriteable.
  ** 
  **     (b) The atomic page write optimization is enabled, and the entire
  **         transaction other than the update of the transaction sequence
  **         number consists of a single page change.
  ** 
  ** (2) The content of a page written into the rollback journal exactly matches
  **     both the content in the database when the rollback journal was written
  **     and the content in the database at the beginning of the current
  **     transaction.
  ** 
  ** (3) Writes to the database file are an integer multiple of the page size
  **     in length and are aligned on a page boundary.
  ** 
  ** (4) Reads from the database file are either aligned on a page boundary and
  **     an integer multiple of the page size in length or are taken from the
  **     first 100 bytes of the database file.
  ** 
  ** (5) All writes to the database file are synced prior to the rollback journal
  **     being deleted, truncated, or zeroed.
  ** 
  ** (6) If a master journal file is used, then all writes to the database file
  **     are synced prior to the master journal being deleted.
  ** 
  ** Definition: Two databases (or the same database at two points it time)
  ** are said to be "logically equivalent" if they give the same answer to
  ** all queries.  Note in particular the content of freelist leaf
  ** pages can be changed arbitrarily without affecting the logical equivalence
  ** of the database.
  ** 
  ** (7) At any time, if any subset, including the empty set and the total set,
  **     of the unsynced changes to a rollback journal are removed and the 
  **     journal is rolled back, the resulting database file will be logically
  **     equivalent to the database file at the beginning of the transaction.
  ** 
  ** (8) When a transaction is rolled back, the xTruncate method of the VFS
  **     is called to restore the database file to the same size it was at
  **     the beginning of the transaction.  (In some VFSes, the xTruncate
  **     method is a no-op, but that does not change the fact the SQLite will
  **     invoke it.)
  ** 
  ** (9) Whenever the database file is modified, at least one bit in the range
  **     of bytes from 24 through 39 inclusive will be changed prior to releasing
  **     the EXCLUSIVE lock, thus signaling other connections on the same
  **     database to flush their caches.
  **
  ** (10) The pattern of bits in bytes 24 through 39 shall not repeat in less
  **      than one billion transactions.
  **
  ** (11) A database file is well-formed at the beginning and at the conclusion
  **      of every transaction.
  **
  ** (12) An EXCLUSIVE lock is held on the database file when writing to
  **      the database file.
  **
  ** (13) A SHARED lock is held on the database file while reading any
  **      content out of the database file.
  **
  ******************************************************************************/
  
  /*
  ** Macros for troubleshooting.  Normally turned off
  */
  #if 0
  int sqlite3PagerTrace=1;  /* True to enable tracing */
  #define sqlite3DebugPrintf printf
  #define PAGERTRACE(X)     if( sqlite3PagerTrace ){ sqlite3DebugPrintf X; }
  #else
  #define PAGERTRACE(X)
  #endif
  
  /*
  ** The following two macros are used within the PAGERTRACE() macros above
  ** to print out file-descriptors. 
  **
  ** PAGERID() takes a pointer to a Pager struct as its argument. The
  ** associated file-descriptor is returned. FILEHANDLEID() takes an sqlite3_file
  ** struct as its argument.
  */
  #define PAGERID(p) ((int)(p->fd))
  #define FILEHANDLEID(fd) ((int)fd)
  
  /*
  ** The Pager.eState variable stores the current 'state' of a pager. A
  ** pager may be in any one of the seven states shown in the following
  ** state diagram.
  **
  **                            OPEN <------+------+
  **                              |         |      |
  **                              V         |      |
  **               +---------> READER-------+      |
  **               |              |                |
  **               |              V                |
  **               |<-------WRITER_LOCKED------> ERROR
  **               |              |                ^  
  **               |              V                |
  **               |<------WRITER_CACHEMOD-------->|
  **               |              |                |
  **               |              V                |
  **               |<-------WRITER_DBMOD---------->|
  **               |              |                |
  **               |              V                |
  **               +<------WRITER_FINISHED-------->+
  **
  **
  ** List of state transitions and the C [function] that performs each:
  ** 
  **   OPEN              -> READER              [sqlite3PagerSharedLock]
  **   READER            -> OPEN                [pager_unlock]
  **
  **   READER            -> WRITER_LOCKED       [sqlite3PagerBegin]
  **   WRITER_LOCKED     -> WRITER_CACHEMOD     [pager_open_journal]
  **   WRITER_CACHEMOD   -> WRITER_DBMOD        [syncJournal]
  **   WRITER_DBMOD      -> WRITER_FINISHED     [sqlite3PagerCommitPhaseOne]
  **   WRITER_***        -> READER              [pager_end_transaction]
  **
  **   WRITER_***        -> ERROR               [pager_error]
  **   ERROR             -> OPEN                [pager_unlock]
  ** 
  **
  **  OPEN:
  **
  **    The pager starts up in this state. Nothing is guaranteed in this
  **    state - the file may or may not be locked and the database size is
  **    unknown. The database may not be read or written.
  **
  **    * No read or write transaction is active.
  **    * Any lock, or no lock at all, may be held on the database file.
  **    * The dbSize, dbOrigSize and dbFileSize variables may not be trusted.
  **
  **  READER:
  **
  **    In this state all the requirements for reading the database in 
  **    rollback (non-WAL) mode are met. Unless the pager is (or recently
  **    was) in exclusive-locking mode, a user-level read transaction is 
  **    open. The database size is known in this state.
  **
  **    A connection running with locking_mode=normal enters this state when
  **    it opens a read-transaction on the database and returns to state
  **    OPEN after the read-transaction is completed. However a connection
  **    running in locking_mode=exclusive (including temp databases) remains in
  **    this state even after the read-transaction is closed. The only way
  **    a locking_mode=exclusive connection can transition from READER to OPEN
  **    is via the ERROR state (see below).
  ** 
  **    * A read transaction may be active (but a write-transaction cannot).
  **    * A SHARED or greater lock is held on the database file.
  **    * The dbSize variable may be trusted (even if a user-level read 
  **      transaction is not active). The dbOrigSize and dbFileSize variables
  **      may not be trusted at this point.
  **    * If the database is a WAL database, then the WAL connection is open.
  **    * Even if a read-transaction is not open, it is guaranteed that 
  **      there is no hot-journal in the file-system.
  **
  **  WRITER_LOCKED:
  **
  **    The pager moves to this state from READER when a write-transaction
  **    is first opened on the database. In WRITER_LOCKED state, all locks 
  **    required to start a write-transaction are held, but no actual 
  **    modifications to the cache or database have taken place.
  **
  **    In rollback mode, a RESERVED or (if the transaction was opened with 
  **    BEGIN EXCLUSIVE) EXCLUSIVE lock is obtained on the database file when
  **    moving to this state, but the journal file is not written to or opened 
  **    to in this state. If the transaction is committed or rolled back while 
  **    in WRITER_LOCKED state, all that is required is to unlock the database 
  **    file.
  **
  **    IN WAL mode, WalBeginWriteTransaction() is called to lock the log file.
  **    If the connection is running with locking_mode=exclusive, an attempt
  **    is made to obtain an EXCLUSIVE lock on the database file.
  **
  **    * A write transaction is active.
  **    * If the connection is open in rollback-mode, a RESERVED or greater 
  **      lock is held on the database file.
  **    * If the connection is open in WAL-mode, a WAL write transaction
  **      is open (i.e. sqlite3WalBeginWriteTransaction() has been successfully
  **      called).
  **    * The dbSize, dbOrigSize and dbFileSize variables are all valid.
  **    * The contents of the pager cache have not been modified.
  **    * The journal file may or may not be open.
  **    * Nothing (not even the first header) has been written to the journal.
  **
  **  WRITER_CACHEMOD:
  **
  **    A pager moves from WRITER_LOCKED state to this state when a page is
  **    first modified by the upper layer. In rollback mode the journal file
  **    is opened (if it is not already open) and a header written to the
  **    start of it. The database file on disk has not been modified.
  **
  **    * A write transaction is active.
  **    * A RESERVED or greater lock is held on the database file.
  **    * The journal file is open and the first header has been written 
  **      to it, but the header has not been synced to disk.
  **    * The contents of the page cache have been modified.
  **
  **  WRITER_DBMOD:
  **
  **    The pager transitions from WRITER_CACHEMOD into WRITER_DBMOD state
  **    when it modifies the contents of the database file. WAL connections
  **    never enter this state (since they do not modify the database file,
  **    just the log file).
  **
  **    * A write transaction is active.
  **    * An EXCLUSIVE or greater lock is held on the database file.
  **    * The journal file is open and the first header has been written 
  **      and synced to disk.
  **    * The contents of the page cache have been modified (and possibly
  **      written to disk).
  **
  **  WRITER_FINISHED:
  **
  **    It is not possible for a WAL connection to enter this state.
  **
  **    A rollback-mode pager changes to WRITER_FINISHED state from WRITER_DBMOD
  **    state after the entire transaction has been successfully written into the
  **    database file. In this state the transaction may be committed simply
  **    by finalizing the journal file. Once in WRITER_FINISHED state, it is 
  **    not possible to modify the database further. At this point, the upper 
  **    layer must either commit or rollback the transaction.
  **
  **    * A write transaction is active.
  **    * An EXCLUSIVE or greater lock is held on the database file.
  **    * All writing and syncing of journal and database data has finished.
  **      If no error occurred, all that remains is to finalize the journal to
  **      commit the transaction. If an error did occur, the caller will need
  **      to rollback the transaction. 
  **
  **  ERROR:
  **
  **    The ERROR state is entered when an IO or disk-full error (including
  **    SQLITE_IOERR_NOMEM) occurs at a point in the code that makes it 
  **    difficult to be sure that the in-memory pager state (cache contents, 
  **    db size etc.) are consistent with the contents of the file-system.
  **
  **    Temporary pager files may enter the ERROR state, but in-memory pagers
  **    cannot.
  **
  **    For example, if an IO error occurs while performing a rollback, 
  **    the contents of the page-cache may be left in an inconsistent state.
  **    At this point it would be dangerous to change back to READER state
  **    (as usually happens after a rollback). Any subsequent readers might
  **    report database corruption (due to the inconsistent cache), and if
  **    they upgrade to writers, they may inadvertently corrupt the database
  **    file. To avoid this hazard, the pager switches into the ERROR state
  **    instead of READER following such an error.
  **
  **    Once it has entered the ERROR state, any attempt to use the pager
  **    to read or write data returns an error. Eventually, once all 
  **    outstanding transactions have been abandoned, the pager is able to
  **    transition back to OPEN state, discarding the contents of the 
  **    page-cache and any other in-memory state at the same time. Everything
  **    is reloaded from disk (and, if necessary, hot-journal rollback peformed)
  **    when a read-transaction is next opened on the pager (transitioning
  **    the pager into READER state). At that point the system has recovered 
  **    from the error.
  **
  **    Specifically, the pager jumps into the ERROR state if:
  **
  **      1. An error occurs while attempting a rollback. This happens in
  **         function sqlite3PagerRollback().
  **
  **      2. An error occurs while attempting to finalize a journal file
  **         following a commit in function sqlite3PagerCommitPhaseTwo().
  **
  **      3. An error occurs while attempting to write to the journal or
  **         database file in function pagerStress() in order to free up
  **         memory.
  **
  **    In other cases, the error is returned to the b-tree layer. The b-tree
  **    layer then attempts a rollback operation. If the error condition 
  **    persists, the pager enters the ERROR state via condition (1) above.
  **
  **    Condition (3) is necessary because it can be triggered by a read-only
  **    statement executed within a transaction. In this case, if the error
  **    code were simply returned to the user, the b-tree layer would not
  **    automatically attempt a rollback, as it assumes that an error in a
  **    read-only statement cannot leave the pager in an internally inconsistent 
  **    state.
  **
  **    * The Pager.errCode variable is set to something other than SQLITE_OK.
  **    * There are one or more outstanding references to pages (after the
  **      last reference is dropped the pager should move back to OPEN state).
  **    * The pager is not an in-memory pager.
  **    
  **
  ** Notes:
  **
  **   * A pager is never in WRITER_DBMOD or WRITER_FINISHED state if the
  **     connection is open in WAL mode. A WAL connection is always in one
  **     of the first four states.
  **
  **   * Normally, a connection open in exclusive mode is never in PAGER_OPEN
  **     state. There are two exceptions: immediately after exclusive-mode has
  **     been turned on (and before any read or write transactions are 
  **     executed), and when the pager is leaving the "error state".
  **
  **   * See also: assert_pager_state().
  */
  #define PAGER_OPEN                  0
  #define PAGER_READER                1
  #define PAGER_WRITER_LOCKED         2
  #define PAGER_WRITER_CACHEMOD       3
  #define PAGER_WRITER_DBMOD          4
  #define PAGER_WRITER_FINISHED       5
  #define PAGER_ERROR                 6
  
  /*
  ** The Pager.eLock variable is almost always set to one of the 
  ** following locking-states, according to the lock currently held on
  ** the database file: NO_LOCK, SHARED_LOCK, RESERVED_LOCK or EXCLUSIVE_LOCK.
  ** This variable is kept up to date as locks are taken and released by
  ** the pagerLockDb() and pagerUnlockDb() wrappers.
  **
  ** If the VFS xLock() or xUnlock() returns an error other than SQLITE_BUSY
  ** (i.e. one of the SQLITE_IOERR subtypes), it is not clear whether or not
  ** the operation was successful. In these circumstances pagerLockDb() and
  ** pagerUnlockDb() take a conservative approach - eLock is always updated
  ** when unlocking the file, and only updated when locking the file if the
  ** VFS call is successful. This way, the Pager.eLock variable may be set
  ** to a less exclusive (lower) value than the lock that is actually held
  ** at the system level, but it is never set to a more exclusive value.
  **
  ** This is usually safe. If an xUnlock fails or appears to fail, there may 
  ** be a few redundant xLock() calls or a lock may be held for longer than
  ** required, but nothing really goes wrong.
  **
  ** The exception is when the database file is unlocked as the pager moves
  ** from ERROR to OPEN state. At this point there may be a hot-journal file 
  ** in the file-system that needs to be rolled back (as part of an OPEN->SHARED
  ** transition, by the same pager or any other). If the call to xUnlock()
  ** fails at this point and the pager is left holding an EXCLUSIVE lock, this
  ** can confuse the call to xCheckReservedLock() call made later as part
  ** of hot-journal detection.
  **
  ** xCheckReservedLock() is defined as returning true "if there is a RESERVED 
  ** lock held by this process or any others". So xCheckReservedLock may 
  ** return true because the caller itself is holding an EXCLUSIVE lock (but
  ** doesn't know it because of a previous error in xUnlock). If this happens
  ** a hot-journal may be mistaken for a journal being created by an active
  ** transaction in another process, causing SQLite to read from the database
  ** without rolling it back.
  **
  ** To work around this, if a call to xUnlock() fails when unlocking the
  ** database in the ERROR state, Pager.eLock is set to UNKNOWN_LOCK. It
  ** is only changed back to a real locking state after a successful call
  ** to xLock(EXCLUSIVE). Also, the code to do the OPEN->SHARED state transition
  ** omits the check for a hot-journal if Pager.eLock is set to UNKNOWN_LOCK 
  ** lock. Instead, it assumes a hot-journal exists and obtains an EXCLUSIVE
  ** lock on the database file before attempting to roll it back. See function
  ** PagerSharedLock() for more detail.
  **
  ** Pager.eLock may only be set to UNKNOWN_LOCK when the pager is in 
  ** PAGER_OPEN state.
  */
  #define UNKNOWN_LOCK                (EXCLUSIVE_LOCK+1)
  
  /*
  ** A macro used for invoking the codec if there is one
  */
  #ifdef SQLITE_HAS_CODEC
  # define CODEC1(P,D,N,X,E) \
      if( P->xCodec && P->xCodec(P->pCodec,D,N,X)==0 ){ E; }
  # define CODEC2(P,D,N,X,E,O) \
      if( P->xCodec==0 ){ O=(char*)D; }else \
      if( (O=(char*)(P->xCodec(P->pCodec,D,N,X)))==0 ){ E; }
  #else
  # define CODEC1(P,D,N,X,E)   /* NO-OP */
  # define CODEC2(P,D,N,X,E,O) O=(char*)D
  #endif
  
  /*
  ** The maximum allowed sector size. 64KiB. If the xSectorsize() method 
  ** returns a value larger than this, then MAX_SECTOR_SIZE is used instead.
  ** This could conceivably cause corruption following a power failure on
  ** such a system. This is currently an undocumented limit.
  */
  #define MAX_SECTOR_SIZE 0x10000
  
  
  /*
  ** An instance of the following structure is allocated for each active
  ** savepoint and statement transaction in the system. All such structures
  ** are stored in the Pager.aSavepoint[] array, which is allocated and
  ** resized using sqlite3Realloc().
  **
  ** When a savepoint is created, the PagerSavepoint.iHdrOffset field is
  ** set to 0. If a journal-header is written into the main journal while
  ** the savepoint is active, then iHdrOffset is set to the byte offset 
  ** immediately following the last journal record written into the main
  ** journal before the journal-header. This is required during savepoint
  ** rollback (see pagerPlaybackSavepoint()).
  */
  typedef struct PagerSavepoint PagerSavepoint;
  struct PagerSavepoint {
    i64 iOffset;                 /* Starting offset in main journal */
    i64 iHdrOffset;              /* See above */
    Bitvec *pInSavepoint;        /* Set of pages in this savepoint */
    Pgno nOrig;                  /* Original number of pages in file */
    Pgno iSubRec;                /* Index of first record in sub-journal */
  #ifndef SQLITE_OMIT_WAL
    u32 aWalData[WAL_SAVEPOINT_NDATA];        /* WAL savepoint context */
  #endif
  };
  
  /*
  ** Bits of the Pager.doNotSpill flag.  See further description below.
  */
  #define SPILLFLAG_OFF         0x01 /* Never spill cache.  Set via pragma */
  #define SPILLFLAG_ROLLBACK    0x02 /* Current rolling back, so do not spill */
  #define SPILLFLAG_NOSYNC      0x04 /* Spill is ok, but do not sync */
  
  /*
  ** An open page cache is an instance of struct Pager. A description of
  ** some of the more important member variables follows:
  **
  ** eState
  **
  **   The current 'state' of the pager object. See the comment and state
  **   diagram above for a description of the pager state.
  **
  ** eLock
  **
  **   For a real on-disk database, the current lock held on the database file -
  **   NO_LOCK, SHARED_LOCK, RESERVED_LOCK or EXCLUSIVE_LOCK.
  **
  **   For a temporary or in-memory database (neither of which require any
  **   locks), this variable is always set to EXCLUSIVE_LOCK. Since such
  **   databases always have Pager.exclusiveMode==1, this tricks the pager
  **   logic into thinking that it already has all the locks it will ever
  **   need (and no reason to release them).
  **
  **   In some (obscure) circumstances, this variable may also be set to
  **   UNKNOWN_LOCK. See the comment above the #define of UNKNOWN_LOCK for
  **   details.
  **
  ** changeCountDone
  **
  **   This boolean variable is used to make sure that the change-counter 
  **   (the 4-byte header field at byte offset 24 of the database file) is 
  **   not updated more often than necessary. 
  **
  **   It is set to true when the change-counter field is updated, which 
  **   can only happen if an exclusive lock is held on the database file.
  **   It is cleared (set to false) whenever an exclusive lock is 
  **   relinquished on the database file. Each time a transaction is committed,
  **   The changeCountDone flag is inspected. If it is true, the work of
  **   updating the change-counter is omitted for the current transaction.
  **
  **   This mechanism means that when running in exclusive mode, a connection 
  **   need only update the change-counter once, for the first transaction
  **   committed.
  **
  ** setMaster
  **
  **   When PagerCommitPhaseOne() is called to commit a transaction, it may
  **   (or may not) specify a master-journal name to be written into the 
  **   journal file before it is synced to disk.
  **
  **   Whether or not a journal file contains a master-journal pointer affects 
  **   the way in which the journal file is finalized after the transaction is 
  **   committed or rolled back when running in "journal_mode=PERSIST" mode.
  **   If a journal file does not contain a master-journal pointer, it is
  **   finalized by overwriting the first journal header with zeroes. If
  **   it does contain a master-journal pointer the journal file is finalized 
  **   by truncating it to zero bytes, just as if the connection were 
  **   running in "journal_mode=truncate" mode.
  **
  **   Journal files that contain master journal pointers cannot be finalized
  **   simply by overwriting the first journal-header with zeroes, as the
  **   master journal pointer could interfere with hot-journal rollback of any
  **   subsequently interrupted transaction that reuses the journal file.
  **
  **   The flag is cleared as soon as the journal file is finalized (either
  **   by PagerCommitPhaseTwo or PagerRollback). If an IO error prevents the
  **   journal file from being successfully finalized, the setMaster flag
  **   is cleared anyway (and the pager will move to ERROR state).
  **
  ** doNotSpill
  **
  **   This variables control the behavior of cache-spills  (calls made by
  **   the pcache module to the pagerStress() routine to write cached data
  **   to the file-system in order to free up memory).
  **
  **   When bits SPILLFLAG_OFF or SPILLFLAG_ROLLBACK of doNotSpill are set,
  **   writing to the database from pagerStress() is disabled altogether.
  **   The SPILLFLAG_ROLLBACK case is done in a very obscure case that
  **   comes up during savepoint rollback that requires the pcache module
  **   to allocate a new page to prevent the journal file from being written
  **   while it is being traversed by code in pager_playback().  The SPILLFLAG_OFF
  **   case is a user preference.
  ** 
  **   If the SPILLFLAG_NOSYNC bit is set, writing to the database from
  **   pagerStress() is permitted, but syncing the journal file is not.
  **   This flag is set by sqlite3PagerWrite() when the file-system sector-size
  **   is larger than the database page-size in order to prevent a journal sync
  **   from happening in between the journalling of two pages on the same sector. 
  **
  ** subjInMemory
  **
  **   This is a boolean variable. If true, then any required sub-journal
  **   is opened as an in-memory journal file. If false, then in-memory
  **   sub-journals are only used for in-memory pager files.
  **
  **   This variable is updated by the upper layer each time a new 
  **   write-transaction is opened.
  **
  ** dbSize, dbOrigSize, dbFileSize
  **
  **   Variable dbSize is set to the number of pages in the database file.
  **   It is valid in PAGER_READER and higher states (all states except for
  **   OPEN and ERROR). 
  **
  **   dbSize is set based on the size of the database file, which may be 
  **   larger than the size of the database (the value stored at offset
  **   28 of the database header by the btree). If the size of the file
  **   is not an integer multiple of the page-size, the value stored in
  **   dbSize is rounded down (i.e. a 5KB file with 2K page-size has dbSize==2).
  **   Except, any file that is greater than 0 bytes in size is considered
  **   to have at least one page. (i.e. a 1KB file with 2K page-size leads
  **   to dbSize==1).
  **
  **   During a write-transaction, if pages with page-numbers greater than
  **   dbSize are modified in the cache, dbSize is updated accordingly.
  **   Similarly, if the database is truncated using PagerTruncateImage(), 
  **   dbSize is updated.
  **
  **   Variables dbOrigSize and dbFileSize are valid in states 
  **   PAGER_WRITER_LOCKED and higher. dbOrigSize is a copy of the dbSize
  **   variable at the start of the transaction. It is used during rollback,
  **   and to determine whether or not pages need to be journalled before
  **   being modified.
  **
  **   Throughout a write-transaction, dbFileSize contains the size of
  **   the file on disk in pages. It is set to a copy of dbSize when the
  **   write-transaction is first opened, and updated when VFS calls are made
  **   to write or truncate the database file on disk. 
  **
  **   The only reason the dbFileSize variable is required is to suppress 
  **   unnecessary calls to xTruncate() after committing a transaction. If, 
  **   when a transaction is committed, the dbFileSize variable indicates 
  **   that the database file is larger than the database image (Pager.dbSize), 
  **   pager_truncate() is called. The pager_truncate() call uses xFilesize()
  **   to measure the database file on disk, and then truncates it if required.
  **   dbFileSize is not used when rolling back a transaction. In this case
  **   pager_truncate() is called unconditionally (which means there may be
  **   a call to xFilesize() that is not strictly required). In either case,
  **   pager_truncate() may cause the file to become smaller or larger.
  **
  ** dbHintSize
  **
  **   The dbHintSize variable is used to limit the number of calls made to
  **   the VFS xFileControl(FCNTL_SIZE_HINT) method. 
  **
  **   dbHintSize is set to a copy of the dbSize variable when a
  **   write-transaction is opened (at the same time as dbFileSize and
  **   dbOrigSize). If the xFileControl(FCNTL_SIZE_HINT) method is called,
  **   dbHintSize is increased to the number of pages that correspond to the
  **   size-hint passed to the method call. See pager_write_pagelist() for 
  **   details.
  **
  ** errCode
  **
  **   The Pager.errCode variable is only ever used in PAGER_ERROR state. It
  **   is set to zero in all other states. In PAGER_ERROR state, Pager.errCode 
  **   is always set to SQLITE_FULL, SQLITE_IOERR or one of the SQLITE_IOERR_XXX 
  **   sub-codes.
  */
  struct Pager {
    sqlite3_vfs *pVfs;          /* OS functions to use for IO */
    u8 exclusiveMode;           /* Boolean. True if locking_mode==EXCLUSIVE */
    u8 journalMode;             /* One of the PAGER_JOURNALMODE_* values */
    u8 useJournal;              /* Use a rollback journal on this file */
    u8 noSync;                  /* Do not sync the journal if true */
    u8 fullSync;                /* Do extra syncs of the journal for robustness */
    u8 extraSync;               /* sync directory after journal delete */
    u8 ckptSyncFlags;           /* SYNC_NORMAL or SYNC_FULL for checkpoint */
    u8 walSyncFlags;            /* SYNC_NORMAL or SYNC_FULL for wal writes */
    u8 syncFlags;               /* SYNC_NORMAL or SYNC_FULL otherwise */
    u8 tempFile;                /* zFilename is a temporary or immutable file */
    u8 noLock;                  /* Do not lock (except in WAL mode) */
    u8 readOnly;                /* True for a read-only database */
    u8 memDb;                   /* True to inhibit all file I/O */
  
    /**************************************************************************
    ** The following block contains those class members that change during
    ** routine operation.  Class members not in this block are either fixed
    ** when the pager is first created or else only change when there is a
    ** significant mode change (such as changing the page_size, locking_mode,
    ** or the journal_mode).  From another view, these class members describe
    ** the "state" of the pager, while other class members describe the
    ** "configuration" of the pager.
    */
    u8 eState;                  /* Pager state (OPEN, READER, WRITER_LOCKED..) */
    u8 eLock;                   /* Current lock held on database file */
    u8 changeCountDone;         /* Set after incrementing the change-counter */
    u8 setMaster;               /* True if a m-j name has been written to jrnl */
    u8 doNotSpill;              /* Do not spill the cache when non-zero */
    u8 subjInMemory;            /* True to use in-memory sub-journals */
    u8 bUseFetch;               /* True to use xFetch() */
    u8 hasHeldSharedLock;       /* True if a shared lock has ever been held */
    Pgno dbSize;                /* Number of pages in the database */
    Pgno dbOrigSize;            /* dbSize before the current transaction */
    Pgno dbFileSize;            /* Number of pages in the database file */
    Pgno dbHintSize;            /* Value passed to FCNTL_SIZE_HINT call */
    int errCode;                /* One of several kinds of errors */
    int nRec;                   /* Pages journalled since last j-header written */
    u32 cksumInit;              /* Quasi-random value added to every checksum */
    u32 nSubRec;                /* Number of records written to sub-journal */
    Bitvec *pInJournal;         /* One bit for each page in the database file */
    sqlite3_file *fd;           /* File descriptor for database */
    sqlite3_file *jfd;          /* File descriptor for main journal */
    sqlite3_file *sjfd;         /* File descriptor for sub-journal */
    i64 journalOff;             /* Current write offset in the journal file */
    i64 journalHdr;             /* Byte offset to previous journal header */
    sqlite3_backup *pBackup;    /* Pointer to list of ongoing backup processes */
    PagerSavepoint *aSavepoint; /* Array of active savepoints */
    int nSavepoint;             /* Number of elements in aSavepoint[] */
    u32 iDataVersion;           /* Changes whenever database content changes */
    char dbFileVers[16];        /* Changes whenever database file changes */
  
    int nMmapOut;               /* Number of mmap pages currently outstanding */
    sqlite3_int64 szMmap;       /* Desired maximum mmap size */
    PgHdr *pMmapFreelist;       /* List of free mmap page headers (pDirty) */
    /*
    ** End of the routinely-changing class members
    ***************************************************************************/
  
    u16 nExtra;                 /* Add this many bytes to each in-memory page */
    i16 nReserve;               /* Number of unused bytes at end of each page */
    u32 vfsFlags;               /* Flags for sqlite3_vfs.xOpen() */
    u32 sectorSize;             /* Assumed sector size during rollback */
    int pageSize;               /* Number of bytes in a page */
    Pgno mxPgno;                /* Maximum allowed size of the database */
    i64 journalSizeLimit;       /* Size limit for persistent journal files */
    char *zFilename;            /* Name of the database file */
    char *zJournal;             /* Name of the journal file */
    int (*xBusyHandler)(void*); /* Function to call when busy */
    void *pBusyHandlerArg;      /* Context argument for xBusyHandler */
    int aStat[3];               /* Total cache hits, misses and writes */
  #ifdef SQLITE_TEST
    int nRead;                  /* Database pages read */
  #endif
    void (*xReiniter)(DbPage*); /* Call this routine when reloading pages */
  #ifdef SQLITE_HAS_CODEC
    void *(*xCodec)(void*,void*,Pgno,int); /* Routine for en/decoding data */
    void (*xCodecSizeChng)(void*,int,int); /* Notify of page size changes */
    void (*xCodecFree)(void*);             /* Destructor for the codec */
    void *pCodec;               /* First argument to xCodec... methods */
  #endif
    char *pTmpSpace;            /* Pager.pageSize bytes of space for tmp use */
    PCache *pPCache;            /* Pointer to page cache object */
  #ifndef SQLITE_OMIT_WAL
    Wal *pWal;                  /* Write-ahead log used by "journal_mode=wal" */
    char *zWal;                 /* File name for write-ahead log */
  #endif
  };
  
  /*
  ** Indexes for use with Pager.aStat[]. The Pager.aStat[] array contains
  ** the values accessed by passing SQLITE_DBSTATUS_CACHE_HIT, CACHE_MISS 
  ** or CACHE_WRITE to sqlite3_db_status().
  */
  #define PAGER_STAT_HIT   0
  #define PAGER_STAT_MISS  1
  #define PAGER_STAT_WRITE 2
  
  /*
  ** The following global variables hold counters used for
  ** testing purposes only.  These variables do not exist in
  ** a non-testing build.  These variables are not thread-safe.
  */
  #ifdef SQLITE_TEST
  int sqlite3_pager_readdb_count = 0;    /* Number of full pages read from DB */
  int sqlite3_pager_writedb_count = 0;   /* Number of full pages written to DB */
  int sqlite3_pager_writej_count = 0;    /* Number of pages written to journal */
  # define PAGER_INCR(v)  v++
  #else
  # define PAGER_INCR(v)
  #endif
  
  
  
  /*
  ** Journal files begin with the following magic string.  The data
  ** was obtained from /dev/random.  It is used only as a sanity check.
  **
  ** Since version 2.8.0, the journal format contains additional sanity
  ** checking information.  If the power fails while the journal is being
  ** written, semi-random garbage data might appear in the journal
  ** file after power is restored.  If an attempt is then made
  ** to roll the journal back, the database could be corrupted.  The additional
  ** sanity checking data is an attempt to discover the garbage in the
  ** journal and ignore it.
  **
  ** The sanity checking information for the new journal format consists
  ** of a 32-bit checksum on each page of data.  The checksum covers both
  ** the page number and the pPager->pageSize bytes of data for the page.
  ** This cksum is initialized to a 32-bit random value that appears in the
  ** journal file right after the header.  The random initializer is important,
  ** because garbage data that appears at the end of a journal is likely
  ** data that was once in other files that have now been deleted.  If the
  ** garbage data came from an obsolete journal file, the checksums might
  ** be correct.  But by initializing the checksum to random value which
  ** is different for every journal, we minimize that risk.
  */
  static const unsigned char aJournalMagic[] = {
    0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd7,
  };
  
  /*
  ** The size of the of each page record in the journal is given by
  ** the following macro.
  */
  #define JOURNAL_PG_SZ(pPager)  ((pPager->pageSize) + 8)
  
  /*
  ** The journal header size for this pager. This is usually the same 
  ** size as a single disk sector. See also setSectorSize().
  */
  #define JOURNAL_HDR_SZ(pPager) (pPager->sectorSize)
  
  /*
  ** The macro MEMDB is true if we are dealing with an in-memory database.
  ** We do this as a macro so that if the SQLITE_OMIT_MEMORYDB macro is set,
  ** the value of MEMDB will be a constant and the compiler will optimize
  ** out code that would never execute.
  */
  #ifdef SQLITE_OMIT_MEMORYDB
  # define MEMDB 0
  #else
  # define MEMDB pPager->memDb
  #endif
  
  /*
  ** The macro USEFETCH is true if we are allowed to use the xFetch and xUnfetch
  ** interfaces to access the database using memory-mapped I/O.
  */
  #if SQLITE_MAX_MMAP_SIZE>0
  # define USEFETCH(x) ((x)->bUseFetch)
  #else
  # define USEFETCH(x) 0
  #endif
  
  /*
  ** The maximum legal page number is (2^31 - 1).
  */
  #define PAGER_MAX_PGNO 2147483647
  
  /*
  ** The argument to this macro is a file descriptor (type sqlite3_file*).
  ** Return 0 if it is not open, or non-zero (but not 1) if it is.
  **
  ** This is so that expressions can be written as:
  **
  **   if( isOpen(pPager->jfd) ){ ...
  **
  ** instead of
  **
  **   if( pPager->jfd->pMethods ){ ...
  */
  #define isOpen(pFd) ((pFd)->pMethods!=0)
  
  /*
  ** Return true if this pager uses a write-ahead log instead of the usual
  ** rollback journal. Otherwise false.
  */
  #ifndef SQLITE_OMIT_WAL
  static int pagerUseWal(Pager *pPager){
    return (pPager->pWal!=0);
  }
  #else
  # define pagerUseWal(x) 0
  # define pagerRollbackWal(x) 0
  # define pagerWalFrames(v,w,x,y) 0
  # define pagerOpenWalIfPresent(z) SQLITE_OK
  # define pagerBeginReadTransaction(z) SQLITE_OK
  #endif
  
  #ifndef NDEBUG 
  /*
  ** Usage:
  **
  **   assert( assert_pager_state(pPager) );
  **
  ** This function runs many asserts to try to find inconsistencies in
  ** the internal state of the Pager object.
  */
  static int assert_pager_state(Pager *p){
    Pager *pPager = p;
  
    /* State must be valid. */
    assert( p->eState==PAGER_OPEN
         || p->eState==PAGER_READER
         || p->eState==PAGER_WRITER_LOCKED
         || p->eState==PAGER_WRITER_CACHEMOD
         || p->eState==PAGER_WRITER_DBMOD
         || p->eState==PAGER_WRITER_FINISHED
         || p->eState==PAGER_ERROR
    );
  
    /* Regardless of the current state, a temp-file connection always behaves
    ** as if it has an exclusive lock on the database file. It never updates
    ** the change-counter field, so the changeCountDone flag is always set.
    */
    assert( p->tempFile==0 || p->eLock==EXCLUSIVE_LOCK );
    assert( p->tempFile==0 || pPager->changeCountDone );
  
    /* If the useJournal flag is clear, the journal-mode must be "OFF". 
    ** And if the journal-mode is "OFF", the journal file must not be open.
    */
    assert( p->journalMode==PAGER_JOURNALMODE_OFF || p->useJournal );
    assert( p->journalMode!=PAGER_JOURNALMODE_OFF || !isOpen(p->jfd) );
  
    /* Check that MEMDB implies noSync. And an in-memory journal. Since 
    ** this means an in-memory pager performs no IO at all, it cannot encounter 
    ** either SQLITE_IOERR or SQLITE_FULL during rollback or while finalizing 
    ** a journal file. (although the in-memory journal implementation may 
    ** return SQLITE_IOERR_NOMEM while the journal file is being written). It 
    ** is therefore not possible for an in-memory pager to enter the ERROR 
    ** state.
    */
    if( MEMDB ){
      assert( !isOpen(p->fd) );
      assert( p->noSync );
      assert( p->journalMode==PAGER_JOURNALMODE_OFF 
           || p->journalMode==PAGER_JOURNALMODE_MEMORY 
      );
      assert( p->eState!=PAGER_ERROR && p->eState!=PAGER_OPEN );
      assert( pagerUseWal(p)==0 );
    }
  
    /* If changeCountDone is set, a RESERVED lock or greater must be held
    ** on the file.
    */
    assert( pPager->changeCountDone==0 || pPager->eLock>=RESERVED_LOCK );
    assert( p->eLock!=PENDING_LOCK );
  
    switch( p->eState ){
      case PAGER_OPEN:
        assert( !MEMDB );
        assert( pPager->errCode==SQLITE_OK );
        assert( sqlite3PcacheRefCount(pPager->pPCache)==0 || pPager->tempFile );
        break;
  
      case PAGER_READER:
        assert( pPager->errCode==SQLITE_OK );
        assert( p->eLock!=UNKNOWN_LOCK );
        assert( p->eLock>=SHARED_LOCK );
        break;
  
      case PAGER_WRITER_LOCKED:
        assert( p->eLock!=UNKNOWN_LOCK );
        assert( pPager->errCode==SQLITE_OK );
        if( !pagerUseWal(pPager) ){
          assert( p->eLock>=RESERVED_LOCK );
        }
        assert( pPager->dbSize==pPager->dbOrigSize );
        assert( pPager->dbOrigSize==pPager->dbFileSize );
        assert( pPager->dbOrigSize==pPager->dbHintSize );
        assert( pPager->setMaster==0 );
        break;
  
      case PAGER_WRITER_CACHEMOD:
        assert( p->eLock!=UNKNOWN_LOCK );
        assert( pPager->errCode==SQLITE_OK );
        if( !pagerUseWal(pPager) ){
          /* It is possible that if journal_mode=wal here that neither the
          ** journal file nor the WAL file are open. This happens during
          ** a rollback transaction that switches from journal_mode=off
          ** to journal_mode=wal.
          */
          assert( p->eLock>=RESERVED_LOCK );
          assert( isOpen(p->jfd) 
               || p->journalMode==PAGER_JOURNALMODE_OFF 
               || p->journalMode==PAGER_JOURNALMODE_WAL 
          );
        }
        assert( pPager->dbOrigSize==pPager->dbFileSize );
        assert( pPager->dbOrigSize==pPager->dbHintSize );
        break;
  
      case PAGER_WRITER_DBMOD:
        assert( p->eLock==EXCLUSIVE_LOCK );
        assert( pPager->errCode==SQLITE_OK );
        assert( !pagerUseWal(pPager) );
        assert( p->eLock>=EXCLUSIVE_LOCK );
        assert( isOpen(p->jfd) 
             || p->journalMode==PAGER_JOURNALMODE_OFF 
             || p->journalMode==PAGER_JOURNALMODE_WAL 
        );
        assert( pPager->dbOrigSize<=pPager->dbHintSize );
        break;
  
      case PAGER_WRITER_FINISHED:
        assert( p->eLock==EXCLUSIVE_LOCK );
        assert( pPager->errCode==SQLITE_OK );
        assert( !pagerUseWal(pPager) );
        assert( isOpen(p->jfd) 
             || p->journalMode==PAGER_JOURNALMODE_OFF 
             || p->journalMode==PAGER_JOURNALMODE_WAL 
        );
        break;
  
      case PAGER_ERROR:
        /* There must be at least one outstanding reference to the pager if
        ** in ERROR state. Otherwise the pager should have already dropped
        ** back to OPEN state.
        */
        assert( pPager->errCode!=SQLITE_OK );
        assert( sqlite3PcacheRefCount(pPager->pPCache)>0 || pPager->tempFile );
        break;
    }
  
    return 1;
  }
  #endif /* ifndef NDEBUG */
  
  #ifdef SQLITE_DEBUG 
  /*
  ** Return a pointer to a human readable string in a static buffer
  ** containing the state of the Pager object passed as an argument. This
  ** is intended to be used within debuggers. For example, as an alternative
  ** to "print *pPager" in gdb:
  **
  ** (gdb) printf "%s", print_pager_state(pPager)
  */
  static char *print_pager_state(Pager *p){
    static char zRet[1024];
  
    sqlite3_snprintf(1024, zRet,
        "Filename:      %s\n"
        "State:         %s errCode=%d\n"
        "Lock:          %s\n"
        "Locking mode:  locking_mode=%s\n"
        "Journal mode:  journal_mode=%s\n"
        "Backing store: tempFile=%d memDb=%d useJournal=%d\n"
        "Journal:       journalOff=%lld journalHdr=%lld\n"
        "Size:          dbsize=%d dbOrigSize=%d dbFileSize=%d\n"
        , p->zFilename
        , p->eState==PAGER_OPEN            ? "OPEN" :
          p->eState==PAGER_READER          ? "READER" :
          p->eState==PAGER_WRITER_LOCKED   ? "WRITER_LOCKED" :
          p->eState==PAGER_WRITER_CACHEMOD ? "WRITER_CACHEMOD" :
          p->eState==PAGER_WRITER_DBMOD    ? "WRITER_DBMOD" :
          p->eState==PAGER_WRITER_FINISHED ? "WRITER_FINISHED" :
          p->eState==PAGER_ERROR           ? "ERROR" : "?error?"
        , (int)p->errCode
        , p->eLock==NO_LOCK         ? "NO_LOCK" :
          p->eLock==RESERVED_LOCK   ? "RESERVED" :
          p->eLock==EXCLUSIVE_LOCK  ? "EXCLUSIVE" :
          p->eLock==SHARED_LOCK     ? "SHARED" :
          p->eLock==UNKNOWN_LOCK    ? "UNKNOWN" : "?error?"
        , p->exclusiveMode ? "exclusive" : "normal"
        , p->journalMode==PAGER_JOURNALMODE_MEMORY   ? "memory" :
          p->journalMode==PAGER_JOURNALMODE_OFF      ? "off" :
          p->journalMode==PAGER_JOURNALMODE_DELETE   ? "delete" :
          p->journalMode==PAGER_JOURNALMODE_PERSIST  ? "persist" :
          p->journalMode==PAGER_JOURNALMODE_TRUNCATE ? "truncate" :
          p->journalMode==PAGER_JOURNALMODE_WAL      ? "wal" : "?error?"
        , (int)p->tempFile, (int)p->memDb, (int)p->useJournal
        , p->journalOff, p->journalHdr
        , (int)p->dbSize, (int)p->dbOrigSize, (int)p->dbFileSize
    );
  
    return zRet;
  }
  #endif
  
  /*
  ** Return true if it is necessary to write page *pPg into the sub-journal.
  ** A page needs to be written into the sub-journal if there exists one
  ** or more open savepoints for which:
  **
  **   * The page-number is less than or equal to PagerSavepoint.nOrig, and
  **   * The bit corresponding to the page-number is not set in
  **     PagerSavepoint.pInSavepoint.
  */
  static int subjRequiresPage(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    PagerSavepoint *p;
    Pgno pgno = pPg->pgno;
    int i;
    for(i=0; i<pPager->nSavepoint; i++){
      p = &pPager->aSavepoint[i];
      if( p->nOrig>=pgno && 0==sqlite3BitvecTestNotNull(p->pInSavepoint, pgno) ){
        return 1;
      }
    }
    return 0;
  }
  
  #ifdef SQLITE_DEBUG
  /*
  ** Return true if the page is already in the journal file.
  */
  static int pageInJournal(Pager *pPager, PgHdr *pPg){
    return sqlite3BitvecTest(pPager->pInJournal, pPg->pgno);
  }
  #endif
  
  /*
  ** Read a 32-bit integer from the given file descriptor.  Store the integer
  ** that is read in *pRes.  Return SQLITE_OK if everything worked, or an
  ** error code is something goes wrong.
  **
  ** All values are stored on disk as big-endian.
  */
  static int read32bits(sqlite3_file *fd, i64 offset, u32 *pRes){
    unsigned char ac[4];
    int rc = sqlite3OsRead(fd, ac, sizeof(ac), offset);
    if( rc==SQLITE_OK ){
      *pRes = sqlite3Get4byte(ac);
    }
    return rc;
  }
  
  /*
  ** Write a 32-bit integer into a string buffer in big-endian byte order.
  */
  #define put32bits(A,B)  sqlite3Put4byte((u8*)A,B)
  
  
  /*
  ** Write a 32-bit integer into the given file descriptor.  Return SQLITE_OK
  ** on success or an error code is something goes wrong.
  */
  static int write32bits(sqlite3_file *fd, i64 offset, u32 val){
    char ac[4];
    put32bits(ac, val);
    return sqlite3OsWrite(fd, ac, 4, offset);
  }
  
  /*
  ** Unlock the database file to level eLock, which must be either NO_LOCK
  ** or SHARED_LOCK. Regardless of whether or not the call to xUnlock()
  ** succeeds, set the Pager.eLock variable to match the (attempted) new lock.
  **
  ** Except, if Pager.eLock is set to UNKNOWN_LOCK when this function is
  ** called, do not modify it. See the comment above the #define of 
  ** UNKNOWN_LOCK for an explanation of this.
  */
  static int pagerUnlockDb(Pager *pPager, int eLock){
    int rc = SQLITE_OK;
  
    assert( !pPager->exclusiveMode || pPager->eLock==eLock );
    assert( eLock==NO_LOCK || eLock==SHARED_LOCK );
    assert( eLock!=NO_LOCK || pagerUseWal(pPager)==0 );
    if( isOpen(pPager->fd) ){
      assert( pPager->eLock>=eLock );
      rc = pPager->noLock ? SQLITE_OK : sqlite3OsUnlock(pPager->fd, eLock);
      if( pPager->eLock!=UNKNOWN_LOCK ){
        pPager->eLock = (u8)eLock;
      }
      IOTRACE(("UNLOCK %p %d\n", pPager, eLock))
    }
    return rc;
  }
  
  /*
  ** Lock the database file to level eLock, which must be either SHARED_LOCK,
  ** RESERVED_LOCK or EXCLUSIVE_LOCK. If the caller is successful, set the
  ** Pager.eLock variable to the new locking state. 
  **
  ** Except, if Pager.eLock is set to UNKNOWN_LOCK when this function is 
  ** called, do not modify it unless the new locking state is EXCLUSIVE_LOCK. 
  ** See the comment above the #define of UNKNOWN_LOCK for an explanation 
  ** of this.
  */
  static int pagerLockDb(Pager *pPager, int eLock){
    int rc = SQLITE_OK;
  
    assert( eLock==SHARED_LOCK || eLock==RESERVED_LOCK || eLock==EXCLUSIVE_LOCK );
    if( pPager->eLock<eLock || pPager->eLock==UNKNOWN_LOCK ){
      rc = pPager->noLock ? SQLITE_OK : sqlite3OsLock(pPager->fd, eLock);
      if( rc==SQLITE_OK && (pPager->eLock!=UNKNOWN_LOCK||eLock==EXCLUSIVE_LOCK) ){
        pPager->eLock = (u8)eLock;
        IOTRACE(("LOCK %p %d\n", pPager, eLock))
      }
    }
    return rc;
  }
  
  /*
  ** This function determines whether or not the atomic-write optimization
  ** can be used with this pager. The optimization can be used if:
  **
  **  (a) the value returned by OsDeviceCharacteristics() indicates that
  **      a database page may be written atomically, and
  **  (b) the value returned by OsSectorSize() is less than or equal
  **      to the page size.
  **
  ** The optimization is also always enabled for temporary files. It is
  ** an error to call this function if pPager is opened on an in-memory
  ** database.
  **
  ** If the optimization cannot be used, 0 is returned. If it can be used,
  ** then the value returned is the size of the journal file when it
  ** contains rollback data for exactly one page.
  */
  #ifdef SQLITE_ENABLE_ATOMIC_WRITE
  static int jrnlBufferSize(Pager *pPager){
    assert( !MEMDB );
    if( !pPager->tempFile ){
      int dc;                           /* Device characteristics */
      int nSector;                      /* Sector size */
      int szPage;                       /* Page size */
  
      assert( isOpen(pPager->fd) );
      dc = sqlite3OsDeviceCharacteristics(pPager->fd);
      nSector = pPager->sectorSize;
      szPage = pPager->pageSize;
  
      assert(SQLITE_IOCAP_ATOMIC512==(512>>8));
      assert(SQLITE_IOCAP_ATOMIC64K==(65536>>8));
      if( 0==(dc&(SQLITE_IOCAP_ATOMIC|(szPage>>8)) || nSector>szPage) ){
        return 0;
      }
    }
  
    return JOURNAL_HDR_SZ(pPager) + JOURNAL_PG_SZ(pPager);
  }
  #else
  # define jrnlBufferSize(x) 0
  #endif
  
  /*
  ** If SQLITE_CHECK_PAGES is defined then we do some sanity checking
  ** on the cache using a hash function.  This is used for testing
  ** and debugging only.
  */
  #ifdef SQLITE_CHECK_PAGES
  /*
  ** Return a 32-bit hash of the page data for pPage.
  */
  static u32 pager_datahash(int nByte, unsigned char *pData){
    u32 hash = 0;
    int i;
    for(i=0; i<nByte; i++){
      hash = (hash*1039) + pData[i];
    }
    return hash;
  }
  static u32 pager_pagehash(PgHdr *pPage){
    return pager_datahash(pPage->pPager->pageSize, (unsigned char *)pPage->pData);
  }
  static void pager_set_pagehash(PgHdr *pPage){
    pPage->pageHash = pager_pagehash(pPage);
  }
  
  /*
  ** The CHECK_PAGE macro takes a PgHdr* as an argument. If SQLITE_CHECK_PAGES
  ** is defined, and NDEBUG is not defined, an assert() statement checks
  ** that the page is either dirty or still matches the calculated page-hash.
  */
  #define CHECK_PAGE(x) checkPage(x)
  static void checkPage(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    assert( pPager->eState!=PAGER_ERROR );
    assert( (pPg->flags&PGHDR_DIRTY) || pPg->pageHash==pager_pagehash(pPg) );
  }
  
  #else
  #define pager_datahash(X,Y)  0
  #define pager_pagehash(X)  0
  #define pager_set_pagehash(X)
  #define CHECK_PAGE(x)
  #endif  /* SQLITE_CHECK_PAGES */
  
  /*
  ** When this is called the journal file for pager pPager must be open.
  ** This function attempts to read a master journal file name from the 
  ** end of the file and, if successful, copies it into memory supplied 
  ** by the caller. See comments above writeMasterJournal() for the format
  ** used to store a master journal file name at the end of a journal file.
  **
  ** zMaster must point to a buffer of at least nMaster bytes allocated by
  ** the caller. This should be sqlite3_vfs.mxPathname+1 (to ensure there is
  ** enough space to write the master journal name). If the master journal
  ** name in the journal is longer than nMaster bytes (including a
  ** nul-terminator), then this is handled as if no master journal name
  ** were present in the journal.
  **
  ** If a master journal file name is present at the end of the journal
  ** file, then it is copied into the buffer pointed to by zMaster. A
  ** nul-terminator byte is appended to the buffer following the master
  ** journal file name.
  **
  ** If it is determined that no master journal file name is present 
  ** zMaster[0] is set to 0 and SQLITE_OK returned.
  **
  ** If an error occurs while reading from the journal file, an SQLite
  ** error code is returned.
  */
  static int readMasterJournal(sqlite3_file *pJrnl, char *zMaster, u32 nMaster){
    int rc;                    /* Return code */
    u32 len;                   /* Length in bytes of master journal name */
    i64 szJ;                   /* Total size in bytes of journal file pJrnl */
    u32 cksum;                 /* MJ checksum value read from journal */
    u32 u;                     /* Unsigned loop counter */
    unsigned char aMagic[8];   /* A buffer to hold the magic header */
    zMaster[0] = '\0';
  
    if( SQLITE_OK!=(rc = sqlite3OsFileSize(pJrnl, &szJ))
     || szJ<16
     || SQLITE_OK!=(rc = read32bits(pJrnl, szJ-16, &len))
     || len>=nMaster 
     || len==0 
     || SQLITE_OK!=(rc = read32bits(pJrnl, szJ-12, &cksum))
     || SQLITE_OK!=(rc = sqlite3OsRead(pJrnl, aMagic, 8, szJ-8))
     || memcmp(aMagic, aJournalMagic, 8)
     || SQLITE_OK!=(rc = sqlite3OsRead(pJrnl, zMaster, len, szJ-16-len))
    ){
      return rc;
    }
  
    /* See if the checksum matches the master journal name */
    for(u=0; u<len; u++){
      cksum -= zMaster[u];
    }
    if( cksum ){
      /* If the checksum doesn't add up, then one or more of the disk sectors
      ** containing the master journal filename is corrupted. This means
      ** definitely roll back, so just return SQLITE_OK and report a (nul)
      ** master-journal filename.
      */
      len = 0;
    }
    zMaster[len] = '\0';
     
    return SQLITE_OK;
  }
  
  /*
  ** Return the offset of the sector boundary at or immediately 
  ** following the value in pPager->journalOff, assuming a sector 
  ** size of pPager->sectorSize bytes.
  **
  ** i.e for a sector size of 512:
  **
  **   Pager.journalOff          Return value
  **   ---------------------------------------
  **   0                         0
  **   512                       512
  **   100                       512
  **   2000                      2048
  ** 
  */
  static i64 journalHdrOffset(Pager *pPager){
    i64 offset = 0;
    i64 c = pPager->journalOff;
    if( c ){
      offset = ((c-1)/JOURNAL_HDR_SZ(pPager) + 1) * JOURNAL_HDR_SZ(pPager);
    }
    assert( offset%JOURNAL_HDR_SZ(pPager)==0 );
    assert( offset>=c );
    assert( (offset-c)<JOURNAL_HDR_SZ(pPager) );
    return offset;
  }
  
  /*
  ** The journal file must be open when this function is called.
  **
  ** This function is a no-op if the journal file has not been written to
  ** within the current transaction (i.e. if Pager.journalOff==0).
  **
  ** If doTruncate is non-zero or the Pager.journalSizeLimit variable is
  ** set to 0, then truncate the journal file to zero bytes in size. Otherwise,
  ** zero the 28-byte header at the start of the journal file. In either case, 
  ** if the pager is not in no-sync mode, sync the journal file immediately 
  ** after writing or truncating it.
  **
  ** If Pager.journalSizeLimit is set to a positive, non-zero value, and
  ** following the truncation or zeroing described above the size of the 
  ** journal file in bytes is larger than this value, then truncate the
  ** journal file to Pager.journalSizeLimit bytes. The journal file does
  ** not need to be synced following this operation.
  **
  ** If an IO error occurs, abandon processing and return the IO error code.
  ** Otherwise, return SQLITE_OK.
  */
  static int zeroJournalHdr(Pager *pPager, int doTruncate){
    int rc = SQLITE_OK;                               /* Return code */
    assert( isOpen(pPager->jfd) );
    assert( !sqlite3JournalIsInMemory(pPager->jfd) );
    if( pPager->journalOff ){
      const i64 iLimit = pPager->journalSizeLimit;    /* Local cache of jsl */
  
      IOTRACE(("JZEROHDR %p\n", pPager))
      if( doTruncate || iLimit==0 ){
        rc = sqlite3OsTruncate(pPager->jfd, 0);
      }else{
        static const char zeroHdr[28] = {0};
        rc = sqlite3OsWrite(pPager->jfd, zeroHdr, sizeof(zeroHdr), 0);
      }
      if( rc==SQLITE_OK && !pPager->noSync ){
        rc = sqlite3OsSync(pPager->jfd, SQLITE_SYNC_DATAONLY|pPager->syncFlags);
      }
  
      /* At this point the transaction is committed but the write lock 
      ** is still held on the file. If there is a size limit configured for 
      ** the persistent journal and the journal file currently consumes more
      ** space than that limit allows for, truncate it now. There is no need
      ** to sync the file following this operation.
      */
      if( rc==SQLITE_OK && iLimit>0 ){
        i64 sz;
        rc = sqlite3OsFileSize(pPager->jfd, &sz);
        if( rc==SQLITE_OK && sz>iLimit ){
          rc = sqlite3OsTruncate(pPager->jfd, iLimit);
        }
      }
    }
    return rc;
  }
  
  /*
  ** The journal file must be open when this routine is called. A journal
  ** header (JOURNAL_HDR_SZ bytes) is written into the journal file at the
  ** current location.
  **
  ** The format for the journal header is as follows:
  ** - 8 bytes: Magic identifying journal format.
  ** - 4 bytes: Number of records in journal, or -1 no-sync mode is on.
  ** - 4 bytes: Random number used for page hash.
  ** - 4 bytes: Initial database page count.
  ** - 4 bytes: Sector size used by the process that wrote this journal.
  ** - 4 bytes: Database page size.
  ** 
  ** Followed by (JOURNAL_HDR_SZ - 28) bytes of unused space.
  */
  static int writeJournalHdr(Pager *pPager){
    int rc = SQLITE_OK;                 /* Return code */
    char *zHeader = pPager->pTmpSpace;  /* Temporary space used to build header */
    u32 nHeader = (u32)pPager->pageSize;/* Size of buffer pointed to by zHeader */
    u32 nWrite;                         /* Bytes of header sector written */
    int ii;                             /* Loop counter */
  
    assert( isOpen(pPager->jfd) );      /* Journal file must be open. */
  
    if( nHeader>JOURNAL_HDR_SZ(pPager) ){
      nHeader = JOURNAL_HDR_SZ(pPager);
    }
  
    /* If there are active savepoints and any of them were created 
    ** since the most recent journal header was written, update the 
    ** PagerSavepoint.iHdrOffset fields now.
    */
    for(ii=0; ii<pPager->nSavepoint; ii++){
      if( pPager->aSavepoint[ii].iHdrOffset==0 ){
        pPager->aSavepoint[ii].iHdrOffset = pPager->journalOff;
      }
    }
  
    pPager->journalHdr = pPager->journalOff = journalHdrOffset(pPager);
  
    /* 
    ** Write the nRec Field - the number of page records that follow this
    ** journal header. Normally, zero is written to this value at this time.
    ** After the records are added to the journal (and the journal synced, 
    ** if in full-sync mode), the zero is overwritten with the true number
    ** of records (see syncJournal()).
    **
    ** A faster alternative is to write 0xFFFFFFFF to the nRec field. When
    ** reading the journal this value tells SQLite to assume that the
    ** rest of the journal file contains valid page records. This assumption
    ** is dangerous, as if a failure occurred whilst writing to the journal
    ** file it may contain some garbage data. There are two scenarios
    ** where this risk can be ignored:
    **
    **   * When the pager is in no-sync mode. Corruption can follow a
    **     power failure in this case anyway.
    **
    **   * When the SQLITE_IOCAP_SAFE_APPEND flag is set. This guarantees
    **     that garbage data is never appended to the journal file.
    */
    assert( isOpen(pPager->fd) || pPager->noSync );
    if( pPager->noSync || (pPager->journalMode==PAGER_JOURNALMODE_MEMORY)
     || (sqlite3OsDeviceCharacteristics(pPager->fd)&SQLITE_IOCAP_SAFE_APPEND) 
    ){
      memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
      put32bits(&zHeader[sizeof(aJournalMagic)], 0xffffffff);
    }else{
      memset(zHeader, 0, sizeof(aJournalMagic)+4);
    }
  
    /* The random check-hash initializer */ 
    sqlite3_randomness(sizeof(pPager->cksumInit), &pPager->cksumInit);
    put32bits(&zHeader[sizeof(aJournalMagic)+4], pPager->cksumInit);
    /* The initial database size */
    put32bits(&zHeader[sizeof(aJournalMagic)+8], pPager->dbOrigSize);
    /* The assumed sector size for this process */
    put32bits(&zHeader[sizeof(aJournalMagic)+12], pPager->sectorSize);
  
    /* The page size */
    put32bits(&zHeader[sizeof(aJournalMagic)+16], pPager->pageSize);
  
    /* Initializing the tail of the buffer is not necessary.  Everything
    ** works find if the following memset() is omitted.  But initializing
    ** the memory prevents valgrind from complaining, so we are willing to
    ** take the performance hit.
    */
    memset(&zHeader[sizeof(aJournalMagic)+20], 0,
           nHeader-(sizeof(aJournalMagic)+20));
  
    /* In theory, it is only necessary to write the 28 bytes that the 
    ** journal header consumes to the journal file here. Then increment the 
    ** Pager.journalOff variable by JOURNAL_HDR_SZ so that the next 
    ** record is written to the following sector (leaving a gap in the file
    ** that will be implicitly filled in by the OS).
    **
    ** However it has been discovered that on some systems this pattern can 
    ** be significantly slower than contiguously writing data to the file,
    ** even if that means explicitly writing data to the block of 
    ** (JOURNAL_HDR_SZ - 28) bytes that will not be used. So that is what
    ** is done. 
    **
    ** The loop is required here in case the sector-size is larger than the 
    ** database page size. Since the zHeader buffer is only Pager.pageSize
    ** bytes in size, more than one call to sqlite3OsWrite() may be required
    ** to populate the entire journal header sector.
    */ 
    for(nWrite=0; rc==SQLITE_OK&&nWrite<JOURNAL_HDR_SZ(pPager); nWrite+=nHeader){
      IOTRACE(("JHDR %p %lld %d\n", pPager, pPager->journalHdr, nHeader))
      rc = sqlite3OsWrite(pPager->jfd, zHeader, nHeader, pPager->journalOff);
      assert( pPager->journalHdr <= pPager->journalOff );
      pPager->journalOff += nHeader;
    }
  
    return rc;
  }
  
  /*
  ** The journal file must be open when this is called. A journal header file
  ** (JOURNAL_HDR_SZ bytes) is read from the current location in the journal
  ** file. The current location in the journal file is given by
  ** pPager->journalOff. See comments above function writeJournalHdr() for
  ** a description of the journal header format.
  **
  ** If the header is read successfully, *pNRec is set to the number of
  ** page records following this header and *pDbSize is set to the size of the
  ** database before the transaction began, in pages. Also, pPager->cksumInit
  ** is set to the value read from the journal header. SQLITE_OK is returned
  ** in this case.
  **
  ** If the journal header file appears to be corrupted, SQLITE_DONE is
  ** returned and *pNRec and *PDbSize are undefined.  If JOURNAL_HDR_SZ bytes
  ** cannot be read from the journal file an error code is returned.
  */
  static int readJournalHdr(
    Pager *pPager,               /* Pager object */
    int isHot,
    i64 journalSize,             /* Size of the open journal file in bytes */
    u32 *pNRec,                  /* OUT: Value read from the nRec field */
    u32 *pDbSize                 /* OUT: Value of original database size field */
  ){
    int rc;                      /* Return code */
    unsigned char aMagic[8];     /* A buffer to hold the magic header */
    i64 iHdrOff;                 /* Offset of journal header being read */
  
    assert( isOpen(pPager->jfd) );      /* Journal file must be open. */
  
    /* Advance Pager.journalOff to the start of the next sector. If the
    ** journal file is too small for there to be a header stored at this
    ** point, return SQLITE_DONE.
    */
    pPager->journalOff = journalHdrOffset(pPager);
    if( pPager->journalOff+JOURNAL_HDR_SZ(pPager) > journalSize ){
      return SQLITE_DONE;
    }
    iHdrOff = pPager->journalOff;
  
    /* Read in the first 8 bytes of the journal header. If they do not match
    ** the  magic string found at the start of each journal header, return
    ** SQLITE_DONE. If an IO error occurs, return an error code. Otherwise,
    ** proceed.
    */
    if( isHot || iHdrOff!=pPager->journalHdr ){
      rc = sqlite3OsRead(pPager->jfd, aMagic, sizeof(aMagic), iHdrOff);
      if( rc ){
        return rc;
      }
      if( memcmp(aMagic, aJournalMagic, sizeof(aMagic))!=0 ){
        return SQLITE_DONE;
      }
    }
  
    /* Read the first three 32-bit fields of the journal header: The nRec
    ** field, the checksum-initializer and the database size at the start
    ** of the transaction. Return an error code if anything goes wrong.
    */
    if( SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+8, pNRec))
     || SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+12, &pPager->cksumInit))
     || SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+16, pDbSize))
    ){
      return rc;
    }
  
    if( pPager->journalOff==0 ){
      u32 iPageSize;               /* Page-size field of journal header */
      u32 iSectorSize;             /* Sector-size field of journal header */
  
      /* Read the page-size and sector-size journal header fields. */
      if( SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+20, &iSectorSize))
       || SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+24, &iPageSize))
      ){
        return rc;
      }
  
      /* Versions of SQLite prior to 3.5.8 set the page-size field of the
      ** journal header to zero. In this case, assume that the Pager.pageSize
      ** variable is already set to the correct page size.
      */
      if( iPageSize==0 ){
        iPageSize = pPager->pageSize;
      }
  
      /* Check that the values read from the page-size and sector-size fields
      ** are within range. To be 'in range', both values need to be a power
      ** of two greater than or equal to 512 or 32, and not greater than their 
      ** respective compile time maximum limits.
      */
      if( iPageSize<512                  || iSectorSize<32
       || iPageSize>SQLITE_MAX_PAGE_SIZE || iSectorSize>MAX_SECTOR_SIZE
       || ((iPageSize-1)&iPageSize)!=0   || ((iSectorSize-1)&iSectorSize)!=0 
      ){
        /* If the either the page-size or sector-size in the journal-header is 
        ** invalid, then the process that wrote the journal-header must have 
        ** crashed before the header was synced. In this case stop reading 
        ** the journal file here.
        */
        return SQLITE_DONE;
      }
  
      /* Update the page-size to match the value read from the journal. 
      ** Use a testcase() macro to make sure that malloc failure within 
      ** PagerSetPagesize() is tested.
      */
      rc = sqlite3PagerSetPagesize(pPager, &iPageSize, -1);
      testcase( rc!=SQLITE_OK );
  
      /* Update the assumed sector-size to match the value used by 
      ** the process that created this journal. If this journal was
      ** created by a process other than this one, then this routine
      ** is being called from within pager_playback(). The local value
      ** of Pager.sectorSize is restored at the end of that routine.
      */
      pPager->sectorSize = iSectorSize;
    }
  
    pPager->journalOff += JOURNAL_HDR_SZ(pPager);
    return rc;
  }
  
  
  /*
  ** Write the supplied master journal name into the journal file for pager
  ** pPager at the current location. The master journal name must be the last
  ** thing written to a journal file. If the pager is in full-sync mode, the
  ** journal file descriptor is advanced to the next sector boundary before
  ** anything is written. The format is:
  **
  **   + 4 bytes: PAGER_MJ_PGNO.
  **   + N bytes: Master journal filename in utf-8.
  **   + 4 bytes: N (length of master journal name in bytes, no nul-terminator).
  **   + 4 bytes: Master journal name checksum.
  **   + 8 bytes: aJournalMagic[].
  **
  ** The master journal page checksum is the sum of the bytes in the master
  ** journal name, where each byte is interpreted as a signed 8-bit integer.
  **
  ** If zMaster is a NULL pointer (occurs for a single database transaction), 
  ** this call is a no-op.
  */
  static int writeMasterJournal(Pager *pPager, const char *zMaster){
    int rc;                          /* Return code */
    int nMaster;                     /* Length of string zMaster */
    i64 iHdrOff;                     /* Offset of header in journal file */
    i64 jrnlSize;                    /* Size of journal file on disk */
    u32 cksum = 0;                   /* Checksum of string zMaster */
  
    assert( pPager->setMaster==0 );
    assert( !pagerUseWal(pPager) );
  
    if( !zMaster 
     || pPager->journalMode==PAGER_JOURNALMODE_MEMORY 
     || !isOpen(pPager->jfd)
    ){
      return SQLITE_OK;
    }
    pPager->setMaster = 1;
    assert( pPager->journalHdr <= pPager->journalOff );
  
    /* Calculate the length in bytes and the checksum of zMaster */
    for(nMaster=0; zMaster[nMaster]; nMaster++){
      cksum += zMaster[nMaster];
    }
  
    /* If in full-sync mode, advance to the next disk sector before writing
    ** the master journal name. This is in case the previous page written to
    ** the journal has already been synced.
    */
    if( pPager->fullSync ){
      pPager->journalOff = journalHdrOffset(pPager);
    }
    iHdrOff = pPager->journalOff;
  
    /* Write the master journal data to the end of the journal file. If
    ** an error occurs, return the error code to the caller.
    */
    if( (0 != (rc = write32bits(pPager->jfd, iHdrOff, PAGER_MJ_PGNO(pPager))))
     || (0 != (rc = sqlite3OsWrite(pPager->jfd, zMaster, nMaster, iHdrOff+4)))
     || (0 != (rc = write32bits(pPager->jfd, iHdrOff+4+nMaster, nMaster)))
     || (0 != (rc = write32bits(pPager->jfd, iHdrOff+4+nMaster+4, cksum)))
     || (0 != (rc = sqlite3OsWrite(pPager->jfd, aJournalMagic, 8,
                                   iHdrOff+4+nMaster+8)))
    ){
      return rc;
    }
    pPager->journalOff += (nMaster+20);
  
    /* If the pager is in peristent-journal mode, then the physical 
    ** journal-file may extend past the end of the master-journal name
    ** and 8 bytes of magic data just written to the file. This is 
    ** dangerous because the code to rollback a hot-journal file
    ** will not be able to find the master-journal name to determine 
    ** whether or not the journal is hot. 
    **
    ** Easiest thing to do in this scenario is to truncate the journal 
    ** file to the required size.
    */ 
    if( SQLITE_OK==(rc = sqlite3OsFileSize(pPager->jfd, &jrnlSize))
     && jrnlSize>pPager->journalOff
    ){
      rc = sqlite3OsTruncate(pPager->jfd, pPager->journalOff);
    }
    return rc;
  }
  
  /*
  ** Discard the entire contents of the in-memory page-cache.
  */
  static void pager_reset(Pager *pPager){
    pPager->iDataVersion++;
    sqlite3BackupRestart(pPager->pBackup);
    sqlite3PcacheClear(pPager->pPCache);
  }
  
  /*
  ** Return the pPager->iDataVersion value
  */
  u32 sqlite3PagerDataVersion(Pager *pPager){
    assert( pPager->eState>PAGER_OPEN );
    return pPager->iDataVersion;
  }
  
  /*
  ** Free all structures in the Pager.aSavepoint[] array and set both
  ** Pager.aSavepoint and Pager.nSavepoint to zero. Close the sub-journal
  ** if it is open and the pager is not in exclusive mode.
  */
  static void releaseAllSavepoints(Pager *pPager){
    int ii;               /* Iterator for looping through Pager.aSavepoint */
    for(ii=0; ii<pPager->nSavepoint; ii++){
      sqlite3BitvecDestroy(pPager->aSavepoint[ii].pInSavepoint);
    }
    if( !pPager->exclusiveMode || sqlite3JournalIsInMemory(pPager->sjfd) ){
      sqlite3OsClose(pPager->sjfd);
    }
    sqlite3_free(pPager->aSavepoint);
    pPager->aSavepoint = 0;
    pPager->nSavepoint = 0;
    pPager->nSubRec = 0;
  }
  
  /*
  ** Set the bit number pgno in the PagerSavepoint.pInSavepoint 
  ** bitvecs of all open savepoints. Return SQLITE_OK if successful
  ** or SQLITE_NOMEM if a malloc failure occurs.
  */
  static int addToSavepointBitvecs(Pager *pPager, Pgno pgno){
    int ii;                   /* Loop counter */
    int rc = SQLITE_OK;       /* Result code */
  
    for(ii=0; ii<pPager->nSavepoint; ii++){
      PagerSavepoint *p = &pPager->aSavepoint[ii];
      if( pgno<=p->nOrig ){
        rc |= sqlite3BitvecSet(p->pInSavepoint, pgno);
        testcase( rc==SQLITE_NOMEM );
        assert( rc==SQLITE_OK || rc==SQLITE_NOMEM );
      }
    }
    return rc;
  }
  
  /*
  ** This function is a no-op if the pager is in exclusive mode and not
  ** in the ERROR state. Otherwise, it switches the pager to PAGER_OPEN
  ** state.
  **
  ** If the pager is not in exclusive-access mode, the database file is
  ** completely unlocked. If the file is unlocked and the file-system does
  ** not exhibit the UNDELETABLE_WHEN_OPEN property, the journal file is
  ** closed (if it is open).
  **
  ** If the pager is in ERROR state when this function is called, the 
  ** contents of the pager cache are discarded before switching back to 
  ** the OPEN state. Regardless of whether the pager is in exclusive-mode
  ** or not, any journal file left in the file-system will be treated
  ** as a hot-journal and rolled back the next time a read-transaction
  ** is opened (by this or by any other connection).
  */
  static void pager_unlock(Pager *pPager){
  
    assert( pPager->eState==PAGER_READER 
         || pPager->eState==PAGER_OPEN 
         || pPager->eState==PAGER_ERROR 
    );
  
    sqlite3BitvecDestroy(pPager->pInJournal);
    pPager->pInJournal = 0;
    releaseAllSavepoints(pPager);
  
    if( pagerUseWal(pPager) ){
      assert( !isOpen(pPager->jfd) );
      sqlite3WalEndReadTransaction(pPager->pWal);
      pPager->eState = PAGER_OPEN;
    }else if( !pPager->exclusiveMode ){
      int rc;                       /* Error code returned by pagerUnlockDb() */
      int iDc = isOpen(pPager->fd)?sqlite3OsDeviceCharacteristics(pPager->fd):0;
  
      /* If the operating system support deletion of open files, then
      ** close the journal file when dropping the database lock.  Otherwise
      ** another connection with journal_mode=delete might delete the file
      ** out from under us.
      */
      assert( (PAGER_JOURNALMODE_MEMORY   & 5)!=1 );
      assert( (PAGER_JOURNALMODE_OFF      & 5)!=1 );
      assert( (PAGER_JOURNALMODE_WAL      & 5)!=1 );
      assert( (PAGER_JOURNALMODE_DELETE   & 5)!=1 );
      assert( (PAGER_JOURNALMODE_TRUNCATE & 5)==1 );
      assert( (PAGER_JOURNALMODE_PERSIST  & 5)==1 );
      if( 0==(iDc & SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN)
       || 1!=(pPager->journalMode & 5)
      ){
        sqlite3OsClose(pPager->jfd);
      }
  
      /* If the pager is in the ERROR state and the call to unlock the database
      ** file fails, set the current lock to UNKNOWN_LOCK. See the comment
      ** above the #define for UNKNOWN_LOCK for an explanation of why this
      ** is necessary.
      */
      rc = pagerUnlockDb(pPager, NO_LOCK);
      if( rc!=SQLITE_OK && pPager->eState==PAGER_ERROR ){
        pPager->eLock = UNKNOWN_LOCK;
      }
  
      /* The pager state may be changed from PAGER_ERROR to PAGER_OPEN here
      ** without clearing the error code. This is intentional - the error
      ** code is cleared and the cache reset in the block below.
      */
      assert( pPager->errCode || pPager->eState!=PAGER_ERROR );
      pPager->changeCountDone = 0;
      pPager->eState = PAGER_OPEN;
    }
  
    /* If Pager.errCode is set, the contents of the pager cache cannot be
    ** trusted. Now that there are no outstanding references to the pager,
    ** it can safely move back to PAGER_OPEN state. This happens in both
    ** normal and exclusive-locking mode.
    */
    assert( pPager->errCode==SQLITE_OK || !MEMDB );
    if( pPager->errCode ){
      if( pPager->tempFile==0 ){
        pager_reset(pPager);
        pPager->changeCountDone = 0;
        pPager->eState = PAGER_OPEN;
      }else{
        pPager->eState = (isOpen(pPager->jfd) ? PAGER_OPEN : PAGER_READER);
      }
      if( USEFETCH(pPager) ) sqlite3OsUnfetch(pPager->fd, 0, 0);
      pPager->errCode = SQLITE_OK;
    }
  
    pPager->journalOff = 0;
    pPager->journalHdr = 0;
    pPager->setMaster = 0;
  }
  
  /*
  ** This function is called whenever an IOERR or FULL error that requires
  ** the pager to transition into the ERROR state may ahve occurred.
  ** The first argument is a pointer to the pager structure, the second 
  ** the error-code about to be returned by a pager API function. The 
  ** value returned is a copy of the second argument to this function. 
  **
  ** If the second argument is SQLITE_FULL, SQLITE_IOERR or one of the
  ** IOERR sub-codes, the pager enters the ERROR state and the error code
  ** is stored in Pager.errCode. While the pager remains in the ERROR state,
  ** all major API calls on the Pager will immediately return Pager.errCode.
  **
  ** The ERROR state indicates that the contents of the pager-cache 
  ** cannot be trusted. This state can be cleared by completely discarding 
  ** the contents of the pager-cache. If a transaction was active when
  ** the persistent error occurred, then the rollback journal may need
  ** to be replayed to restore the contents of the database file (as if
  ** it were a hot-journal).
  */
  static int pager_error(Pager *pPager, int rc){
    int rc2 = rc & 0xff;
    assert( rc==SQLITE_OK || !MEMDB );
    assert(
         pPager->errCode==SQLITE_FULL ||
         pPager->errCode==SQLITE_OK ||
         (pPager->errCode & 0xff)==SQLITE_IOERR
    );
    if( rc2==SQLITE_FULL || rc2==SQLITE_IOERR ){
      pPager->errCode = rc;
      pPager->eState = PAGER_ERROR;
    }
    return rc;
  }
  
  static int pager_truncate(Pager *pPager, Pgno nPage);
  
  /*
  ** The write transaction open on pPager is being committed (bCommit==1)
  ** or rolled back (bCommit==0).
  **
  ** Return TRUE if and only if all dirty pages should be flushed to disk.
  **
  ** Rules:
  **
  **   *  For non-TEMP databases, always sync to disk.  This is necessary
  **      for transactions to be durable.
  **
  **   *  Sync TEMP database only on a COMMIT (not a ROLLBACK) when the backing
  **      file has been created already (via a spill on pagerStress()) and
  **      when the number of dirty pages in memory exceeds 25% of the total
  **      cache size.
  */
  static int pagerFlushOnCommit(Pager *pPager, int bCommit){
    if( pPager->tempFile==0 ) return 1;
    if( !bCommit ) return 0;
    if( !isOpen(pPager->fd) ) return 0;
    return (sqlite3PCachePercentDirty(pPager->pPCache)>=25);
  }
  
  /*
  ** This routine ends a transaction. A transaction is usually ended by 
  ** either a COMMIT or a ROLLBACK operation. This routine may be called 
  ** after rollback of a hot-journal, or if an error occurs while opening
  ** the journal file or writing the very first journal-header of a
  ** database transaction.
  ** 
  ** This routine is never called in PAGER_ERROR state. If it is called
  ** in PAGER_NONE or PAGER_SHARED state and the lock held is less
  ** exclusive than a RESERVED lock, it is a no-op.
  **
  ** Otherwise, any active savepoints are released.
  **
  ** If the journal file is open, then it is "finalized". Once a journal 
  ** file has been finalized it is not possible to use it to roll back a 
  ** transaction. Nor will it be considered to be a hot-journal by this
  ** or any other database connection. Exactly how a journal is finalized
  ** depends on whether or not the pager is running in exclusive mode and
  ** the current journal-mode (Pager.journalMode value), as follows:
  **
  **   journalMode==MEMORY
  **     Journal file descriptor is simply closed. This destroys an 
  **     in-memory journal.
  **
  **   journalMode==TRUNCATE
  **     Journal file is truncated to zero bytes in size.
  **
  **   journalMode==PERSIST
  **     The first 28 bytes of the journal file are zeroed. This invalidates
  **     the first journal header in the file, and hence the entire journal
  **     file. An invalid journal file cannot be rolled back.
  **
  **   journalMode==DELETE
  **     The journal file is closed and deleted using sqlite3OsDelete().
  **
  **     If the pager is running in exclusive mode, this method of finalizing
  **     the journal file is never used. Instead, if the journalMode is
  **     DELETE and the pager is in exclusive mode, the method described under
  **     journalMode==PERSIST is used instead.
  **
  ** After the journal is finalized, the pager moves to PAGER_READER state.
  ** If running in non-exclusive rollback mode, the lock on the file is 
  ** downgraded to a SHARED_LOCK.
  **
  ** SQLITE_OK is returned if no error occurs. If an error occurs during
  ** any of the IO operations to finalize the journal file or unlock the
  ** database then the IO error code is returned to the user. If the 
  ** operation to finalize the journal file fails, then the code still
  ** tries to unlock the database file if not in exclusive mode. If the
  ** unlock operation fails as well, then the first error code related
  ** to the first error encountered (the journal finalization one) is
  ** returned.
  */
  static int pager_end_transaction(Pager *pPager, int hasMaster, int bCommit){
    int rc = SQLITE_OK;      /* Error code from journal finalization operation */
    int rc2 = SQLITE_OK;     /* Error code from db file unlock operation */
  
    /* Do nothing if the pager does not have an open write transaction
    ** or at least a RESERVED lock. This function may be called when there
    ** is no write-transaction active but a RESERVED or greater lock is
    ** held under two circumstances:
    **
    **   1. After a successful hot-journal rollback, it is called with
    **      eState==PAGER_NONE and eLock==EXCLUSIVE_LOCK.
    **
    **   2. If a connection with locking_mode=exclusive holding an EXCLUSIVE 
    **      lock switches back to locking_mode=normal and then executes a
    **      read-transaction, this function is called with eState==PAGER_READER 
    **      and eLock==EXCLUSIVE_LOCK when the read-transaction is closed.
    */
    assert( assert_pager_state(pPager) );
    assert( pPager->eState!=PAGER_ERROR );
    if( pPager->eState<PAGER_WRITER_LOCKED && pPager->eLock<RESERVED_LOCK ){
      return SQLITE_OK;
    }
  
    releaseAllSavepoints(pPager);
    assert( isOpen(pPager->jfd) || pPager->pInJournal==0 );
    if( isOpen(pPager->jfd) ){
      assert( !pagerUseWal(pPager) );
  
      /* Finalize the journal file. */
      if( sqlite3JournalIsInMemory(pPager->jfd) ){
        /* assert( pPager->journalMode==PAGER_JOURNALMODE_MEMORY ); */
        sqlite3OsClose(pPager->jfd);
      }else if( pPager->journalMode==PAGER_JOURNALMODE_TRUNCATE ){
        if( pPager->journalOff==0 ){
          rc = SQLITE_OK;
        }else{
          rc = sqlite3OsTruncate(pPager->jfd, 0);
          if( rc==SQLITE_OK && pPager->fullSync ){
            /* Make sure the new file size is written into the inode right away.
            ** Otherwise the journal might resurrect following a power loss and
            ** cause the last transaction to roll back.  See
            ** https://bugzilla.mozilla.org/show_bug.cgi?id=1072773
            */
            rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags);
          }
        }
        pPager->journalOff = 0;
      }else if( pPager->journalMode==PAGER_JOURNALMODE_PERSIST
        || (pPager->exclusiveMode && pPager->journalMode!=PAGER_JOURNALMODE_WAL)
      ){
        rc = zeroJournalHdr(pPager, hasMaster||pPager->tempFile);
        pPager->journalOff = 0;
      }else{
        /* This branch may be executed with Pager.journalMode==MEMORY if
        ** a hot-journal was just rolled back. In this case the journal
        ** file should be closed and deleted. If this connection writes to
        ** the database file, it will do so using an in-memory journal.
        */
        int bDelete = !pPager->tempFile;
        assert( sqlite3JournalIsInMemory(pPager->jfd)==0 );
        assert( pPager->journalMode==PAGER_JOURNALMODE_DELETE 
             || pPager->journalMode==PAGER_JOURNALMODE_MEMORY 
             || pPager->journalMode==PAGER_JOURNALMODE_WAL 
        );
        sqlite3OsClose(pPager->jfd);
        if( bDelete ){
          rc = sqlite3OsDelete(pPager->pVfs, pPager->zJournal, pPager->extraSync);
        }
      }
    }
  
  #ifdef SQLITE_CHECK_PAGES
    sqlite3PcacheIterateDirty(pPager->pPCache, pager_set_pagehash);
    if( pPager->dbSize==0 && sqlite3PcacheRefCount(pPager->pPCache)>0 ){
      PgHdr *p = sqlite3PagerLookup(pPager, 1);
      if( p ){
        p->pageHash = 0;
        sqlite3PagerUnrefNotNull(p);
      }
    }
  #endif
  
    sqlite3BitvecDestroy(pPager->pInJournal);
    pPager->pInJournal = 0;
    pPager->nRec = 0;
    if( rc==SQLITE_OK ){
      if( pagerFlushOnCommit(pPager, bCommit) ){
        sqlite3PcacheCleanAll(pPager->pPCache);
      }else{
        sqlite3PcacheClearWritable(pPager->pPCache);
      }
      sqlite3PcacheTruncate(pPager->pPCache, pPager->dbSize);
    }
  
    if( pagerUseWal(pPager) ){
      /* Drop the WAL write-lock, if any. Also, if the connection was in 
      ** locking_mode=exclusive mode but is no longer, drop the EXCLUSIVE 
      ** lock held on the database file.
      */
      rc2 = sqlite3WalEndWriteTransaction(pPager->pWal);
      assert( rc2==SQLITE_OK );
    }else if( rc==SQLITE_OK && bCommit && pPager->dbFileSize>pPager->dbSize ){
      /* This branch is taken when committing a transaction in rollback-journal
      ** mode if the database file on disk is larger than the database image.
      ** At this point the journal has been finalized and the transaction 
      ** successfully committed, but the EXCLUSIVE lock is still held on the
      ** file. So it is safe to truncate the database file to its minimum
      ** required size.  */
      assert( pPager->eLock==EXCLUSIVE_LOCK );
      rc = pager_truncate(pPager, pPager->dbSize);
    }
  
    if( rc==SQLITE_OK && bCommit && isOpen(pPager->fd) ){
      rc = sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_COMMIT_PHASETWO, 0);
      if( rc==SQLITE_NOTFOUND ) rc = SQLITE_OK;
    }
  
    if( !pPager->exclusiveMode 
     && (!pagerUseWal(pPager) || sqlite3WalExclusiveMode(pPager->pWal, 0))
    ){
      rc2 = pagerUnlockDb(pPager, SHARED_LOCK);
      pPager->changeCountDone = 0;
    }
    pPager->eState = PAGER_READER;
    pPager->setMaster = 0;
  
    return (rc==SQLITE_OK?rc2:rc);
  }
  
  /*
  ** Execute a rollback if a transaction is active and unlock the 
  ** database file. 
  **
  ** If the pager has already entered the ERROR state, do not attempt 
  ** the rollback at this time. Instead, pager_unlock() is called. The
  ** call to pager_unlock() will discard all in-memory pages, unlock
  ** the database file and move the pager back to OPEN state. If this 
  ** means that there is a hot-journal left in the file-system, the next 
  ** connection to obtain a shared lock on the pager (which may be this one) 
  ** will roll it back.
  **
  ** If the pager has not already entered the ERROR state, but an IO or
  ** malloc error occurs during a rollback, then this will itself cause 
  ** the pager to enter the ERROR state. Which will be cleared by the
  ** call to pager_unlock(), as described above.
  */
  static void pagerUnlockAndRollback(Pager *pPager){
    if( pPager->eState!=PAGER_ERROR && pPager->eState!=PAGER_OPEN ){
      assert( assert_pager_state(pPager) );
      if( pPager->eState>=PAGER_WRITER_LOCKED ){
        sqlite3BeginBenignMalloc();
        sqlite3PagerRollback(pPager);
        sqlite3EndBenignMalloc();
      }else if( !pPager->exclusiveMode ){
        assert( pPager->eState==PAGER_READER );
        pager_end_transaction(pPager, 0, 0);
      }
    }
    pager_unlock(pPager);
  }
  
  /*
  ** Parameter aData must point to a buffer of pPager->pageSize bytes
  ** of data. Compute and return a checksum based ont the contents of the 
  ** page of data and the current value of pPager->cksumInit.
  **
  ** This is not a real checksum. It is really just the sum of the 
  ** random initial value (pPager->cksumInit) and every 200th byte
  ** of the page data, starting with byte offset (pPager->pageSize%200).
  ** Each byte is interpreted as an 8-bit unsigned integer.
  **
  ** Changing the formula used to compute this checksum results in an
  ** incompatible journal file format.
  **
  ** If journal corruption occurs due to a power failure, the most likely 
  ** scenario is that one end or the other of the record will be changed. 
  ** It is much less likely that the two ends of the journal record will be
  ** correct and the middle be corrupt.  Thus, this "checksum" scheme,
  ** though fast and simple, catches the mostly likely kind of corruption.
  */
  static u32 pager_cksum(Pager *pPager, const u8 *aData){
    u32 cksum = pPager->cksumInit;         /* Checksum value to return */
    int i = pPager->pageSize-200;          /* Loop counter */
    while( i>0 ){
      cksum += aData[i];
      i -= 200;
    }
    return cksum;
  }
  
  /*
  ** Report the current page size and number of reserved bytes back
  ** to the codec.
  */
  #ifdef SQLITE_HAS_CODEC
  static void pagerReportSize(Pager *pPager){
    if( pPager->xCodecSizeChng ){
      pPager->xCodecSizeChng(pPager->pCodec, pPager->pageSize,
                             (int)pPager->nReserve);
    }
  }
  #else
  # define pagerReportSize(X)     /* No-op if we do not support a codec */
  #endif
  
  #ifdef SQLITE_HAS_CODEC
  /*
  ** Make sure the number of reserved bits is the same in the destination
  ** pager as it is in the source.  This comes up when a VACUUM changes the
  ** number of reserved bits to the "optimal" amount.
  */
  void sqlite3PagerAlignReserve(Pager *pDest, Pager *pSrc){
    if( pDest->nReserve!=pSrc->nReserve ){
      pDest->nReserve = pSrc->nReserve;
      pagerReportSize(pDest);
    }
  }
  #endif
  
  /*
  ** Read a single page from either the journal file (if isMainJrnl==1) or
  ** from the sub-journal (if isMainJrnl==0) and playback that page.
  ** The page begins at offset *pOffset into the file. The *pOffset
  ** value is increased to the start of the next page in the journal.
  **
  ** The main rollback journal uses checksums - the statement journal does 
  ** not.
  **
  ** If the page number of the page record read from the (sub-)journal file
  ** is greater than the current value of Pager.dbSize, then playback is
  ** skipped and SQLITE_OK is returned.
  **
  ** If pDone is not NULL, then it is a record of pages that have already
  ** been played back.  If the page at *pOffset has already been played back
  ** (if the corresponding pDone bit is set) then skip the playback.
  ** Make sure the pDone bit corresponding to the *pOffset page is set
  ** prior to returning.
  **
  ** If the page record is successfully read from the (sub-)journal file
  ** and played back, then SQLITE_OK is returned. If an IO error occurs
  ** while reading the record from the (sub-)journal file or while writing
  ** to the database file, then the IO error code is returned. If data
  ** is successfully read from the (sub-)journal file but appears to be
  ** corrupted, SQLITE_DONE is returned. Data is considered corrupted in
  ** two circumstances:
  ** 
  **   * If the record page-number is illegal (0 or PAGER_MJ_PGNO), or
  **   * If the record is being rolled back from the main journal file
  **     and the checksum field does not match the record content.
  **
  ** Neither of these two scenarios are possible during a savepoint rollback.
  **
  ** If this is a savepoint rollback, then memory may have to be dynamically
  ** allocated by this function. If this is the case and an allocation fails,
  ** SQLITE_NOMEM is returned.
  */
  static int pager_playback_one_page(
    Pager *pPager,                /* The pager being played back */
    i64 *pOffset,                 /* Offset of record to playback */
    Bitvec *pDone,                /* Bitvec of pages already played back */
    int isMainJrnl,               /* 1 -> main journal. 0 -> sub-journal. */
    int isSavepnt                 /* True for a savepoint rollback */
  ){
    int rc;
    PgHdr *pPg;                   /* An existing page in the cache */
    Pgno pgno;                    /* The page number of a page in journal */
    u32 cksum;                    /* Checksum used for sanity checking */
    char *aData;                  /* Temporary storage for the page */
    sqlite3_file *jfd;            /* The file descriptor for the journal file */
    int isSynced;                 /* True if journal page is synced */
  
    assert( (isMainJrnl&~1)==0 );      /* isMainJrnl is 0 or 1 */
    assert( (isSavepnt&~1)==0 );       /* isSavepnt is 0 or 1 */
    assert( isMainJrnl || pDone );     /* pDone always used on sub-journals */
    assert( isSavepnt || pDone==0 );   /* pDone never used on non-savepoint */
  
    aData = pPager->pTmpSpace;
    assert( aData );         /* Temp storage must have already been allocated */
    assert( pagerUseWal(pPager)==0 || (!isMainJrnl && isSavepnt) );
  
    /* Either the state is greater than PAGER_WRITER_CACHEMOD (a transaction 
    ** or savepoint rollback done at the request of the caller) or this is
    ** a hot-journal rollback. If it is a hot-journal rollback, the pager
    ** is in state OPEN and holds an EXCLUSIVE lock. Hot-journal rollback
    ** only reads from the main journal, not the sub-journal.
    */
    assert( pPager->eState>=PAGER_WRITER_CACHEMOD
         || (pPager->eState==PAGER_OPEN && pPager->eLock==EXCLUSIVE_LOCK)
    );
    assert( pPager->eState>=PAGER_WRITER_CACHEMOD || isMainJrnl );
  
    /* Read the page number and page data from the journal or sub-journal
    ** file. Return an error code to the caller if an IO error occurs.
    */
    jfd = isMainJrnl ? pPager->jfd : pPager->sjfd;
    rc = read32bits(jfd, *pOffset, &pgno);
    if( rc!=SQLITE_OK ) return rc;
    rc = sqlite3OsRead(jfd, (u8*)aData, pPager->pageSize, (*pOffset)+4);
    if( rc!=SQLITE_OK ) return rc;
    *pOffset += pPager->pageSize + 4 + isMainJrnl*4;
  
    /* Sanity checking on the page.  This is more important that I originally
    ** thought.  If a power failure occurs while the journal is being written,
    ** it could cause invalid data to be written into the journal.  We need to
    ** detect this invalid data (with high probability) and ignore it.
    */
    if( pgno==0 || pgno==PAGER_MJ_PGNO(pPager) ){
      assert( !isSavepnt );
      return SQLITE_DONE;
    }
    if( pgno>(Pgno)pPager->dbSize || sqlite3BitvecTest(pDone, pgno) ){
      return SQLITE_OK;
    }
    if( isMainJrnl ){
      rc = read32bits(jfd, (*pOffset)-4, &cksum);
      if( rc ) return rc;
      if( !isSavepnt && pager_cksum(pPager, (u8*)aData)!=cksum ){
        return SQLITE_DONE;
      }
    }
  
    /* If this page has already been played back before during the current
    ** rollback, then don't bother to play it back again.
    */
    if( pDone && (rc = sqlite3BitvecSet(pDone, pgno))!=SQLITE_OK ){
      return rc;
    }
  
    /* When playing back page 1, restore the nReserve setting
    */
    if( pgno==1 && pPager->nReserve!=((u8*)aData)[20] ){
      pPager->nReserve = ((u8*)aData)[20];
      pagerReportSize(pPager);
    }
  
    /* If the pager is in CACHEMOD state, then there must be a copy of this
    ** page in the pager cache. In this case just update the pager cache,
    ** not the database file. The page is left marked dirty in this case.
    **
    ** An exception to the above rule: If the database is in no-sync mode
    ** and a page is moved during an incremental vacuum then the page may
    ** not be in the pager cache. Later: if a malloc() or IO error occurs
    ** during a Movepage() call, then the page may not be in the cache
    ** either. So the condition described in the above paragraph is not
    ** assert()able.
    **
    ** If in WRITER_DBMOD, WRITER_FINISHED or OPEN state, then we update the
    ** pager cache if it exists and the main file. The page is then marked 
    ** not dirty. Since this code is only executed in PAGER_OPEN state for
    ** a hot-journal rollback, it is guaranteed that the page-cache is empty
    ** if the pager is in OPEN state.
    **
    ** Ticket #1171:  The statement journal might contain page content that is
    ** different from the page content at the start of the transaction.
    ** This occurs when a page is changed prior to the start of a statement
    ** then changed again within the statement.  When rolling back such a
    ** statement we must not write to the original database unless we know
    ** for certain that original page contents are synced into the main rollback
    ** journal.  Otherwise, a power loss might leave modified data in the
    ** database file without an entry in the rollback journal that can
    ** restore the database to its original form.  Two conditions must be
    ** met before writing to the database files. (1) the database must be
    ** locked.  (2) we know that the original page content is fully synced
    ** in the main journal either because the page is not in cache or else
    ** the page is marked as needSync==0.
    **
    ** 2008-04-14:  When attempting to vacuum a corrupt database file, it
    ** is possible to fail a statement on a database that does not yet exist.
    ** Do not attempt to write if database file has never been opened.
    */
    if( pagerUseWal(pPager) ){
      pPg = 0;
    }else{
      pPg = sqlite3PagerLookup(pPager, pgno);
    }
    assert( pPg || !MEMDB );
    assert( pPager->eState!=PAGER_OPEN || pPg==0 || pPager->tempFile );
    PAGERTRACE(("PLAYBACK %d page %d hash(%08x) %s\n",
             PAGERID(pPager), pgno, pager_datahash(pPager->pageSize, (u8*)aData),
             (isMainJrnl?"main-journal":"sub-journal")
    ));
    if( isMainJrnl ){
      isSynced = pPager->noSync || (*pOffset <= pPager->journalHdr);
    }else{
      isSynced = (pPg==0 || 0==(pPg->flags & PGHDR_NEED_SYNC));
    }
    if( isOpen(pPager->fd)
     && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN)
     && isSynced
    ){
      i64 ofst = (pgno-1)*(i64)pPager->pageSize;
      testcase( !isSavepnt && pPg!=0 && (pPg->flags&PGHDR_NEED_SYNC)!=0 );
      assert( !pagerUseWal(pPager) );
      rc = sqlite3OsWrite(pPager->fd, (u8 *)aData, pPager->pageSize, ofst);
      if( pgno>pPager->dbFileSize ){
        pPager->dbFileSize = pgno;
      }
      if( pPager->pBackup ){
        CODEC1(pPager, aData, pgno, 3, rc=SQLITE_NOMEM_BKPT);
        sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)aData);
        CODEC2(pPager, aData, pgno, 7, rc=SQLITE_NOMEM_BKPT, aData);
      }
    }else if( !isMainJrnl && pPg==0 ){
      /* If this is a rollback of a savepoint and data was not written to
      ** the database and the page is not in-memory, there is a potential
      ** problem. When the page is next fetched by the b-tree layer, it 
      ** will be read from the database file, which may or may not be 
      ** current. 
      **
      ** There are a couple of different ways this can happen. All are quite
      ** obscure. When running in synchronous mode, this can only happen 
      ** if the page is on the free-list at the start of the transaction, then
      ** populated, then moved using sqlite3PagerMovepage().
      **
      ** The solution is to add an in-memory page to the cache containing
      ** the data just read from the sub-journal. Mark the page as dirty 
      ** and if the pager requires a journal-sync, then mark the page as 
      ** requiring a journal-sync before it is written.
      */
      assert( isSavepnt );
      assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)==0 );
      pPager->doNotSpill |= SPILLFLAG_ROLLBACK;
      rc = sqlite3PagerGet(pPager, pgno, &pPg, 1);
      assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)!=0 );
      pPager->doNotSpill &= ~SPILLFLAG_ROLLBACK;
      if( rc!=SQLITE_OK ) return rc;
      sqlite3PcacheMakeDirty(pPg);
    }
    if( pPg ){
      /* No page should ever be explicitly rolled back that is in use, except
      ** for page 1 which is held in use in order to keep the lock on the
      ** database active. However such a page may be rolled back as a result
      ** of an internal error resulting in an automatic call to
      ** sqlite3PagerRollback().
      */
      void *pData;
      pData = pPg->pData;
      memcpy(pData, (u8*)aData, pPager->pageSize);
      pPager->xReiniter(pPg);
      /* It used to be that sqlite3PcacheMakeClean(pPg) was called here.  But
      ** that call was dangerous and had no detectable benefit since the cache
      ** is normally cleaned by sqlite3PcacheCleanAll() after rollback and so
      ** has been removed. */
      pager_set_pagehash(pPg);
  
      /* If this was page 1, then restore the value of Pager.dbFileVers.
      ** Do this before any decoding. */
      if( pgno==1 ){
        memcpy(&pPager->dbFileVers, &((u8*)pData)[24],sizeof(pPager->dbFileVers));
      }
  
      /* Decode the page just read from disk */
      CODEC1(pPager, pData, pPg->pgno, 3, rc=SQLITE_NOMEM_BKPT);
      sqlite3PcacheRelease(pPg);
    }
    return rc;
  }
  
  /*
  ** Parameter zMaster is the name of a master journal file. A single journal
  ** file that referred to the master journal file has just been rolled back.
  ** This routine checks if it is possible to delete the master journal file,
  ** and does so if it is.
  **
  ** Argument zMaster may point to Pager.pTmpSpace. So that buffer is not 
  ** available for use within this function.
  **
  ** When a master journal file is created, it is populated with the names 
  ** of all of its child journals, one after another, formatted as utf-8 
  ** encoded text. The end of each child journal file is marked with a 
  ** nul-terminator byte (0x00). i.e. the entire contents of a master journal
  ** file for a transaction involving two databases might be:
  **
  **   "/home/bill/a.db-journal\x00/home/bill/b.db-journal\x00"
  **
  ** A master journal file may only be deleted once all of its child 
  ** journals have been rolled back.
  **
  ** This function reads the contents of the master-journal file into 
  ** memory and loops through each of the child journal names. For
  ** each child journal, it checks if:
  **
  **   * if the child journal exists, and if so
  **   * if the child journal contains a reference to master journal 
  **     file zMaster
  **
  ** If a child journal can be found that matches both of the criteria
  ** above, this function returns without doing anything. Otherwise, if
  ** no such child journal can be found, file zMaster is deleted from
  ** the file-system using sqlite3OsDelete().
  **
  ** If an IO error within this function, an error code is returned. This
  ** function allocates memory by calling sqlite3Malloc(). If an allocation
  ** fails, SQLITE_NOMEM is returned. Otherwise, if no IO or malloc errors 
  ** occur, SQLITE_OK is returned.
  **
  ** TODO: This function allocates a single block of memory to load
  ** the entire contents of the master journal file. This could be
  ** a couple of kilobytes or so - potentially larger than the page 
  ** size.
  */
  static int pager_delmaster(Pager *pPager, const char *zMaster){
    sqlite3_vfs *pVfs = pPager->pVfs;
    int rc;                   /* Return code */
    sqlite3_file *pMaster;    /* Malloc'd master-journal file descriptor */
    sqlite3_file *pJournal;   /* Malloc'd child-journal file descriptor */
    char *zMasterJournal = 0; /* Contents of master journal file */
    i64 nMasterJournal;       /* Size of master journal file */
    char *zJournal;           /* Pointer to one journal within MJ file */
    char *zMasterPtr;         /* Space to hold MJ filename from a journal file */
    int nMasterPtr;           /* Amount of space allocated to zMasterPtr[] */
  
    /* Allocate space for both the pJournal and pMaster file descriptors.
    ** If successful, open the master journal file for reading.
    */
    pMaster = (sqlite3_file *)sqlite3MallocZero(pVfs->szOsFile * 2);
    pJournal = (sqlite3_file *)(((u8 *)pMaster) + pVfs->szOsFile);
    if( !pMaster ){
      rc = SQLITE_NOMEM_BKPT;
    }else{
      const int flags = (SQLITE_OPEN_READONLY|SQLITE_OPEN_MASTER_JOURNAL);
      rc = sqlite3OsOpen(pVfs, zMaster, pMaster, flags, 0);
    }
    if( rc!=SQLITE_OK ) goto delmaster_out;
  
    /* Load the entire master journal file into space obtained from
    ** sqlite3_malloc() and pointed to by zMasterJournal.   Also obtain
    ** sufficient space (in zMasterPtr) to hold the names of master
    ** journal files extracted from regular rollback-journals.
    */
    rc = sqlite3OsFileSize(pMaster, &nMasterJournal);
    if( rc!=SQLITE_OK ) goto delmaster_out;
    nMasterPtr = pVfs->mxPathname+1;
    zMasterJournal = sqlite3Malloc(nMasterJournal + nMasterPtr + 1);
    if( !zMasterJournal ){
      rc = SQLITE_NOMEM_BKPT;
      goto delmaster_out;
    }
    zMasterPtr = &zMasterJournal[nMasterJournal+1];
    rc = sqlite3OsRead(pMaster, zMasterJournal, (int)nMasterJournal, 0);
    if( rc!=SQLITE_OK ) goto delmaster_out;
    zMasterJournal[nMasterJournal] = 0;
  
    zJournal = zMasterJournal;
    while( (zJournal-zMasterJournal)<nMasterJournal ){
      int exists;
      rc = sqlite3OsAccess(pVfs, zJournal, SQLITE_ACCESS_EXISTS, &exists);
      if( rc!=SQLITE_OK ){
        goto delmaster_out;
      }
      if( exists ){
        /* One of the journals pointed to by the master journal exists.
        ** Open it and check if it points at the master journal. If
        ** so, return without deleting the master journal file.
        */
        int c;
        int flags = (SQLITE_OPEN_READONLY|SQLITE_OPEN_MAIN_JOURNAL);
        rc = sqlite3OsOpen(pVfs, zJournal, pJournal, flags, 0);
        if( rc!=SQLITE_OK ){
          goto delmaster_out;
        }
  
        rc = readMasterJournal(pJournal, zMasterPtr, nMasterPtr);
        sqlite3OsClose(pJournal);
        if( rc!=SQLITE_OK ){
          goto delmaster_out;
        }
  
        c = zMasterPtr[0]!=0 && strcmp(zMasterPtr, zMaster)==0;
        if( c ){
          /* We have a match. Do not delete the master journal file. */
          goto delmaster_out;
        }
      }
      zJournal += (sqlite3Strlen30(zJournal)+1);
    }
   
    sqlite3OsClose(pMaster);
    rc = sqlite3OsDelete(pVfs, zMaster, 0);
  
  delmaster_out:
    sqlite3_free(zMasterJournal);
    if( pMaster ){
      sqlite3OsClose(pMaster);
      assert( !isOpen(pJournal) );
      sqlite3_free(pMaster);
    }
    return rc;
  }
  
  
  /*
  ** This function is used to change the actual size of the database 
  ** file in the file-system. This only happens when committing a transaction,
  ** or rolling back a transaction (including rolling back a hot-journal).
  **
  ** If the main database file is not open, or the pager is not in either
  ** DBMOD or OPEN state, this function is a no-op. Otherwise, the size 
  ** of the file is changed to nPage pages (nPage*pPager->pageSize bytes). 
  ** If the file on disk is currently larger than nPage pages, then use the VFS
  ** xTruncate() method to truncate it.
  **
  ** Or, it might be the case that the file on disk is smaller than 
  ** nPage pages. Some operating system implementations can get confused if 
  ** you try to truncate a file to some size that is larger than it 
  ** currently is, so detect this case and write a single zero byte to 
  ** the end of the new file instead.
  **
  ** If successful, return SQLITE_OK. If an IO error occurs while modifying
  ** the database file, return the error code to the caller.
  */
  static int pager_truncate(Pager *pPager, Pgno nPage){
    int rc = SQLITE_OK;
    assert( pPager->eState!=PAGER_ERROR );
    assert( pPager->eState!=PAGER_READER );
    
    if( isOpen(pPager->fd) 
     && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN) 
    ){
      i64 currentSize, newSize;
      int szPage = pPager->pageSize;
      assert( pPager->eLock==EXCLUSIVE_LOCK );
      /* TODO: Is it safe to use Pager.dbFileSize here? */
      rc = sqlite3OsFileSize(pPager->fd, &currentSize);
      newSize = szPage*(i64)nPage;
      if( rc==SQLITE_OK && currentSize!=newSize ){
        if( currentSize>newSize ){
          rc = sqlite3OsTruncate(pPager->fd, newSize);
        }else if( (currentSize+szPage)<=newSize ){
          char *pTmp = pPager->pTmpSpace;
          memset(pTmp, 0, szPage);
          testcase( (newSize-szPage) == currentSize );
          testcase( (newSize-szPage) >  currentSize );
          rc = sqlite3OsWrite(pPager->fd, pTmp, szPage, newSize-szPage);
        }
        if( rc==SQLITE_OK ){
          pPager->dbFileSize = nPage;
        }
      }
    }
    return rc;
  }
  
  /*
  ** Return a sanitized version of the sector-size of OS file pFile. The
  ** return value is guaranteed to lie between 32 and MAX_SECTOR_SIZE.
  */
  int sqlite3SectorSize(sqlite3_file *pFile){
    int iRet = sqlite3OsSectorSize(pFile);
    if( iRet<32 ){
      iRet = 512;
    }else if( iRet>MAX_SECTOR_SIZE ){
      assert( MAX_SECTOR_SIZE>=512 );
      iRet = MAX_SECTOR_SIZE;
    }
    return iRet;
  }
  
  /*
  ** Set the value of the Pager.sectorSize variable for the given
  ** pager based on the value returned by the xSectorSize method
  ** of the open database file. The sector size will be used 
  ** to determine the size and alignment of journal header and 
  ** master journal pointers within created journal files.
  **
  ** For temporary files the effective sector size is always 512 bytes.
  **
  ** Otherwise, for non-temporary files, the effective sector size is
  ** the value returned by the xSectorSize() method rounded up to 32 if
  ** it is less than 32, or rounded down to MAX_SECTOR_SIZE if it
  ** is greater than MAX_SECTOR_SIZE.
  **
  ** If the file has the SQLITE_IOCAP_POWERSAFE_OVERWRITE property, then set
  ** the effective sector size to its minimum value (512).  The purpose of
  ** pPager->sectorSize is to define the "blast radius" of bytes that
  ** might change if a crash occurs while writing to a single byte in
  ** that range.  But with POWERSAFE_OVERWRITE, the blast radius is zero
  ** (that is what POWERSAFE_OVERWRITE means), so we minimize the sector
  ** size.  For backwards compatibility of the rollback journal file format,
  ** we cannot reduce the effective sector size below 512.
  */
  static void setSectorSize(Pager *pPager){
    assert( isOpen(pPager->fd) || pPager->tempFile );
  
    if( pPager->tempFile
     || (sqlite3OsDeviceCharacteristics(pPager->fd) & 
                SQLITE_IOCAP_POWERSAFE_OVERWRITE)!=0
    ){
      /* Sector size doesn't matter for temporary files. Also, the file
      ** may not have been opened yet, in which case the OsSectorSize()
      ** call will segfault. */
      pPager->sectorSize = 512;
    }else{
      pPager->sectorSize = sqlite3SectorSize(pPager->fd);
    }
  }
  
  /*
  ** Playback the journal and thus restore the database file to
  ** the state it was in before we started making changes.  
  **
  ** The journal file format is as follows: 
  **
  **  (1)  8 byte prefix.  A copy of aJournalMagic[].
  **  (2)  4 byte big-endian integer which is the number of valid page records
  **       in the journal.  If this value is 0xffffffff, then compute the
  **       number of page records from the journal size.
  **  (3)  4 byte big-endian integer which is the initial value for the 
  **       sanity checksum.
  **  (4)  4 byte integer which is the number of pages to truncate the
  **       database to during a rollback.
  **  (5)  4 byte big-endian integer which is the sector size.  The header
  **       is this many bytes in size.
  **  (6)  4 byte big-endian integer which is the page size.
  **  (7)  zero padding out to the next sector size.
  **  (8)  Zero or more pages instances, each as follows:
  **        +  4 byte page number.
  **        +  pPager->pageSize bytes of data.
  **        +  4 byte checksum
  **
  ** When we speak of the journal header, we mean the first 7 items above.
  ** Each entry in the journal is an instance of the 8th item.
  **
  ** Call the value from the second bullet "nRec".  nRec is the number of
  ** valid page entries in the journal.  In most cases, you can compute the
  ** value of nRec from the size of the journal file.  But if a power
  ** failure occurred while the journal was being written, it could be the
  ** case that the size of the journal file had already been increased but
  ** the extra entries had not yet made it safely to disk.  In such a case,
  ** the value of nRec computed from the file size would be too large.  For
  ** that reason, we always use the nRec value in the header.
  **
  ** If the nRec value is 0xffffffff it means that nRec should be computed
  ** from the file size.  This value is used when the user selects the
  ** no-sync option for the journal.  A power failure could lead to corruption
  ** in this case.  But for things like temporary table (which will be
  ** deleted when the power is restored) we don't care.  
  **
  ** If the file opened as the journal file is not a well-formed
  ** journal file then all pages up to the first corrupted page are rolled
  ** back (or no pages if the journal header is corrupted). The journal file
  ** is then deleted and SQLITE_OK returned, just as if no corruption had
  ** been encountered.
  **
  ** If an I/O or malloc() error occurs, the journal-file is not deleted
  ** and an error code is returned.
  **
  ** The isHot parameter indicates that we are trying to rollback a journal
  ** that might be a hot journal.  Or, it could be that the journal is 
  ** preserved because of JOURNALMODE_PERSIST or JOURNALMODE_TRUNCATE.
  ** If the journal really is hot, reset the pager cache prior rolling
  ** back any content.  If the journal is merely persistent, no reset is
  ** needed.
  */
  static int pager_playback(Pager *pPager, int isHot){
    sqlite3_vfs *pVfs = pPager->pVfs;
    i64 szJ;                 /* Size of the journal file in bytes */
    u32 nRec;                /* Number of Records in the journal */
    u32 u;                   /* Unsigned loop counter */
    Pgno mxPg = 0;           /* Size of the original file in pages */
    int rc;                  /* Result code of a subroutine */
    int res = 1;             /* Value returned by sqlite3OsAccess() */
    char *zMaster = 0;       /* Name of master journal file if any */
    int needPagerReset;      /* True to reset page prior to first page rollback */
    int nPlayback = 0;       /* Total number of pages restored from journal */
  
    /* Figure out how many records are in the journal.  Abort early if
    ** the journal is empty.
    */
    assert( isOpen(pPager->jfd) );
    rc = sqlite3OsFileSize(pPager->jfd, &szJ);
    if( rc!=SQLITE_OK ){
      goto end_playback;
    }
  
    /* Read the master journal name from the journal, if it is present.
    ** If a master journal file name is specified, but the file is not
    ** present on disk, then the journal is not hot and does not need to be
    ** played back.
    **
    ** TODO: Technically the following is an error because it assumes that
    ** buffer Pager.pTmpSpace is (mxPathname+1) bytes or larger. i.e. that
    ** (pPager->pageSize >= pPager->pVfs->mxPathname+1). Using os_unix.c,
    ** mxPathname is 512, which is the same as the minimum allowable value
    ** for pageSize.
    */
    zMaster = pPager->pTmpSpace;
    rc = readMasterJournal(pPager->jfd, zMaster, pPager->pVfs->mxPathname+1);
    if( rc==SQLITE_OK && zMaster[0] ){
      rc = sqlite3OsAccess(pVfs, zMaster, SQLITE_ACCESS_EXISTS, &res);
    }
    zMaster = 0;
    if( rc!=SQLITE_OK || !res ){
      goto end_playback;
    }
    pPager->journalOff = 0;
    needPagerReset = isHot;
  
    /* This loop terminates either when a readJournalHdr() or 
    ** pager_playback_one_page() call returns SQLITE_DONE or an IO error 
    ** occurs. 
    */
    while( 1 ){
      /* Read the next journal header from the journal file.  If there are
      ** not enough bytes left in the journal file for a complete header, or
      ** it is corrupted, then a process must have failed while writing it.
      ** This indicates nothing more needs to be rolled back.
      */
      rc = readJournalHdr(pPager, isHot, szJ, &nRec, &mxPg);
      if( rc!=SQLITE_OK ){ 
        if( rc==SQLITE_DONE ){
          rc = SQLITE_OK;
        }
        goto end_playback;
      }
  
      /* If nRec is 0xffffffff, then this journal was created by a process
      ** working in no-sync mode. This means that the rest of the journal
      ** file consists of pages, there are no more journal headers. Compute
      ** the value of nRec based on this assumption.
      */
      if( nRec==0xffffffff ){
        assert( pPager->journalOff==JOURNAL_HDR_SZ(pPager) );
        nRec = (int)((szJ - JOURNAL_HDR_SZ(pPager))/JOURNAL_PG_SZ(pPager));
      }
  
      /* If nRec is 0 and this rollback is of a transaction created by this
      ** process and if this is the final header in the journal, then it means
      ** that this part of the journal was being filled but has not yet been
      ** synced to disk.  Compute the number of pages based on the remaining
      ** size of the file.
      **
      ** The third term of the test was added to fix ticket #2565.
      ** When rolling back a hot journal, nRec==0 always means that the next
      ** chunk of the journal contains zero pages to be rolled back.  But
      ** when doing a ROLLBACK and the nRec==0 chunk is the last chunk in
      ** the journal, it means that the journal might contain additional
      ** pages that need to be rolled back and that the number of pages 
      ** should be computed based on the journal file size.
      */
      if( nRec==0 && !isHot &&
          pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff ){
        nRec = (int)((szJ - pPager->journalOff) / JOURNAL_PG_SZ(pPager));
      }
  
      /* If this is the first header read from the journal, truncate the
      ** database file back to its original size.
      */
      if( pPager->journalOff==JOURNAL_HDR_SZ(pPager) ){
        rc = pager_truncate(pPager, mxPg);
        if( rc!=SQLITE_OK ){
          goto end_playback;
        }
        pPager->dbSize = mxPg;
      }
  
      /* Copy original pages out of the journal and back into the 
      ** database file and/or page cache.
      */
      for(u=0; u<nRec; u++){
        if( needPagerReset ){
          pager_reset(pPager);
          needPagerReset = 0;
        }
        rc = pager_playback_one_page(pPager,&pPager->journalOff,0,1,0);
        if( rc==SQLITE_OK ){
          nPlayback++;
        }else{
          if( rc==SQLITE_DONE ){
            pPager->journalOff = szJ;
            break;
          }else if( rc==SQLITE_IOERR_SHORT_READ ){
            /* If the journal has been truncated, simply stop reading and
            ** processing the journal. This might happen if the journal was
            ** not completely written and synced prior to a crash.  In that
            ** case, the database should have never been written in the
            ** first place so it is OK to simply abandon the rollback. */
            rc = SQLITE_OK;
            goto end_playback;
          }else{
            /* If we are unable to rollback, quit and return the error
            ** code.  This will cause the pager to enter the error state
            ** so that no further harm will be done.  Perhaps the next
            ** process to come along will be able to rollback the database.
            */
            goto end_playback;
          }
        }
      }
    }
    /*NOTREACHED*/
    assert( 0 );
  
  end_playback:
    /* Following a rollback, the database file should be back in its original
    ** state prior to the start of the transaction, so invoke the
    ** SQLITE_FCNTL_DB_UNCHANGED file-control method to disable the
    ** assertion that the transaction counter was modified.
    */
  #ifdef SQLITE_DEBUG
    if( pPager->fd->pMethods ){
      sqlite3OsFileControlHint(pPager->fd,SQLITE_FCNTL_DB_UNCHANGED,0);
    }
  #endif
  
    /* If this playback is happening automatically as a result of an IO or 
    ** malloc error that occurred after the change-counter was updated but 
    ** before the transaction was committed, then the change-counter 
    ** modification may just have been reverted. If this happens in exclusive 
    ** mode, then subsequent transactions performed by the connection will not
    ** update the change-counter at all. This may lead to cache inconsistency
    ** problems for other processes at some point in the future. So, just
    ** in case this has happened, clear the changeCountDone flag now.
    */
    pPager->changeCountDone = pPager->tempFile;
  
    if( rc==SQLITE_OK ){
      zMaster = pPager->pTmpSpace;
      rc = readMasterJournal(pPager->jfd, zMaster, pPager->pVfs->mxPathname+1);
      testcase( rc!=SQLITE_OK );
    }
    if( rc==SQLITE_OK
     && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN)
    ){
      rc = sqlite3PagerSync(pPager, 0);
    }
    if( rc==SQLITE_OK ){
      rc = pager_end_transaction(pPager, zMaster[0]!='\0', 0);
      testcase( rc!=SQLITE_OK );
    }
    if( rc==SQLITE_OK && zMaster[0] && res ){
      /* If there was a master journal and this routine will return success,
      ** see if it is possible to delete the master journal.
      */
      rc = pager_delmaster(pPager, zMaster);
      testcase( rc!=SQLITE_OK );
    }
    if( isHot && nPlayback ){
      sqlite3_log(SQLITE_NOTICE_RECOVER_ROLLBACK, "recovered %d pages from %s",
                  nPlayback, pPager->zJournal);
    }
  
    /* The Pager.sectorSize variable may have been updated while rolling
    ** back a journal created by a process with a different sector size
    ** value. Reset it to the correct value for this process.
    */
    setSectorSize(pPager);
    return rc;
  }
  
  
  /*
  ** Read the content for page pPg out of the database file and into 
  ** pPg->pData. A shared lock or greater must be held on the database
  ** file before this function is called.
  **
  ** If page 1 is read, then the value of Pager.dbFileVers[] is set to
  ** the value read from the database file.
  **
  ** If an IO error occurs, then the IO error is returned to the caller.
  ** Otherwise, SQLITE_OK is returned.
  */
  static int readDbPage(PgHdr *pPg, u32 iFrame){
    Pager *pPager = pPg->pPager; /* Pager object associated with page pPg */
    Pgno pgno = pPg->pgno;       /* Page number to read */
    int rc = SQLITE_OK;          /* Return code */
    int pgsz = pPager->pageSize; /* Number of bytes to read */
  
    assert( pPager->eState>=PAGER_READER && !MEMDB );
    assert( isOpen(pPager->fd) );
  
  #ifndef SQLITE_OMIT_WAL
    if( iFrame ){
      /* Try to pull the page from the write-ahead log. */
      rc = sqlite3WalReadFrame(pPager->pWal, iFrame, pgsz, pPg->pData);
    }else
  #endif
    {
      i64 iOffset = (pgno-1)*(i64)pPager->pageSize;
      rc = sqlite3OsRead(pPager->fd, pPg->pData, pgsz, iOffset);
      if( rc==SQLITE_IOERR_SHORT_READ ){
        rc = SQLITE_OK;
      }
    }
  
    if( pgno==1 ){
      if( rc ){
        /* If the read is unsuccessful, set the dbFileVers[] to something
        ** that will never be a valid file version.  dbFileVers[] is a copy
        ** of bytes 24..39 of the database.  Bytes 28..31 should always be
        ** zero or the size of the database in page. Bytes 32..35 and 35..39
        ** should be page numbers which are never 0xffffffff.  So filling
        ** pPager->dbFileVers[] with all 0xff bytes should suffice.
        **
        ** For an encrypted database, the situation is more complex:  bytes
        ** 24..39 of the database are white noise.  But the probability of
        ** white noise equaling 16 bytes of 0xff is vanishingly small so
        ** we should still be ok.
        */
        memset(pPager->dbFileVers, 0xff, sizeof(pPager->dbFileVers));
      }else{
        u8 *dbFileVers = &((u8*)pPg->pData)[24];
        memcpy(&pPager->dbFileVers, dbFileVers, sizeof(pPager->dbFileVers));
      }
    }
    CODEC1(pPager, pPg->pData, pgno, 3, rc = SQLITE_NOMEM_BKPT);
  
    PAGER_INCR(sqlite3_pager_readdb_count);
    PAGER_INCR(pPager->nRead);
    IOTRACE(("PGIN %p %d\n", pPager, pgno));
    PAGERTRACE(("FETCH %d page %d hash(%08x)\n",
                 PAGERID(pPager), pgno, pager_pagehash(pPg)));
  
    return rc;
  }
  
  /*
  ** Update the value of the change-counter at offsets 24 and 92 in
  ** the header and the sqlite version number at offset 96.
  **
  ** This is an unconditional update.  See also the pager_incr_changecounter()
  ** routine which only updates the change-counter if the update is actually
  ** needed, as determined by the pPager->changeCountDone state variable.
  */
  static void pager_write_changecounter(PgHdr *pPg){
    u32 change_counter;
  
    /* Increment the value just read and write it back to byte 24. */
    change_counter = sqlite3Get4byte((u8*)pPg->pPager->dbFileVers)+1;
    put32bits(((char*)pPg->pData)+24, change_counter);
  
    /* Also store the SQLite version number in bytes 96..99 and in
    ** bytes 92..95 store the change counter for which the version number
    ** is valid. */
    put32bits(((char*)pPg->pData)+92, change_counter);
    put32bits(((char*)pPg->pData)+96, SQLITE_VERSION_NUMBER);
  }
  
  #ifndef SQLITE_OMIT_WAL
  /*
  ** This function is invoked once for each page that has already been 
  ** written into the log file when a WAL transaction is rolled back.
  ** Parameter iPg is the page number of said page. The pCtx argument 
  ** is actually a pointer to the Pager structure.
  **
  ** If page iPg is present in the cache, and has no outstanding references,
  ** it is discarded. Otherwise, if there are one or more outstanding
  ** references, the page content is reloaded from the database. If the
  ** attempt to reload content from the database is required and fails, 
  ** return an SQLite error code. Otherwise, SQLITE_OK.
  */
  static int pagerUndoCallback(void *pCtx, Pgno iPg){
    int rc = SQLITE_OK;
    Pager *pPager = (Pager *)pCtx;
    PgHdr *pPg;
  
    assert( pagerUseWal(pPager) );
    pPg = sqlite3PagerLookup(pPager, iPg);
    if( pPg ){
      if( sqlite3PcachePageRefcount(pPg)==1 ){
        sqlite3PcacheDrop(pPg);
      }else{
        u32 iFrame = 0;
        rc = sqlite3WalFindFrame(pPager->pWal, pPg->pgno, &iFrame);
        if( rc==SQLITE_OK ){
          rc = readDbPage(pPg, iFrame);
        }
        if( rc==SQLITE_OK ){
          pPager->xReiniter(pPg);
        }
        sqlite3PagerUnrefNotNull(pPg);
      }
    }
  
    /* Normally, if a transaction is rolled back, any backup processes are
    ** updated as data is copied out of the rollback journal and into the
    ** database. This is not generally possible with a WAL database, as
    ** rollback involves simply truncating the log file. Therefore, if one
    ** or more frames have already been written to the log (and therefore 
    ** also copied into the backup databases) as part of this transaction,
    ** the backups must be restarted.
    */
    sqlite3BackupRestart(pPager->pBackup);
  
    return rc;
  }
  
  /*
  ** This function is called to rollback a transaction on a WAL database.
  */
  static int pagerRollbackWal(Pager *pPager){
    int rc;                         /* Return Code */
    PgHdr *pList;                   /* List of dirty pages to revert */
  
    /* For all pages in the cache that are currently dirty or have already
    ** been written (but not committed) to the log file, do one of the 
    ** following:
    **
    **   + Discard the cached page (if refcount==0), or
    **   + Reload page content from the database (if refcount>0).
    */
    pPager->dbSize = pPager->dbOrigSize;
    rc = sqlite3WalUndo(pPager->pWal, pagerUndoCallback, (void *)pPager);
    pList = sqlite3PcacheDirtyList(pPager->pPCache);
    while( pList && rc==SQLITE_OK ){
      PgHdr *pNext = pList->pDirty;
      rc = pagerUndoCallback((void *)pPager, pList->pgno);
      pList = pNext;
    }
  
    return rc;
  }
  
  /*
  ** This function is a wrapper around sqlite3WalFrames(). As well as logging
  ** the contents of the list of pages headed by pList (connected by pDirty),
  ** this function notifies any active backup processes that the pages have
  ** changed. 
  **
  ** The list of pages passed into this routine is always sorted by page number.
  ** Hence, if page 1 appears anywhere on the list, it will be the first page.
  */ 
  static int pagerWalFrames(
    Pager *pPager,                  /* Pager object */
    PgHdr *pList,                   /* List of frames to log */
    Pgno nTruncate,                 /* Database size after this commit */
    int isCommit                    /* True if this is a commit */
  ){
    int rc;                         /* Return code */
    int nList;                      /* Number of pages in pList */
    PgHdr *p;                       /* For looping over pages */
  
    assert( pPager->pWal );
    assert( pList );
  #ifdef SQLITE_DEBUG
    /* Verify that the page list is in accending order */
    for(p=pList; p && p->pDirty; p=p->pDirty){
      assert( p->pgno < p->pDirty->pgno );
    }
  #endif
  
    assert( pList->pDirty==0 || isCommit );
    if( isCommit ){
      /* If a WAL transaction is being committed, there is no point in writing
      ** any pages with page numbers greater than nTruncate into the WAL file.
      ** They will never be read by any client. So remove them from the pDirty
      ** list here. */
      PgHdr **ppNext = &pList;
      nList = 0;
      for(p=pList; (*ppNext = p)!=0; p=p->pDirty){
        if( p->pgno<=nTruncate ){
          ppNext = &p->pDirty;
          nList++;
        }
      }
      assert( pList );
    }else{
      nList = 1;
    }
    pPager->aStat[PAGER_STAT_WRITE] += nList;
  
    if( pList->pgno==1 ) pager_write_changecounter(pList);
    rc = sqlite3WalFrames(pPager->pWal, 
        pPager->pageSize, pList, nTruncate, isCommit, pPager->walSyncFlags
    );
    if( rc==SQLITE_OK && pPager->pBackup ){
      for(p=pList; p; p=p->pDirty){
        sqlite3BackupUpdate(pPager->pBackup, p->pgno, (u8 *)p->pData);
      }
    }
  
  #ifdef SQLITE_CHECK_PAGES
    pList = sqlite3PcacheDirtyList(pPager->pPCache);
    for(p=pList; p; p=p->pDirty){
      pager_set_pagehash(p);
    }
  #endif
  
    return rc;
  }
  
  /*
  ** Begin a read transaction on the WAL.
  **
  ** This routine used to be called "pagerOpenSnapshot()" because it essentially
  ** makes a snapshot of the database at the current point in time and preserves
  ** that snapshot for use by the reader in spite of concurrently changes by
  ** other writers or checkpointers.
  */
  static int pagerBeginReadTransaction(Pager *pPager){
    int rc;                         /* Return code */
    int changed = 0;                /* True if cache must be reset */
  
    assert( pagerUseWal(pPager) );
    assert( pPager->eState==PAGER_OPEN || pPager->eState==PAGER_READER );
  
    /* sqlite3WalEndReadTransaction() was not called for the previous
    ** transaction in locking_mode=EXCLUSIVE.  So call it now.  If we
    ** are in locking_mode=NORMAL and EndRead() was previously called,
    ** the duplicate call is harmless.
    */
    sqlite3WalEndReadTransaction(pPager->pWal);
  
    rc = sqlite3WalBeginReadTransaction(pPager->pWal, &changed);
    if( rc!=SQLITE_OK || changed ){
      pager_reset(pPager);
      if( USEFETCH(pPager) ) sqlite3OsUnfetch(pPager->fd, 0, 0);
    }
  
    return rc;
  }
  #endif
  
  /*
  ** This function is called as part of the transition from PAGER_OPEN
  ** to PAGER_READER state to determine the size of the database file
  ** in pages (assuming the page size currently stored in Pager.pageSize).
  **
  ** If no error occurs, SQLITE_OK is returned and the size of the database
  ** in pages is stored in *pnPage. Otherwise, an error code (perhaps
  ** SQLITE_IOERR_FSTAT) is returned and *pnPage is left unmodified.
  */
  static int pagerPagecount(Pager *pPager, Pgno *pnPage){
    Pgno nPage;                     /* Value to return via *pnPage */
  
    /* Query the WAL sub-system for the database size. The WalDbsize()
    ** function returns zero if the WAL is not open (i.e. Pager.pWal==0), or
    ** if the database size is not available. The database size is not
    ** available from the WAL sub-system if the log file is empty or
    ** contains no valid committed transactions.
    */
    assert( pPager->eState==PAGER_OPEN );
    assert( pPager->eLock>=SHARED_LOCK );
    assert( isOpen(pPager->fd) );
    assert( pPager->tempFile==0 );
    nPage = sqlite3WalDbsize(pPager->pWal);
  
    /* If the number of pages in the database is not available from the
    ** WAL sub-system, determine the page counte based on the size of
    ** the database file.  If the size of the database file is not an
    ** integer multiple of the page-size, round up the result.
    */
    if( nPage==0 && ALWAYS(isOpen(pPager->fd)) ){
      i64 n = 0;                    /* Size of db file in bytes */
      int rc = sqlite3OsFileSize(pPager->fd, &n);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      nPage = (Pgno)((n+pPager->pageSize-1) / pPager->pageSize);
    }
  
    /* If the current number of pages in the file is greater than the
    ** configured maximum pager number, increase the allowed limit so
    ** that the file can be read.
    */
    if( nPage>pPager->mxPgno ){
      pPager->mxPgno = (Pgno)nPage;
    }
  
    *pnPage = nPage;
    return SQLITE_OK;
  }
  
  #ifndef SQLITE_OMIT_WAL
  /*
  ** Check if the *-wal file that corresponds to the database opened by pPager
  ** exists if the database is not empy, or verify that the *-wal file does
  ** not exist (by deleting it) if the database file is empty.
  **
  ** If the database is not empty and the *-wal file exists, open the pager
  ** in WAL mode.  If the database is empty or if no *-wal file exists and
  ** if no error occurs, make sure Pager.journalMode is not set to
  ** PAGER_JOURNALMODE_WAL.
  **
  ** Return SQLITE_OK or an error code.
  **
  ** The caller must hold a SHARED lock on the database file to call this
  ** function. Because an EXCLUSIVE lock on the db file is required to delete 
  ** a WAL on a none-empty database, this ensures there is no race condition 
  ** between the xAccess() below and an xDelete() being executed by some 
  ** other connection.
  */
  static int pagerOpenWalIfPresent(Pager *pPager){
    int rc = SQLITE_OK;
    assert( pPager->eState==PAGER_OPEN );
    assert( pPager->eLock>=SHARED_LOCK );
  
    if( !pPager->tempFile ){
      int isWal;                    /* True if WAL file exists */
      Pgno nPage;                   /* Size of the database file */
  
      rc = pagerPagecount(pPager, &nPage);
      if( rc ) return rc;
      if( nPage==0 ){
        rc = sqlite3OsDelete(pPager->pVfs, pPager->zWal, 0);
        if( rc==SQLITE_IOERR_DELETE_NOENT ) rc = SQLITE_OK;
        isWal = 0;
      }else{
        rc = sqlite3OsAccess(
            pPager->pVfs, pPager->zWal, SQLITE_ACCESS_EXISTS, &isWal
        );
      }
      if( rc==SQLITE_OK ){
        if( isWal ){
          testcase( sqlite3PcachePagecount(pPager->pPCache)==0 );
          rc = sqlite3PagerOpenWal(pPager, 0);
        }else if( pPager->journalMode==PAGER_JOURNALMODE_WAL ){
          pPager->journalMode = PAGER_JOURNALMODE_DELETE;
        }
      }
    }
    return rc;
  }
  #endif
  
  /*
  ** Playback savepoint pSavepoint. Or, if pSavepoint==NULL, then playback
  ** the entire master journal file. The case pSavepoint==NULL occurs when 
  ** a ROLLBACK TO command is invoked on a SAVEPOINT that is a transaction 
  ** savepoint.
  **
  ** When pSavepoint is not NULL (meaning a non-transaction savepoint is 
  ** being rolled back), then the rollback consists of up to three stages,
  ** performed in the order specified:
  **
  **   * Pages are played back from the main journal starting at byte
  **     offset PagerSavepoint.iOffset and continuing to 
  **     PagerSavepoint.iHdrOffset, or to the end of the main journal
  **     file if PagerSavepoint.iHdrOffset is zero.
  **
  **   * If PagerSavepoint.iHdrOffset is not zero, then pages are played
  **     back starting from the journal header immediately following 
  **     PagerSavepoint.iHdrOffset to the end of the main journal file.
  **
  **   * Pages are then played back from the sub-journal file, starting
  **     with the PagerSavepoint.iSubRec and continuing to the end of
  **     the journal file.
  **
  ** Throughout the rollback process, each time a page is rolled back, the
  ** corresponding bit is set in a bitvec structure (variable pDone in the
  ** implementation below). This is used to ensure that a page is only
  ** rolled back the first time it is encountered in either journal.
  **
  ** If pSavepoint is NULL, then pages are only played back from the main
  ** journal file. There is no need for a bitvec in this case.
  **
  ** In either case, before playback commences the Pager.dbSize variable
  ** is reset to the value that it held at the start of the savepoint 
  ** (or transaction). No page with a page-number greater than this value
  ** is played back. If one is encountered it is simply skipped.
  */
  static int pagerPlaybackSavepoint(Pager *pPager, PagerSavepoint *pSavepoint){
    i64 szJ;                 /* Effective size of the main journal */
    i64 iHdrOff;             /* End of first segment of main-journal records */
    int rc = SQLITE_OK;      /* Return code */
    Bitvec *pDone = 0;       /* Bitvec to ensure pages played back only once */
  
    assert( pPager->eState!=PAGER_ERROR );
    assert( pPager->eState>=PAGER_WRITER_LOCKED );
  
    /* Allocate a bitvec to use to store the set of pages rolled back */
    if( pSavepoint ){
      pDone = sqlite3BitvecCreate(pSavepoint->nOrig);
      if( !pDone ){
        return SQLITE_NOMEM_BKPT;
      }
    }
  
    /* Set the database size back to the value it was before the savepoint 
    ** being reverted was opened.
    */
    pPager->dbSize = pSavepoint ? pSavepoint->nOrig : pPager->dbOrigSize;
    pPager->changeCountDone = pPager->tempFile;
  
    if( !pSavepoint && pagerUseWal(pPager) ){
      return pagerRollbackWal(pPager);
    }
  
    /* Use pPager->journalOff as the effective size of the main rollback
    ** journal.  The actual file might be larger than this in
    ** PAGER_JOURNALMODE_TRUNCATE or PAGER_JOURNALMODE_PERSIST.  But anything
    ** past pPager->journalOff is off-limits to us.
    */
    szJ = pPager->journalOff;
    assert( pagerUseWal(pPager)==0 || szJ==0 );
  
    /* Begin by rolling back records from the main journal starting at
    ** PagerSavepoint.iOffset and continuing to the next journal header.
    ** There might be records in the main journal that have a page number
    ** greater than the current database size (pPager->dbSize) but those
    ** will be skipped automatically.  Pages are added to pDone as they
    ** are played back.
    */
    if( pSavepoint && !pagerUseWal(pPager) ){
      iHdrOff = pSavepoint->iHdrOffset ? pSavepoint->iHdrOffset : szJ;
      pPager->journalOff = pSavepoint->iOffset;
      while( rc==SQLITE_OK && pPager->journalOff<iHdrOff ){
        rc = pager_playback_one_page(pPager, &pPager->journalOff, pDone, 1, 1);
      }
      assert( rc!=SQLITE_DONE );
    }else{
      pPager->journalOff = 0;
    }
  
    /* Continue rolling back records out of the main journal starting at
    ** the first journal header seen and continuing until the effective end
    ** of the main journal file.  Continue to skip out-of-range pages and
    ** continue adding pages rolled back to pDone.
    */
    while( rc==SQLITE_OK && pPager->journalOff<szJ ){
      u32 ii;            /* Loop counter */
      u32 nJRec = 0;     /* Number of Journal Records */
      u32 dummy;
      rc = readJournalHdr(pPager, 0, szJ, &nJRec, &dummy);
      assert( rc!=SQLITE_DONE );
  
      /*
      ** The "pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff"
      ** test is related to ticket #2565.  See the discussion in the
      ** pager_playback() function for additional information.
      */
      if( nJRec==0 
       && pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff
      ){
        nJRec = (u32)((szJ - pPager->journalOff)/JOURNAL_PG_SZ(pPager));
      }
      for(ii=0; rc==SQLITE_OK && ii<nJRec && pPager->journalOff<szJ; ii++){
        rc = pager_playback_one_page(pPager, &pPager->journalOff, pDone, 1, 1);
      }
      assert( rc!=SQLITE_DONE );
    }
    assert( rc!=SQLITE_OK || pPager->journalOff>=szJ );
  
    /* Finally,  rollback pages from the sub-journal.  Page that were
    ** previously rolled back out of the main journal (and are hence in pDone)
    ** will be skipped.  Out-of-range pages are also skipped.
    */
    if( pSavepoint ){
      u32 ii;            /* Loop counter */
      i64 offset = (i64)pSavepoint->iSubRec*(4+pPager->pageSize);
  
      if( pagerUseWal(pPager) ){
        rc = sqlite3WalSavepointUndo(pPager->pWal, pSavepoint->aWalData);
      }
      for(ii=pSavepoint->iSubRec; rc==SQLITE_OK && ii<pPager->nSubRec; ii++){
        assert( offset==(i64)ii*(4+pPager->pageSize) );
        rc = pager_playback_one_page(pPager, &offset, pDone, 0, 1);
      }
      assert( rc!=SQLITE_DONE );
    }
  
    sqlite3BitvecDestroy(pDone);
    if( rc==SQLITE_OK ){
      pPager->journalOff = szJ;
    }
  
    return rc;
  }
  
  /*
  ** Change the maximum number of in-memory pages that are allowed
  ** before attempting to recycle clean and unused pages.
  */
  void sqlite3PagerSetCachesize(Pager *pPager, int mxPage){
    sqlite3PcacheSetCachesize(pPager->pPCache, mxPage);
  }
  
  /*
  ** Change the maximum number of in-memory pages that are allowed
  ** before attempting to spill pages to journal.
  */
  int sqlite3PagerSetSpillsize(Pager *pPager, int mxPage){
    return sqlite3PcacheSetSpillsize(pPager->pPCache, mxPage);
  }
  
  /*
  ** Invoke SQLITE_FCNTL_MMAP_SIZE based on the current value of szMmap.
  */
  static void pagerFixMaplimit(Pager *pPager){
  #if SQLITE_MAX_MMAP_SIZE>0
    sqlite3_file *fd = pPager->fd;
    if( isOpen(fd) && fd->pMethods->iVersion>=3 ){
      sqlite3_int64 sz;
      sz = pPager->szMmap;
      pPager->bUseFetch = (sz>0);
      sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_MMAP_SIZE, &sz);
    }
  #endif
  }
  
  /*
  ** Change the maximum size of any memory mapping made of the database file.
  */
  void sqlite3PagerSetMmapLimit(Pager *pPager, sqlite3_int64 szMmap){
    pPager->szMmap = szMmap;
    pagerFixMaplimit(pPager);
  }
  
  /*
  ** Free as much memory as possible from the pager.
  */
  void sqlite3PagerShrink(Pager *pPager){
    sqlite3PcacheShrink(pPager->pPCache);
  }
  
  /*
  ** Adjust settings of the pager to those specified in the pgFlags parameter.
  **
  ** The "level" in pgFlags & PAGER_SYNCHRONOUS_MASK sets the robustness
  ** of the database to damage due to OS crashes or power failures by
  ** changing the number of syncs()s when writing the journals.
  ** There are four levels:
  **
  **    OFF       sqlite3OsSync() is never called.  This is the default
  **              for temporary and transient files.
  **
  **    NORMAL    The journal is synced once before writes begin on the
  **              database.  This is normally adequate protection, but
  **              it is theoretically possible, though very unlikely,
  **              that an inopertune power failure could leave the journal
  **              in a state which would cause damage to the database
  **              when it is rolled back.
  **
  **    FULL      The journal is synced twice before writes begin on the
  **              database (with some additional information - the nRec field
  **              of the journal header - being written in between the two
  **              syncs).  If we assume that writing a
  **              single disk sector is atomic, then this mode provides
  **              assurance that the journal will not be corrupted to the
  **              point of causing damage to the database during rollback.
  **
  **    EXTRA     This is like FULL except that is also syncs the directory
  **              that contains the rollback journal after the rollback
  **              journal is unlinked.
  **
  ** The above is for a rollback-journal mode.  For WAL mode, OFF continues
  ** to mean that no syncs ever occur.  NORMAL means that the WAL is synced
  ** prior to the start of checkpoint and that the database file is synced
  ** at the conclusion of the checkpoint if the entire content of the WAL
  ** was written back into the database.  But no sync operations occur for
  ** an ordinary commit in NORMAL mode with WAL.  FULL means that the WAL
  ** file is synced following each commit operation, in addition to the
  ** syncs associated with NORMAL.  There is no difference between FULL
  ** and EXTRA for WAL mode.
  **
  ** Do not confuse synchronous=FULL with SQLITE_SYNC_FULL.  The
  ** SQLITE_SYNC_FULL macro means to use the MacOSX-style full-fsync
  ** using fcntl(F_FULLFSYNC).  SQLITE_SYNC_NORMAL means to do an
  ** ordinary fsync() call.  There is no difference between SQLITE_SYNC_FULL
  ** and SQLITE_SYNC_NORMAL on platforms other than MacOSX.  But the
  ** synchronous=FULL versus synchronous=NORMAL setting determines when
  ** the xSync primitive is called and is relevant to all platforms.
  **
  ** Numeric values associated with these states are OFF==1, NORMAL=2,
  ** and FULL=3.
  */
  #ifndef SQLITE_OMIT_PAGER_PRAGMAS
  void sqlite3PagerSetFlags(
    Pager *pPager,        /* The pager to set safety level for */
    unsigned pgFlags      /* Various flags */
  ){
    unsigned level = pgFlags & PAGER_SYNCHRONOUS_MASK;
    if( pPager->tempFile ){
      pPager->noSync = 1;
      pPager->fullSync = 0;
      pPager->extraSync = 0;
    }else{
      pPager->noSync =  level==PAGER_SYNCHRONOUS_OFF ?1:0;
      pPager->fullSync = level>=PAGER_SYNCHRONOUS_FULL ?1:0;
      pPager->extraSync = level==PAGER_SYNCHRONOUS_EXTRA ?1:0;
    }
    if( pPager->noSync ){
      pPager->syncFlags = 0;
      pPager->ckptSyncFlags = 0;
    }else if( pgFlags & PAGER_FULLFSYNC ){
      pPager->syncFlags = SQLITE_SYNC_FULL;
      pPager->ckptSyncFlags = SQLITE_SYNC_FULL;
    }else if( pgFlags & PAGER_CKPT_FULLFSYNC ){
      pPager->syncFlags = SQLITE_SYNC_NORMAL;
      pPager->ckptSyncFlags = SQLITE_SYNC_FULL;
    }else{
      pPager->syncFlags = SQLITE_SYNC_NORMAL;
      pPager->ckptSyncFlags = SQLITE_SYNC_NORMAL;
    }
    pPager->walSyncFlags = pPager->syncFlags;
    if( pPager->fullSync ){
      pPager->walSyncFlags |= WAL_SYNC_TRANSACTIONS;
    }
    if( pgFlags & PAGER_CACHESPILL ){
      pPager->doNotSpill &= ~SPILLFLAG_OFF;
    }else{
      pPager->doNotSpill |= SPILLFLAG_OFF;
    }
  }
  #endif
  
  /*
  ** The following global variable is incremented whenever the library
  ** attempts to open a temporary file.  This information is used for
  ** testing and analysis only.  
  */
  #ifdef SQLITE_TEST
  int sqlite3_opentemp_count = 0;
  #endif
  
  /*
  ** Open a temporary file.
  **
  ** Write the file descriptor into *pFile. Return SQLITE_OK on success 
  ** or some other error code if we fail. The OS will automatically 
  ** delete the temporary file when it is closed.
  **
  ** The flags passed to the VFS layer xOpen() call are those specified
  ** by parameter vfsFlags ORed with the following:
  **
  **     SQLITE_OPEN_READWRITE
  **     SQLITE_OPEN_CREATE
  **     SQLITE_OPEN_EXCLUSIVE
  **     SQLITE_OPEN_DELETEONCLOSE
  */
  static int pagerOpentemp(
    Pager *pPager,        /* The pager object */
    sqlite3_file *pFile,  /* Write the file descriptor here */
    int vfsFlags          /* Flags passed through to the VFS */
  ){
    int rc;               /* Return code */
  
  #ifdef SQLITE_TEST
    sqlite3_opentemp_count++;  /* Used for testing and analysis only */
  #endif
  
    vfsFlags |=  SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE |
              SQLITE_OPEN_EXCLUSIVE | SQLITE_OPEN_DELETEONCLOSE;
    rc = sqlite3OsOpen(pPager->pVfs, 0, pFile, vfsFlags, 0);
    assert( rc!=SQLITE_OK || isOpen(pFile) );
    return rc;
  }
  
  /*
  ** Set the busy handler function.
  **
  ** The pager invokes the busy-handler if sqlite3OsLock() returns 
  ** SQLITE_BUSY when trying to upgrade from no-lock to a SHARED lock,
  ** or when trying to upgrade from a RESERVED lock to an EXCLUSIVE 
  ** lock. It does *not* invoke the busy handler when upgrading from
  ** SHARED to RESERVED, or when upgrading from SHARED to EXCLUSIVE
  ** (which occurs during hot-journal rollback). Summary:
  **
  **   Transition                        | Invokes xBusyHandler
  **   --------------------------------------------------------
  **   NO_LOCK       -> SHARED_LOCK      | Yes
  **   SHARED_LOCK   -> RESERVED_LOCK    | No
  **   SHARED_LOCK   -> EXCLUSIVE_LOCK   | No
  **   RESERVED_LOCK -> EXCLUSIVE_LOCK   | Yes
  **
  ** If the busy-handler callback returns non-zero, the lock is 
  ** retried. If it returns zero, then the SQLITE_BUSY error is
  ** returned to the caller of the pager API function.
  */
  void sqlite3PagerSetBusyhandler(
    Pager *pPager,                       /* Pager object */
    int (*xBusyHandler)(void *),         /* Pointer to busy-handler function */
    void *pBusyHandlerArg                /* Argument to pass to xBusyHandler */
  ){
    pPager->xBusyHandler = xBusyHandler;
    pPager->pBusyHandlerArg = pBusyHandlerArg;
  
    if( isOpen(pPager->fd) ){
      void **ap = (void **)&pPager->xBusyHandler;
      assert( ((int(*)(void *))(ap[0]))==xBusyHandler );
      assert( ap[1]==pBusyHandlerArg );
      sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_BUSYHANDLER, (void *)ap);
    }
  }
  
  /*
  ** Change the page size used by the Pager object. The new page size 
  ** is passed in *pPageSize.
  **
  ** If the pager is in the error state when this function is called, it
  ** is a no-op. The value returned is the error state error code (i.e. 
  ** one of SQLITE_IOERR, an SQLITE_IOERR_xxx sub-code or SQLITE_FULL).
  **
  ** Otherwise, if all of the following are true:
  **
  **   * the new page size (value of *pPageSize) is valid (a power 
  **     of two between 512 and SQLITE_MAX_PAGE_SIZE, inclusive), and
  **
  **   * there are no outstanding page references, and
  **
  **   * the database is either not an in-memory database or it is
  **     an in-memory database that currently consists of zero pages.
  **
  ** then the pager object page size is set to *pPageSize.
  **
  ** If the page size is changed, then this function uses sqlite3PagerMalloc() 
  ** to obtain a new Pager.pTmpSpace buffer. If this allocation attempt 
  ** fails, SQLITE_NOMEM is returned and the page size remains unchanged. 
  ** In all other cases, SQLITE_OK is returned.
  **
  ** If the page size is not changed, either because one of the enumerated
  ** conditions above is not true, the pager was in error state when this
  ** function was called, or because the memory allocation attempt failed, 
  ** then *pPageSize is set to the old, retained page size before returning.
  */
  int sqlite3PagerSetPagesize(Pager *pPager, u32 *pPageSize, int nReserve){
    int rc = SQLITE_OK;
  
    /* It is not possible to do a full assert_pager_state() here, as this
    ** function may be called from within PagerOpen(), before the state
    ** of the Pager object is internally consistent.
    **
    ** At one point this function returned an error if the pager was in 
    ** PAGER_ERROR state. But since PAGER_ERROR state guarantees that
    ** there is at least one outstanding page reference, this function
    ** is a no-op for that case anyhow.
    */
  
    u32 pageSize = *pPageSize;
    assert( pageSize==0 || (pageSize>=512 && pageSize<=SQLITE_MAX_PAGE_SIZE) );
    if( (pPager->memDb==0 || pPager->dbSize==0)
     && sqlite3PcacheRefCount(pPager->pPCache)==0 
     && pageSize && pageSize!=(u32)pPager->pageSize 
    ){
      char *pNew = NULL;             /* New temp space */
      i64 nByte = 0;
  
      if( pPager->eState>PAGER_OPEN && isOpen(pPager->fd) ){
        rc = sqlite3OsFileSize(pPager->fd, &nByte);
      }
      if( rc==SQLITE_OK ){
        pNew = (char *)sqlite3PageMalloc(pageSize);
        if( !pNew ) rc = SQLITE_NOMEM_BKPT;
      }
  
      if( rc==SQLITE_OK ){
        pager_reset(pPager);
        rc = sqlite3PcacheSetPageSize(pPager->pPCache, pageSize);
      }
      if( rc==SQLITE_OK ){
        sqlite3PageFree(pPager->pTmpSpace);
        pPager->pTmpSpace = pNew;
        pPager->dbSize = (Pgno)((nByte+pageSize-1)/pageSize);
        pPager->pageSize = pageSize;
      }else{
        sqlite3PageFree(pNew);
      }
    }
  
    *pPageSize = pPager->pageSize;
    if( rc==SQLITE_OK ){
      if( nReserve<0 ) nReserve = pPager->nReserve;
      assert( nReserve>=0 && nReserve<1000 );
      pPager->nReserve = (i16)nReserve;
      pagerReportSize(pPager);
      pagerFixMaplimit(pPager);
    }
    return rc;
  }
  
  /*
  ** Return a pointer to the "temporary page" buffer held internally
  ** by the pager.  This is a buffer that is big enough to hold the
  ** entire content of a database page.  This buffer is used internally
  ** during rollback and will be overwritten whenever a rollback
  ** occurs.  But other modules are free to use it too, as long as
  ** no rollbacks are happening.
  */
  void *sqlite3PagerTempSpace(Pager *pPager){
    return pPager->pTmpSpace;
  }
  
  /*
  ** Attempt to set the maximum database page count if mxPage is positive. 
  ** Make no changes if mxPage is zero or negative.  And never reduce the
  ** maximum page count below the current size of the database.
  **
  ** Regardless of mxPage, return the current maximum page count.
  */
  int sqlite3PagerMaxPageCount(Pager *pPager, int mxPage){
    if( mxPage>0 ){
      pPager->mxPgno = mxPage;
    }
    assert( pPager->eState!=PAGER_OPEN );      /* Called only by OP_MaxPgcnt */
    assert( pPager->mxPgno>=pPager->dbSize );  /* OP_MaxPgcnt enforces this */
    return pPager->mxPgno;
  }
  
  /*
  ** The following set of routines are used to disable the simulated
  ** I/O error mechanism.  These routines are used to avoid simulated
  ** errors in places where we do not care about errors.
  **
  ** Unless -DSQLITE_TEST=1 is used, these routines are all no-ops
  ** and generate no code.
  */
  #ifdef SQLITE_TEST
  extern int sqlite3_io_error_pending;
  extern int sqlite3_io_error_hit;
  static int saved_cnt;
  void disable_simulated_io_errors(void){
    saved_cnt = sqlite3_io_error_pending;
    sqlite3_io_error_pending = -1;
  }
  void enable_simulated_io_errors(void){
    sqlite3_io_error_pending = saved_cnt;
  }
  #else
  # define disable_simulated_io_errors()
  # define enable_simulated_io_errors()
  #endif
  
  /*
  ** Read the first N bytes from the beginning of the file into memory
  ** that pDest points to. 
  **
  ** If the pager was opened on a transient file (zFilename==""), or
  ** opened on a file less than N bytes in size, the output buffer is
  ** zeroed and SQLITE_OK returned. The rationale for this is that this 
  ** function is used to read database headers, and a new transient or
  ** zero sized database has a header than consists entirely of zeroes.
  **
  ** If any IO error apart from SQLITE_IOERR_SHORT_READ is encountered,
  ** the error code is returned to the caller and the contents of the
  ** output buffer undefined.
  */
  int sqlite3PagerReadFileheader(Pager *pPager, int N, unsigned char *pDest){
    int rc = SQLITE_OK;
    memset(pDest, 0, N);
    assert( isOpen(pPager->fd) || pPager->tempFile );
  
    /* This routine is only called by btree immediately after creating
    ** the Pager object.  There has not been an opportunity to transition
    ** to WAL mode yet.
    */
    assert( !pagerUseWal(pPager) );
  
    if( isOpen(pPager->fd) ){
      IOTRACE(("DBHDR %p 0 %d\n", pPager, N))
      rc = sqlite3OsRead(pPager->fd, pDest, N, 0);
      if( rc==SQLITE_IOERR_SHORT_READ ){
        rc = SQLITE_OK;
      }
    }
    return rc;
  }
  
  /*
  ** This function may only be called when a read-transaction is open on
  ** the pager. It returns the total number of pages in the database.
  **
  ** However, if the file is between 1 and <page-size> bytes in size, then 
  ** this is considered a 1 page file.
  */
  void sqlite3PagerPagecount(Pager *pPager, int *pnPage){
    assert( pPager->eState>=PAGER_READER );
    assert( pPager->eState!=PAGER_WRITER_FINISHED );
    *pnPage = (int)pPager->dbSize;
  }
  
  
  /*
  ** Try to obtain a lock of type locktype on the database file. If
  ** a similar or greater lock is already held, this function is a no-op
  ** (returning SQLITE_OK immediately).
  **
  ** Otherwise, attempt to obtain the lock using sqlite3OsLock(). Invoke 
  ** the busy callback if the lock is currently not available. Repeat 
  ** until the busy callback returns false or until the attempt to 
  ** obtain the lock succeeds.
  **
  ** Return SQLITE_OK on success and an error code if we cannot obtain
  ** the lock. If the lock is obtained successfully, set the Pager.state 
  ** variable to locktype before returning.
  */
  static int pager_wait_on_lock(Pager *pPager, int locktype){
    int rc;                              /* Return code */
  
    /* Check that this is either a no-op (because the requested lock is 
    ** already held), or one of the transitions that the busy-handler
    ** may be invoked during, according to the comment above
    ** sqlite3PagerSetBusyhandler().
    */
    assert( (pPager->eLock>=locktype)
         || (pPager->eLock==NO_LOCK && locktype==SHARED_LOCK)
         || (pPager->eLock==RESERVED_LOCK && locktype==EXCLUSIVE_LOCK)
    );
  
    do {
      rc = pagerLockDb(pPager, locktype);
    }while( rc==SQLITE_BUSY && pPager->xBusyHandler(pPager->pBusyHandlerArg) );
    return rc;
  }
  
  /*
  ** Function assertTruncateConstraint(pPager) checks that one of the 
  ** following is true for all dirty pages currently in the page-cache:
  **
  **   a) The page number is less than or equal to the size of the 
  **      current database image, in pages, OR
  **
  **   b) if the page content were written at this time, it would not
  **      be necessary to write the current content out to the sub-journal
  **      (as determined by function subjRequiresPage()).
  **
  ** If the condition asserted by this function were not true, and the
  ** dirty page were to be discarded from the cache via the pagerStress()
  ** routine, pagerStress() would not write the current page content to
  ** the database file. If a savepoint transaction were rolled back after
  ** this happened, the correct behavior would be to restore the current
  ** content of the page. However, since this content is not present in either
  ** the database file or the portion of the rollback journal and 
  ** sub-journal rolled back the content could not be restored and the
  ** database image would become corrupt. It is therefore fortunate that 
  ** this circumstance cannot arise.
  */
  #if defined(SQLITE_DEBUG)
  static void assertTruncateConstraintCb(PgHdr *pPg){
    assert( pPg->flags&PGHDR_DIRTY );
    assert( !subjRequiresPage(pPg) || pPg->pgno<=pPg->pPager->dbSize );
  }
  static void assertTruncateConstraint(Pager *pPager){
    sqlite3PcacheIterateDirty(pPager->pPCache, assertTruncateConstraintCb);
  }
  #else
  # define assertTruncateConstraint(pPager)
  #endif
  
  /*
  ** Truncate the in-memory database file image to nPage pages. This 
  ** function does not actually modify the database file on disk. It 
  ** just sets the internal state of the pager object so that the 
  ** truncation will be done when the current transaction is committed.
  **
  ** This function is only called right before committing a transaction.
  ** Once this function has been called, the transaction must either be
  ** rolled back or committed. It is not safe to call this function and
  ** then continue writing to the database.
  */
  void sqlite3PagerTruncateImage(Pager *pPager, Pgno nPage){
    assert( pPager->dbSize>=nPage );
    assert( pPager->eState>=PAGER_WRITER_CACHEMOD );
    pPager->dbSize = nPage;
  
    /* At one point the code here called assertTruncateConstraint() to
    ** ensure that all pages being truncated away by this operation are,
    ** if one or more savepoints are open, present in the savepoint 
    ** journal so that they can be restored if the savepoint is rolled
    ** back. This is no longer necessary as this function is now only
    ** called right before committing a transaction. So although the 
    ** Pager object may still have open savepoints (Pager.nSavepoint!=0), 
    ** they cannot be rolled back. So the assertTruncateConstraint() call
    ** is no longer correct. */
  }
  
  
  /*
  ** This function is called before attempting a hot-journal rollback. It
  ** syncs the journal file to disk, then sets pPager->journalHdr to the
  ** size of the journal file so that the pager_playback() routine knows
  ** that the entire journal file has been synced.
  **
  ** Syncing a hot-journal to disk before attempting to roll it back ensures 
  ** that if a power-failure occurs during the rollback, the process that
  ** attempts rollback following system recovery sees the same journal
  ** content as this process.
  **
  ** If everything goes as planned, SQLITE_OK is returned. Otherwise, 
  ** an SQLite error code.
  */
  static int pagerSyncHotJournal(Pager *pPager){
    int rc = SQLITE_OK;
    if( !pPager->noSync ){
      rc = sqlite3OsSync(pPager->jfd, SQLITE_SYNC_NORMAL);
    }
    if( rc==SQLITE_OK ){
      rc = sqlite3OsFileSize(pPager->jfd, &pPager->journalHdr);
    }
    return rc;
  }
  
  /*
  ** Obtain a reference to a memory mapped page object for page number pgno. 
  ** The new object will use the pointer pData, obtained from xFetch().
  ** If successful, set *ppPage to point to the new page reference
  ** and return SQLITE_OK. Otherwise, return an SQLite error code and set
  ** *ppPage to zero.
  **
  ** Page references obtained by calling this function should be released
  ** by calling pagerReleaseMapPage().
  */
  static int pagerAcquireMapPage(
    Pager *pPager,                  /* Pager object */
    Pgno pgno,                      /* Page number */
    void *pData,                    /* xFetch()'d data for this page */
    PgHdr **ppPage                  /* OUT: Acquired page object */
  ){
    PgHdr *p;                       /* Memory mapped page to return */
    
    if( pPager->pMmapFreelist ){
      *ppPage = p = pPager->pMmapFreelist;
      pPager->pMmapFreelist = p->pDirty;
      p->pDirty = 0;
      memset(p->pExtra, 0, pPager->nExtra);
    }else{
      *ppPage = p = (PgHdr *)sqlite3MallocZero(sizeof(PgHdr) + pPager->nExtra);
      if( p==0 ){
        sqlite3OsUnfetch(pPager->fd, (i64)(pgno-1) * pPager->pageSize, pData);
        return SQLITE_NOMEM_BKPT;
      }
      p->pExtra = (void *)&p[1];
      p->flags = PGHDR_MMAP;
      p->nRef = 1;
      p->pPager = pPager;
    }
  
    assert( p->pExtra==(void *)&p[1] );
    assert( p->pPage==0 );
    assert( p->flags==PGHDR_MMAP );
    assert( p->pPager==pPager );
    assert( p->nRef==1 );
  
    p->pgno = pgno;
    p->pData = pData;
    pPager->nMmapOut++;
  
    return SQLITE_OK;
  }
  
  /*
  ** Release a reference to page pPg. pPg must have been returned by an 
  ** earlier call to pagerAcquireMapPage().
  */
  static void pagerReleaseMapPage(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    pPager->nMmapOut--;
    pPg->pDirty = pPager->pMmapFreelist;
    pPager->pMmapFreelist = pPg;
  
    assert( pPager->fd->pMethods->iVersion>=3 );
    sqlite3OsUnfetch(pPager->fd, (i64)(pPg->pgno-1)*pPager->pageSize, pPg->pData);
  }
  
  /*
  ** Free all PgHdr objects stored in the Pager.pMmapFreelist list.
  */
  static void pagerFreeMapHdrs(Pager *pPager){
    PgHdr *p;
    PgHdr *pNext;
    for(p=pPager->pMmapFreelist; p; p=pNext){
      pNext = p->pDirty;
      sqlite3_free(p);
    }
  }
  
  
  /*
  ** Shutdown the page cache.  Free all memory and close all files.
  **
  ** If a transaction was in progress when this routine is called, that
  ** transaction is rolled back.  All outstanding pages are invalidated
  ** and their memory is freed.  Any attempt to use a page associated
  ** with this page cache after this function returns will likely
  ** result in a coredump.
  **
  ** This function always succeeds. If a transaction is active an attempt
  ** is made to roll it back. If an error occurs during the rollback 
  ** a hot journal may be left in the filesystem but no error is returned
  ** to the caller.
  */
  int sqlite3PagerClose(Pager *pPager){
    u8 *pTmp = (u8 *)pPager->pTmpSpace;
  
    assert( assert_pager_state(pPager) );
    disable_simulated_io_errors();
    sqlite3BeginBenignMalloc();
    pagerFreeMapHdrs(pPager);
    /* pPager->errCode = 0; */
    pPager->exclusiveMode = 0;
  #ifndef SQLITE_OMIT_WAL
    sqlite3WalClose(pPager->pWal, pPager->ckptSyncFlags, pPager->pageSize, pTmp);
    pPager->pWal = 0;
  #endif
    pager_reset(pPager);
    if( MEMDB ){
      pager_unlock(pPager);
    }else{
      /* If it is open, sync the journal file before calling UnlockAndRollback.
      ** If this is not done, then an unsynced portion of the open journal 
      ** file may be played back into the database. If a power failure occurs 
      ** while this is happening, the database could become corrupt.
      **
      ** If an error occurs while trying to sync the journal, shift the pager
      ** into the ERROR state. This causes UnlockAndRollback to unlock the
      ** database and close the journal file without attempting to roll it
      ** back or finalize it. The next database user will have to do hot-journal
      ** rollback before accessing the database file.
      */
      if( isOpen(pPager->jfd) ){
        pager_error(pPager, pagerSyncHotJournal(pPager));
      }
      pagerUnlockAndRollback(pPager);
    }
    sqlite3EndBenignMalloc();
    enable_simulated_io_errors();
    PAGERTRACE(("CLOSE %d\n", PAGERID(pPager)));
    IOTRACE(("CLOSE %p\n", pPager))
    sqlite3OsClose(pPager->jfd);
    sqlite3OsClose(pPager->fd);
    sqlite3PageFree(pTmp);
    sqlite3PcacheClose(pPager->pPCache);
  
  #ifdef SQLITE_HAS_CODEC
    if( pPager->xCodecFree ) pPager->xCodecFree(pPager->pCodec);
  #endif
  
    assert( !pPager->aSavepoint && !pPager->pInJournal );
    assert( !isOpen(pPager->jfd) && !isOpen(pPager->sjfd) );
  
    sqlite3_free(pPager);
    return SQLITE_OK;
  }
  
  #if !defined(NDEBUG) || defined(SQLITE_TEST)
  /*
  ** Return the page number for page pPg.
  */
  Pgno sqlite3PagerPagenumber(DbPage *pPg){
    return pPg->pgno;
  }
  #endif
  
  /*
  ** Increment the reference count for page pPg.
  */
  void sqlite3PagerRef(DbPage *pPg){
    sqlite3PcacheRef(pPg);
  }
  
  /*
  ** Sync the journal. In other words, make sure all the pages that have
  ** been written to the journal have actually reached the surface of the
  ** disk and can be restored in the event of a hot-journal rollback.
  **
  ** If the Pager.noSync flag is set, then this function is a no-op.
  ** Otherwise, the actions required depend on the journal-mode and the 
  ** device characteristics of the file-system, as follows:
  **
  **   * If the journal file is an in-memory journal file, no action need
  **     be taken.
  **
  **   * Otherwise, if the device does not support the SAFE_APPEND property,
  **     then the nRec field of the most recently written journal header
  **     is updated to contain the number of journal records that have
  **     been written following it. If the pager is operating in full-sync
  **     mode, then the journal file is synced before this field is updated.
  **
  **   * If the device does not support the SEQUENTIAL property, then 
  **     journal file is synced.
  **
  ** Or, in pseudo-code:
  **
  **   if( NOT <in-memory journal> ){
  **     if( NOT SAFE_APPEND ){
  **       if( <full-sync mode> ) xSync(<journal file>);
  **       <update nRec field>
  **     } 
  **     if( NOT SEQUENTIAL ) xSync(<journal file>);
  **   }
  **
  ** If successful, this routine clears the PGHDR_NEED_SYNC flag of every 
  ** page currently held in memory before returning SQLITE_OK. If an IO
  ** error is encountered, then the IO error code is returned to the caller.
  */
  static int syncJournal(Pager *pPager, int newHdr){
    int rc;                         /* Return code */
  
    assert( pPager->eState==PAGER_WRITER_CACHEMOD
         || pPager->eState==PAGER_WRITER_DBMOD
    );
    assert( assert_pager_state(pPager) );
    assert( !pagerUseWal(pPager) );
  
    rc = sqlite3PagerExclusiveLock(pPager);
    if( rc!=SQLITE_OK ) return rc;
  
    if( !pPager->noSync ){
      assert( !pPager->tempFile );
      if( isOpen(pPager->jfd) && pPager->journalMode!=PAGER_JOURNALMODE_MEMORY ){
        const int iDc = sqlite3OsDeviceCharacteristics(pPager->fd);
        assert( isOpen(pPager->jfd) );
  
        if( 0==(iDc&SQLITE_IOCAP_SAFE_APPEND) ){
          /* This block deals with an obscure problem. If the last connection
          ** that wrote to this database was operating in persistent-journal
          ** mode, then the journal file may at this point actually be larger
          ** than Pager.journalOff bytes. If the next thing in the journal
          ** file happens to be a journal-header (written as part of the
          ** previous connection's transaction), and a crash or power-failure 
          ** occurs after nRec is updated but before this connection writes 
          ** anything else to the journal file (or commits/rolls back its 
          ** transaction), then SQLite may become confused when doing the 
          ** hot-journal rollback following recovery. It may roll back all
          ** of this connections data, then proceed to rolling back the old,
          ** out-of-date data that follows it. Database corruption.
          **
          ** To work around this, if the journal file does appear to contain
          ** a valid header following Pager.journalOff, then write a 0x00
          ** byte to the start of it to prevent it from being recognized.
          **
          ** Variable iNextHdrOffset is set to the offset at which this
          ** problematic header will occur, if it exists. aMagic is used 
          ** as a temporary buffer to inspect the first couple of bytes of
          ** the potential journal header.
          */
          i64 iNextHdrOffset;
          u8 aMagic[8];
          u8 zHeader[sizeof(aJournalMagic)+4];
  
          memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
          put32bits(&zHeader[sizeof(aJournalMagic)], pPager->nRec);
  
          iNextHdrOffset = journalHdrOffset(pPager);
          rc = sqlite3OsRead(pPager->jfd, aMagic, 8, iNextHdrOffset);
          if( rc==SQLITE_OK && 0==memcmp(aMagic, aJournalMagic, 8) ){
            static const u8 zerobyte = 0;
            rc = sqlite3OsWrite(pPager->jfd, &zerobyte, 1, iNextHdrOffset);
          }
          if( rc!=SQLITE_OK && rc!=SQLITE_IOERR_SHORT_READ ){
            return rc;
          }
  
          /* Write the nRec value into the journal file header. If in
          ** full-synchronous mode, sync the journal first. This ensures that
          ** all data has really hit the disk before nRec is updated to mark
          ** it as a candidate for rollback.
          **
          ** This is not required if the persistent media supports the
          ** SAFE_APPEND property. Because in this case it is not possible 
          ** for garbage data to be appended to the file, the nRec field
          ** is populated with 0xFFFFFFFF when the journal header is written
          ** and never needs to be updated.
          */
          if( pPager->fullSync && 0==(iDc&SQLITE_IOCAP_SEQUENTIAL) ){
            PAGERTRACE(("SYNC journal of %d\n", PAGERID(pPager)));
            IOTRACE(("JSYNC %p\n", pPager))
            rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags);
            if( rc!=SQLITE_OK ) return rc;
          }
          IOTRACE(("JHDR %p %lld\n", pPager, pPager->journalHdr));
          rc = sqlite3OsWrite(
              pPager->jfd, zHeader, sizeof(zHeader), pPager->journalHdr
          );
          if( rc!=SQLITE_OK ) return rc;
        }
        if( 0==(iDc&SQLITE_IOCAP_SEQUENTIAL) ){
          PAGERTRACE(("SYNC journal of %d\n", PAGERID(pPager)));
          IOTRACE(("JSYNC %p\n", pPager))
          rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags| 
            (pPager->syncFlags==SQLITE_SYNC_FULL?SQLITE_SYNC_DATAONLY:0)
          );
          if( rc!=SQLITE_OK ) return rc;
        }
  
        pPager->journalHdr = pPager->journalOff;
        if( newHdr && 0==(iDc&SQLITE_IOCAP_SAFE_APPEND) ){
          pPager->nRec = 0;
          rc = writeJournalHdr(pPager);
          if( rc!=SQLITE_OK ) return rc;
        }
      }else{
        pPager->journalHdr = pPager->journalOff;
      }
    }
  
    /* Unless the pager is in noSync mode, the journal file was just 
    ** successfully synced. Either way, clear the PGHDR_NEED_SYNC flag on 
    ** all pages.
    */
    sqlite3PcacheClearSyncFlags(pPager->pPCache);
    pPager->eState = PAGER_WRITER_DBMOD;
    assert( assert_pager_state(pPager) );
    return SQLITE_OK;
  }
  
  /*
  ** The argument is the first in a linked list of dirty pages connected
  ** by the PgHdr.pDirty pointer. This function writes each one of the
  ** in-memory pages in the list to the database file. The argument may
  ** be NULL, representing an empty list. In this case this function is
  ** a no-op.
  **
  ** The pager must hold at least a RESERVED lock when this function
  ** is called. Before writing anything to the database file, this lock
  ** is upgraded to an EXCLUSIVE lock. If the lock cannot be obtained,
  ** SQLITE_BUSY is returned and no data is written to the database file.
  ** 
  ** If the pager is a temp-file pager and the actual file-system file
  ** is not yet open, it is created and opened before any data is 
  ** written out.
  **
  ** Once the lock has been upgraded and, if necessary, the file opened,
  ** the pages are written out to the database file in list order. Writing
  ** a page is skipped if it meets either of the following criteria:
  **
  **   * The page number is greater than Pager.dbSize, or
  **   * The PGHDR_DONT_WRITE flag is set on the page.
  **
  ** If writing out a page causes the database file to grow, Pager.dbFileSize
  ** is updated accordingly. If page 1 is written out, then the value cached
  ** in Pager.dbFileVers[] is updated to match the new value stored in
  ** the database file.
  **
  ** If everything is successful, SQLITE_OK is returned. If an IO error 
  ** occurs, an IO error code is returned. Or, if the EXCLUSIVE lock cannot
  ** be obtained, SQLITE_BUSY is returned.
  */
  static int pager_write_pagelist(Pager *pPager, PgHdr *pList){
    int rc = SQLITE_OK;                  /* Return code */
  
    /* This function is only called for rollback pagers in WRITER_DBMOD state. */
    assert( !pagerUseWal(pPager) );
    assert( pPager->tempFile || pPager->eState==PAGER_WRITER_DBMOD );
    assert( pPager->eLock==EXCLUSIVE_LOCK );
    assert( isOpen(pPager->fd) || pList->pDirty==0 );
  
    /* If the file is a temp-file has not yet been opened, open it now. It
    ** is not possible for rc to be other than SQLITE_OK if this branch
    ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
    */
    if( !isOpen(pPager->fd) ){
      assert( pPager->tempFile && rc==SQLITE_OK );
      rc = pagerOpentemp(pPager, pPager->fd, pPager->vfsFlags);
    }
  
    /* Before the first write, give the VFS a hint of what the final
    ** file size will be.
    */
    assert( rc!=SQLITE_OK || isOpen(pPager->fd) );
    if( rc==SQLITE_OK 
     && pPager->dbHintSize<pPager->dbSize
     && (pList->pDirty || pList->pgno>pPager->dbHintSize)
    ){
      sqlite3_int64 szFile = pPager->pageSize * (sqlite3_int64)pPager->dbSize;
      sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_SIZE_HINT, &szFile);
      pPager->dbHintSize = pPager->dbSize;
    }
  
    while( rc==SQLITE_OK && pList ){
      Pgno pgno = pList->pgno;
  
      /* If there are dirty pages in the page cache with page numbers greater
      ** than Pager.dbSize, this means sqlite3PagerTruncateImage() was called to
      ** make the file smaller (presumably by auto-vacuum code). Do not write
      ** any such pages to the file.
      **
      ** Also, do not write out any page that has the PGHDR_DONT_WRITE flag
      ** set (set by sqlite3PagerDontWrite()).
      */
      if( pgno<=pPager->dbSize && 0==(pList->flags&PGHDR_DONT_WRITE) ){
        i64 offset = (pgno-1)*(i64)pPager->pageSize;   /* Offset to write */
        char *pData;                                   /* Data to write */    
  
        assert( (pList->flags&PGHDR_NEED_SYNC)==0 );
        if( pList->pgno==1 ) pager_write_changecounter(pList);
  
        /* Encode the database */
        CODEC2(pPager, pList->pData, pgno, 6, return SQLITE_NOMEM_BKPT, pData);
  
        /* Write out the page data. */
        rc = sqlite3OsWrite(pPager->fd, pData, pPager->pageSize, offset);
  
        /* If page 1 was just written, update Pager.dbFileVers to match
        ** the value now stored in the database file. If writing this 
        ** page caused the database file to grow, update dbFileSize. 
        */
        if( pgno==1 ){
          memcpy(&pPager->dbFileVers, &pData[24], sizeof(pPager->dbFileVers));
        }
        if( pgno>pPager->dbFileSize ){
          pPager->dbFileSize = pgno;
        }
        pPager->aStat[PAGER_STAT_WRITE]++;
  
        /* Update any backup objects copying the contents of this pager. */
        sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)pList->pData);
  
        PAGERTRACE(("STORE %d page %d hash(%08x)\n",
                     PAGERID(pPager), pgno, pager_pagehash(pList)));
        IOTRACE(("PGOUT %p %d\n", pPager, pgno));
        PAGER_INCR(sqlite3_pager_writedb_count);
      }else{
        PAGERTRACE(("NOSTORE %d page %d\n", PAGERID(pPager), pgno));
      }
      pager_set_pagehash(pList);
      pList = pList->pDirty;
    }
  
    return rc;
  }
  
  /*
  ** Ensure that the sub-journal file is open. If it is already open, this 
  ** function is a no-op.
  **
  ** SQLITE_OK is returned if everything goes according to plan. An 
  ** SQLITE_IOERR_XXX error code is returned if a call to sqlite3OsOpen() 
  ** fails.
  */
  static int openSubJournal(Pager *pPager){
    int rc = SQLITE_OK;
    if( !isOpen(pPager->sjfd) ){
      const int flags =  SQLITE_OPEN_SUBJOURNAL | SQLITE_OPEN_READWRITE 
        | SQLITE_OPEN_CREATE | SQLITE_OPEN_EXCLUSIVE 
        | SQLITE_OPEN_DELETEONCLOSE;
      int nStmtSpill = sqlite3Config.nStmtSpill;
      if( pPager->journalMode==PAGER_JOURNALMODE_MEMORY || pPager->subjInMemory ){
        nStmtSpill = -1;
      }
      rc = sqlite3JournalOpen(pPager->pVfs, 0, pPager->sjfd, flags, nStmtSpill);
    }
    return rc;
  }
  
  /*
  ** Append a record of the current state of page pPg to the sub-journal. 
  **
  ** If successful, set the bit corresponding to pPg->pgno in the bitvecs
  ** for all open savepoints before returning.
  **
  ** This function returns SQLITE_OK if everything is successful, an IO
  ** error code if the attempt to write to the sub-journal fails, or 
  ** SQLITE_NOMEM if a malloc fails while setting a bit in a savepoint
  ** bitvec.
  */
  static int subjournalPage(PgHdr *pPg){
    int rc = SQLITE_OK;
    Pager *pPager = pPg->pPager;
    if( pPager->journalMode!=PAGER_JOURNALMODE_OFF ){
  
      /* Open the sub-journal, if it has not already been opened */
      assert( pPager->useJournal );
      assert( isOpen(pPager->jfd) || pagerUseWal(pPager) );
      assert( isOpen(pPager->sjfd) || pPager->nSubRec==0 );
      assert( pagerUseWal(pPager) 
           || pageInJournal(pPager, pPg) 
           || pPg->pgno>pPager->dbOrigSize 
      );
      rc = openSubJournal(pPager);
  
      /* If the sub-journal was opened successfully (or was already open),
      ** write the journal record into the file.  */
      if( rc==SQLITE_OK ){
        void *pData = pPg->pData;
        i64 offset = (i64)pPager->nSubRec*(4+pPager->pageSize);
        char *pData2;
    
        CODEC2(pPager, pData, pPg->pgno, 7, return SQLITE_NOMEM_BKPT, pData2);
        PAGERTRACE(("STMT-JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno));
        rc = write32bits(pPager->sjfd, offset, pPg->pgno);
        if( rc==SQLITE_OK ){
          rc = sqlite3OsWrite(pPager->sjfd, pData2, pPager->pageSize, offset+4);
        }
      }
    }
    if( rc==SQLITE_OK ){
      pPager->nSubRec++;
      assert( pPager->nSavepoint>0 );
      rc = addToSavepointBitvecs(pPager, pPg->pgno);
    }
    return rc;
  }
  static int subjournalPageIfRequired(PgHdr *pPg){
    if( subjRequiresPage(pPg) ){
      return subjournalPage(pPg);
    }else{
      return SQLITE_OK;
    }
  }
  
  /*
  ** This function is called by the pcache layer when it has reached some
  ** soft memory limit. The first argument is a pointer to a Pager object
  ** (cast as a void*). The pager is always 'purgeable' (not an in-memory
  ** database). The second argument is a reference to a page that is 
  ** currently dirty but has no outstanding references. The page
  ** is always associated with the Pager object passed as the first 
  ** argument.
  **
  ** The job of this function is to make pPg clean by writing its contents
  ** out to the database file, if possible. This may involve syncing the
  ** journal file. 
  **
  ** If successful, sqlite3PcacheMakeClean() is called on the page and
  ** SQLITE_OK returned. If an IO error occurs while trying to make the
  ** page clean, the IO error code is returned. If the page cannot be
  ** made clean for some other reason, but no error occurs, then SQLITE_OK
  ** is returned by sqlite3PcacheMakeClean() is not called.
  */
  static int pagerStress(void *p, PgHdr *pPg){
    Pager *pPager = (Pager *)p;
    int rc = SQLITE_OK;
  
    assert( pPg->pPager==pPager );
    assert( pPg->flags&PGHDR_DIRTY );
  
    /* The doNotSpill NOSYNC bit is set during times when doing a sync of
    ** journal (and adding a new header) is not allowed.  This occurs
    ** during calls to sqlite3PagerWrite() while trying to journal multiple
    ** pages belonging to the same sector.
    **
    ** The doNotSpill ROLLBACK and OFF bits inhibits all cache spilling
    ** regardless of whether or not a sync is required.  This is set during
    ** a rollback or by user request, respectively.
    **
    ** Spilling is also prohibited when in an error state since that could
    ** lead to database corruption.   In the current implementation it 
    ** is impossible for sqlite3PcacheFetch() to be called with createFlag==3
    ** while in the error state, hence it is impossible for this routine to
    ** be called in the error state.  Nevertheless, we include a NEVER()
    ** test for the error state as a safeguard against future changes.
    */
    if( NEVER(pPager->errCode) ) return SQLITE_OK;
    testcase( pPager->doNotSpill & SPILLFLAG_ROLLBACK );
    testcase( pPager->doNotSpill & SPILLFLAG_OFF );
    testcase( pPager->doNotSpill & SPILLFLAG_NOSYNC );
    if( pPager->doNotSpill
     && ((pPager->doNotSpill & (SPILLFLAG_ROLLBACK|SPILLFLAG_OFF))!=0
        || (pPg->flags & PGHDR_NEED_SYNC)!=0)
    ){
      return SQLITE_OK;
    }
  
    pPg->pDirty = 0;
    if( pagerUseWal(pPager) ){
      /* Write a single frame for this page to the log. */
      rc = subjournalPageIfRequired(pPg); 
      if( rc==SQLITE_OK ){
        rc = pagerWalFrames(pPager, pPg, 0, 0);
      }
    }else{
    
      /* Sync the journal file if required. */
      if( pPg->flags&PGHDR_NEED_SYNC 
       || pPager->eState==PAGER_WRITER_CACHEMOD
      ){
        rc = syncJournal(pPager, 1);
      }
    
      /* Write the contents of the page out to the database file. */
      if( rc==SQLITE_OK ){
        assert( (pPg->flags&PGHDR_NEED_SYNC)==0 );
        rc = pager_write_pagelist(pPager, pPg);
      }
    }
  
    /* Mark the page as clean. */
    if( rc==SQLITE_OK ){
      PAGERTRACE(("STRESS %d page %d\n", PAGERID(pPager), pPg->pgno));
      sqlite3PcacheMakeClean(pPg);
    }
  
    return pager_error(pPager, rc); 
  }
  
  /*
  ** Flush all unreferenced dirty pages to disk.
  */
  int sqlite3PagerFlush(Pager *pPager){
    int rc = pPager->errCode;
    if( !MEMDB ){
      PgHdr *pList = sqlite3PcacheDirtyList(pPager->pPCache);
      assert( assert_pager_state(pPager) );
      while( rc==SQLITE_OK && pList ){
        PgHdr *pNext = pList->pDirty;
        if( pList->nRef==0 ){
          rc = pagerStress((void*)pPager, pList);
        }
        pList = pNext;
      }
    }
  
    return rc;
  }
  
  /*
  ** Allocate and initialize a new Pager object and put a pointer to it
  ** in *ppPager. The pager should eventually be freed by passing it
  ** to sqlite3PagerClose().
  **
  ** The zFilename argument is the path to the database file to open.
  ** If zFilename is NULL then a randomly-named temporary file is created
  ** and used as the file to be cached. Temporary files are be deleted
  ** automatically when they are closed. If zFilename is ":memory:" then 
  ** all information is held in cache. It is never written to disk. 
  ** This can be used to implement an in-memory database.
  **
  ** The nExtra parameter specifies the number of bytes of space allocated
  ** along with each page reference. This space is available to the user
  ** via the sqlite3PagerGetExtra() API.
  **
  ** The flags argument is used to specify properties that affect the
  ** operation of the pager. It should be passed some bitwise combination
  ** of the PAGER_* flags.
  **
  ** The vfsFlags parameter is a bitmask to pass to the flags parameter
  ** of the xOpen() method of the supplied VFS when opening files. 
  **
  ** If the pager object is allocated and the specified file opened 
  ** successfully, SQLITE_OK is returned and *ppPager set to point to
  ** the new pager object. If an error occurs, *ppPager is set to NULL
  ** and error code returned. This function may return SQLITE_NOMEM
  ** (sqlite3Malloc() is used to allocate memory), SQLITE_CANTOPEN or 
  ** various SQLITE_IO_XXX errors.
  */
  int sqlite3PagerOpen(
    sqlite3_vfs *pVfs,       /* The virtual file system to use */
    Pager **ppPager,         /* OUT: Return the Pager structure here */
    const char *zFilename,   /* Name of the database file to open */
    int nExtra,              /* Extra bytes append to each in-memory page */
    int flags,               /* flags controlling this file */
    int vfsFlags,            /* flags passed through to sqlite3_vfs.xOpen() */
    void (*xReinit)(DbPage*) /* Function to reinitialize pages */
  ){
    u8 *pPtr;
    Pager *pPager = 0;       /* Pager object to allocate and return */
    int rc = SQLITE_OK;      /* Return code */
    int tempFile = 0;        /* True for temp files (incl. in-memory files) */
    int memDb = 0;           /* True if this is an in-memory file */
    int readOnly = 0;        /* True if this is a read-only file */
    int journalFileSize;     /* Bytes to allocate for each journal fd */
    char *zPathname = 0;     /* Full path to database file */
    int nPathname = 0;       /* Number of bytes in zPathname */
    int useJournal = (flags & PAGER_OMIT_JOURNAL)==0; /* False to omit journal */
    int pcacheSize = sqlite3PcacheSize();       /* Bytes to allocate for PCache */
    u32 szPageDflt = SQLITE_DEFAULT_PAGE_SIZE;  /* Default page size */
    const char *zUri = 0;    /* URI args to copy */
    int nUri = 0;            /* Number of bytes of URI args at *zUri */
  
    /* Figure out how much space is required for each journal file-handle
    ** (there are two of them, the main journal and the sub-journal).  */
    journalFileSize = ROUND8(sqlite3JournalSize(pVfs));
  
    /* Set the output variable to NULL in case an error occurs. */
    *ppPager = 0;
  
  #ifndef SQLITE_OMIT_MEMORYDB
    if( flags & PAGER_MEMORY ){
      memDb = 1;
      if( zFilename && zFilename[0] ){
        zPathname = sqlite3DbStrDup(0, zFilename);
        if( zPathname==0  ) return SQLITE_NOMEM_BKPT;
        nPathname = sqlite3Strlen30(zPathname);
        zFilename = 0;
      }
    }
  #endif
  
    /* Compute and store the full pathname in an allocated buffer pointed
    ** to by zPathname, length nPathname. Or, if this is a temporary file,
    ** leave both nPathname and zPathname set to 0.
    */
    if( zFilename && zFilename[0] ){
      const char *z;
      nPathname = pVfs->mxPathname+1;
      zPathname = sqlite3DbMallocRaw(0, nPathname*2);
      if( zPathname==0 ){
        return SQLITE_NOMEM_BKPT;
      }
      zPathname[0] = 0; /* Make sure initialized even if FullPathname() fails */
      rc = sqlite3OsFullPathname(pVfs, zFilename, nPathname, zPathname);
      nPathname = sqlite3Strlen30(zPathname);
      z = zUri = &zFilename[sqlite3Strlen30(zFilename)+1];
      while( *z ){
        z += sqlite3Strlen30(z)+1;
        z += sqlite3Strlen30(z)+1;
      }
      nUri = (int)(&z[1] - zUri);
      assert( nUri>=0 );
      if( rc==SQLITE_OK && nPathname+8>pVfs->mxPathname ){
        /* This branch is taken when the journal path required by
        ** the database being opened will be more than pVfs->mxPathname
        ** bytes in length. This means the database cannot be opened,
        ** as it will not be possible to open the journal file or even
        ** check for a hot-journal before reading.
        */
        rc = SQLITE_CANTOPEN_BKPT;
      }
      if( rc!=SQLITE_OK ){
        sqlite3DbFree(0, zPathname);
        return rc;
      }
    }
  
    /* Allocate memory for the Pager structure, PCache object, the
    ** three file descriptors, the database file name and the journal 
    ** file name. The layout in memory is as follows:
    **
    **     Pager object                    (sizeof(Pager) bytes)
    **     PCache object                   (sqlite3PcacheSize() bytes)
    **     Database file handle            (pVfs->szOsFile bytes)
    **     Sub-journal file handle         (journalFileSize bytes)
    **     Main journal file handle        (journalFileSize bytes)
    **     Database file name              (nPathname+1 bytes)
    **     Journal file name               (nPathname+8+1 bytes)
    */
    pPtr = (u8 *)sqlite3MallocZero(
      ROUND8(sizeof(*pPager)) +      /* Pager structure */
      ROUND8(pcacheSize) +           /* PCache object */
      ROUND8(pVfs->szOsFile) +       /* The main db file */
      journalFileSize * 2 +          /* The two journal files */ 
      nPathname + 1 + nUri +         /* zFilename */
      nPathname + 8 + 2              /* zJournal */
  #ifndef SQLITE_OMIT_WAL
      + nPathname + 4 + 2            /* zWal */
  #endif
    );
    assert( EIGHT_BYTE_ALIGNMENT(SQLITE_INT_TO_PTR(journalFileSize)) );
    if( !pPtr ){
      sqlite3DbFree(0, zPathname);
      return SQLITE_NOMEM_BKPT;
    }
    pPager =              (Pager*)(pPtr);
    pPager->pPCache =    (PCache*)(pPtr += ROUND8(sizeof(*pPager)));
    pPager->fd =   (sqlite3_file*)(pPtr += ROUND8(pcacheSize));
    pPager->sjfd = (sqlite3_file*)(pPtr += ROUND8(pVfs->szOsFile));
    pPager->jfd =  (sqlite3_file*)(pPtr += journalFileSize);
    pPager->zFilename =    (char*)(pPtr += journalFileSize);
    assert( EIGHT_BYTE_ALIGNMENT(pPager->jfd) );
  
    /* Fill in the Pager.zFilename and Pager.zJournal buffers, if required. */
    if( zPathname ){
      assert( nPathname>0 );
      pPager->zJournal =   (char*)(pPtr += nPathname + 1 + nUri);
      memcpy(pPager->zFilename, zPathname, nPathname);
      if( nUri ) memcpy(&pPager->zFilename[nPathname+1], zUri, nUri);
      memcpy(pPager->zJournal, zPathname, nPathname);
      memcpy(&pPager->zJournal[nPathname], "-journal\000", 8+2);
      sqlite3FileSuffix3(pPager->zFilename, pPager->zJournal);
  #ifndef SQLITE_OMIT_WAL
      pPager->zWal = &pPager->zJournal[nPathname+8+1];
      memcpy(pPager->zWal, zPathname, nPathname);
      memcpy(&pPager->zWal[nPathname], "-wal\000", 4+1);
      sqlite3FileSuffix3(pPager->zFilename, pPager->zWal);
  #endif
      sqlite3DbFree(0, zPathname);
    }
    pPager->pVfs = pVfs;
    pPager->vfsFlags = vfsFlags;
  
    /* Open the pager file.
    */
    if( zFilename && zFilename[0] ){
      int fout = 0;                    /* VFS flags returned by xOpen() */
      rc = sqlite3OsOpen(pVfs, pPager->zFilename, pPager->fd, vfsFlags, &fout);
      assert( !memDb );
      readOnly = (fout&SQLITE_OPEN_READONLY);
  
      /* If the file was successfully opened for read/write access,
      ** choose a default page size in case we have to create the
      ** database file. The default page size is the maximum of:
      **
      **    + SQLITE_DEFAULT_PAGE_SIZE,
      **    + The value returned by sqlite3OsSectorSize()
      **    + The largest page size that can be written atomically.
      */
      if( rc==SQLITE_OK ){
        int iDc = sqlite3OsDeviceCharacteristics(pPager->fd);
        if( !readOnly ){
          setSectorSize(pPager);
          assert(SQLITE_DEFAULT_PAGE_SIZE<=SQLITE_MAX_DEFAULT_PAGE_SIZE);
          if( szPageDflt<pPager->sectorSize ){
            if( pPager->sectorSize>SQLITE_MAX_DEFAULT_PAGE_SIZE ){
              szPageDflt = SQLITE_MAX_DEFAULT_PAGE_SIZE;
            }else{
              szPageDflt = (u32)pPager->sectorSize;
            }
          }
  #ifdef SQLITE_ENABLE_ATOMIC_WRITE
          {
            int ii;
            assert(SQLITE_IOCAP_ATOMIC512==(512>>8));
            assert(SQLITE_IOCAP_ATOMIC64K==(65536>>8));
            assert(SQLITE_MAX_DEFAULT_PAGE_SIZE<=65536);
            for(ii=szPageDflt; ii<=SQLITE_MAX_DEFAULT_PAGE_SIZE; ii=ii*2){
              if( iDc&(SQLITE_IOCAP_ATOMIC|(ii>>8)) ){
                szPageDflt = ii;
              }
            }
          }
  #endif
        }
        pPager->noLock = sqlite3_uri_boolean(zFilename, "nolock", 0);
        if( (iDc & SQLITE_IOCAP_IMMUTABLE)!=0
         || sqlite3_uri_boolean(zFilename, "immutable", 0) ){
            vfsFlags |= SQLITE_OPEN_READONLY;
            goto act_like_temp_file;
        }
      }
    }else{
      /* If a temporary file is requested, it is not opened immediately.
      ** In this case we accept the default page size and delay actually
      ** opening the file until the first call to OsWrite().
      **
      ** This branch is also run for an in-memory database. An in-memory
      ** database is the same as a temp-file that is never written out to
      ** disk and uses an in-memory rollback journal.
      **
      ** This branch also runs for files marked as immutable.
      */ 
  act_like_temp_file:
      tempFile = 1;
      pPager->eState = PAGER_READER;     /* Pretend we already have a lock */
      pPager->eLock = EXCLUSIVE_LOCK;    /* Pretend we are in EXCLUSIVE mode */
      pPager->noLock = 1;                /* Do no locking */
      readOnly = (vfsFlags&SQLITE_OPEN_READONLY);
    }
  
    /* The following call to PagerSetPagesize() serves to set the value of 
    ** Pager.pageSize and to allocate the Pager.pTmpSpace buffer.
    */
    if( rc==SQLITE_OK ){
      assert( pPager->memDb==0 );
      rc = sqlite3PagerSetPagesize(pPager, &szPageDflt, -1);
      testcase( rc!=SQLITE_OK );
    }
  
    /* Initialize the PCache object. */
    if( rc==SQLITE_OK ){
      assert( nExtra<1000 );
      nExtra = ROUND8(nExtra);
      rc = sqlite3PcacheOpen(szPageDflt, nExtra, !memDb,
                         !memDb?pagerStress:0, (void *)pPager, pPager->pPCache);
    }
  
    /* If an error occurred above, free the  Pager structure and close the file.
    */
    if( rc!=SQLITE_OK ){
      sqlite3OsClose(pPager->fd);
      sqlite3PageFree(pPager->pTmpSpace);
      sqlite3_free(pPager);
      return rc;
    }
  
    PAGERTRACE(("OPEN %d %s\n", FILEHANDLEID(pPager->fd), pPager->zFilename));
    IOTRACE(("OPEN %p %s\n", pPager, pPager->zFilename))
  
    pPager->useJournal = (u8)useJournal;
    /* pPager->stmtOpen = 0; */
    /* pPager->stmtInUse = 0; */
    /* pPager->nRef = 0; */
    /* pPager->stmtSize = 0; */
    /* pPager->stmtJSize = 0; */
    /* pPager->nPage = 0; */
    pPager->mxPgno = SQLITE_MAX_PAGE_COUNT;
    /* pPager->state = PAGER_UNLOCK; */
    /* pPager->errMask = 0; */
    pPager->tempFile = (u8)tempFile;
    assert( tempFile==PAGER_LOCKINGMODE_NORMAL 
            || tempFile==PAGER_LOCKINGMODE_EXCLUSIVE );
    assert( PAGER_LOCKINGMODE_EXCLUSIVE==1 );
    pPager->exclusiveMode = (u8)tempFile; 
    pPager->changeCountDone = pPager->tempFile;
    pPager->memDb = (u8)memDb;
    pPager->readOnly = (u8)readOnly;
    assert( useJournal || pPager->tempFile );
    pPager->noSync = pPager->tempFile;
    if( pPager->noSync ){
      assert( pPager->fullSync==0 );
      assert( pPager->extraSync==0 );
      assert( pPager->syncFlags==0 );
      assert( pPager->walSyncFlags==0 );
      assert( pPager->ckptSyncFlags==0 );
    }else{
      pPager->fullSync = 1;
      pPager->extraSync = 0;
      pPager->syncFlags = SQLITE_SYNC_NORMAL;
      pPager->walSyncFlags = SQLITE_SYNC_NORMAL | WAL_SYNC_TRANSACTIONS;
      pPager->ckptSyncFlags = SQLITE_SYNC_NORMAL;
    }
    /* pPager->pFirst = 0; */
    /* pPager->pFirstSynced = 0; */
    /* pPager->pLast = 0; */
    pPager->nExtra = (u16)nExtra;
    pPager->journalSizeLimit = SQLITE_DEFAULT_JOURNAL_SIZE_LIMIT;
    assert( isOpen(pPager->fd) || tempFile );
    setSectorSize(pPager);
    if( !useJournal ){
      pPager->journalMode = PAGER_JOURNALMODE_OFF;
    }else if( memDb ){
      pPager->journalMode = PAGER_JOURNALMODE_MEMORY;
    }
    /* pPager->xBusyHandler = 0; */
    /* pPager->pBusyHandlerArg = 0; */
    pPager->xReiniter = xReinit;
    /* memset(pPager->aHash, 0, sizeof(pPager->aHash)); */
    /* pPager->szMmap = SQLITE_DEFAULT_MMAP_SIZE // will be set by btree.c */
  
    *ppPager = pPager;
    return SQLITE_OK;
  }
  
  
  /* Verify that the database file has not be deleted or renamed out from
  ** under the pager.  Return SQLITE_OK if the database is still were it ought
  ** to be on disk.  Return non-zero (SQLITE_READONLY_DBMOVED or some other error
  ** code from sqlite3OsAccess()) if the database has gone missing.
  */
  static int databaseIsUnmoved(Pager *pPager){
    int bHasMoved = 0;
    int rc;
  
    if( pPager->tempFile ) return SQLITE_OK;
    if( pPager->dbSize==0 ) return SQLITE_OK;
    assert( pPager->zFilename && pPager->zFilename[0] );
    rc = sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_HAS_MOVED, &bHasMoved);
    if( rc==SQLITE_NOTFOUND ){
      /* If the HAS_MOVED file-control is unimplemented, assume that the file
      ** has not been moved.  That is the historical behavior of SQLite: prior to
      ** version 3.8.3, it never checked */
      rc = SQLITE_OK;
    }else if( rc==SQLITE_OK && bHasMoved ){
      rc = SQLITE_READONLY_DBMOVED;
    }
    return rc;
  }
  
  
  /*
  ** This function is called after transitioning from PAGER_UNLOCK to
  ** PAGER_SHARED state. It tests if there is a hot journal present in
  ** the file-system for the given pager. A hot journal is one that 
  ** needs to be played back. According to this function, a hot-journal
  ** file exists if the following criteria are met:
  **
  **   * The journal file exists in the file system, and
  **   * No process holds a RESERVED or greater lock on the database file, and
  **   * The database file itself is greater than 0 bytes in size, and
  **   * The first byte of the journal file exists and is not 0x00.
  **
  ** If the current size of the database file is 0 but a journal file
  ** exists, that is probably an old journal left over from a prior
  ** database with the same name. In this case the journal file is
  ** just deleted using OsDelete, *pExists is set to 0 and SQLITE_OK
  ** is returned.
  **
  ** This routine does not check if there is a master journal filename
  ** at the end of the file. If there is, and that master journal file
  ** does not exist, then the journal file is not really hot. In this
  ** case this routine will return a false-positive. The pager_playback()
  ** routine will discover that the journal file is not really hot and 
  ** will not roll it back. 
  **
  ** If a hot-journal file is found to exist, *pExists is set to 1 and 
  ** SQLITE_OK returned. If no hot-journal file is present, *pExists is
  ** set to 0 and SQLITE_OK returned. If an IO error occurs while trying
  ** to determine whether or not a hot-journal file exists, the IO error
  ** code is returned and the value of *pExists is undefined.
  */
  static int hasHotJournal(Pager *pPager, int *pExists){
    sqlite3_vfs * const pVfs = pPager->pVfs;
    int rc = SQLITE_OK;           /* Return code */
    int exists = 1;               /* True if a journal file is present */
    int jrnlOpen = !!isOpen(pPager->jfd);
  
    assert( pPager->useJournal );
    assert( isOpen(pPager->fd) );
    assert( pPager->eState==PAGER_OPEN );
  
    assert( jrnlOpen==0 || ( sqlite3OsDeviceCharacteristics(pPager->jfd) &
      SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN
    ));
  
    *pExists = 0;
    if( !jrnlOpen ){
      rc = sqlite3OsAccess(pVfs, pPager->zJournal, SQLITE_ACCESS_EXISTS, &exists);
    }
    if( rc==SQLITE_OK && exists ){
      int locked = 0;             /* True if some process holds a RESERVED lock */
  
      /* Race condition here:  Another process might have been holding the
      ** the RESERVED lock and have a journal open at the sqlite3OsAccess() 
      ** call above, but then delete the journal and drop the lock before
      ** we get to the following sqlite3OsCheckReservedLock() call.  If that
      ** is the case, this routine might think there is a hot journal when
      ** in fact there is none.  This results in a false-positive which will
      ** be dealt with by the playback routine.  Ticket #3883.
      */
      rc = sqlite3OsCheckReservedLock(pPager->fd, &locked);
      if( rc==SQLITE_OK && !locked ){
        Pgno nPage;                 /* Number of pages in database file */
  
        assert( pPager->tempFile==0 );
        rc = pagerPagecount(pPager, &nPage);
        if( rc==SQLITE_OK ){
          /* If the database is zero pages in size, that means that either (1) the
          ** journal is a remnant from a prior database with the same name where
          ** the database file but not the journal was deleted, or (2) the initial
          ** transaction that populates a new database is being rolled back.
          ** In either case, the journal file can be deleted.  However, take care
          ** not to delete the journal file if it is already open due to
          ** journal_mode=PERSIST.
          */
          if( nPage==0 && !jrnlOpen ){
            sqlite3BeginBenignMalloc();
            if( pagerLockDb(pPager, RESERVED_LOCK)==SQLITE_OK ){
              sqlite3OsDelete(pVfs, pPager->zJournal, 0);
              if( !pPager->exclusiveMode ) pagerUnlockDb(pPager, SHARED_LOCK);
            }
            sqlite3EndBenignMalloc();
          }else{
            /* The journal file exists and no other connection has a reserved
            ** or greater lock on the database file. Now check that there is
            ** at least one non-zero bytes at the start of the journal file.
            ** If there is, then we consider this journal to be hot. If not, 
            ** it can be ignored.
            */
            if( !jrnlOpen ){
              int f = SQLITE_OPEN_READONLY|SQLITE_OPEN_MAIN_JOURNAL;
              rc = sqlite3OsOpen(pVfs, pPager->zJournal, pPager->jfd, f, &f);
            }
            if( rc==SQLITE_OK ){
              u8 first = 0;
              rc = sqlite3OsRead(pPager->jfd, (void *)&first, 1, 0);
              if( rc==SQLITE_IOERR_SHORT_READ ){
                rc = SQLITE_OK;
              }
              if( !jrnlOpen ){
                sqlite3OsClose(pPager->jfd);
              }
              *pExists = (first!=0);
            }else if( rc==SQLITE_CANTOPEN ){
              /* If we cannot open the rollback journal file in order to see if
              ** it has a zero header, that might be due to an I/O error, or
              ** it might be due to the race condition described above and in
              ** ticket #3883.  Either way, assume that the journal is hot.
              ** This might be a false positive.  But if it is, then the
              ** automatic journal playback and recovery mechanism will deal
              ** with it under an EXCLUSIVE lock where we do not need to
              ** worry so much with race conditions.
              */
              *pExists = 1;
              rc = SQLITE_OK;
            }
          }
        }
      }
    }
  
    return rc;
  }
  
  /*
  ** This function is called to obtain a shared lock on the database file.
  ** It is illegal to call sqlite3PagerGet() until after this function
  ** has been successfully called. If a shared-lock is already held when
  ** this function is called, it is a no-op.
  **
  ** The following operations are also performed by this function.
  **
  **   1) If the pager is currently in PAGER_OPEN state (no lock held
  **      on the database file), then an attempt is made to obtain a
  **      SHARED lock on the database file. Immediately after obtaining
  **      the SHARED lock, the file-system is checked for a hot-journal,
  **      which is played back if present. Following any hot-journal 
  **      rollback, the contents of the cache are validated by checking
  **      the 'change-counter' field of the database file header and
  **      discarded if they are found to be invalid.
  **
  **   2) If the pager is running in exclusive-mode, and there are currently
  **      no outstanding references to any pages, and is in the error state,
  **      then an attempt is made to clear the error state by discarding
  **      the contents of the page cache and rolling back any open journal
  **      file.
  **
  ** If everything is successful, SQLITE_OK is returned. If an IO error 
  ** occurs while locking the database, checking for a hot-journal file or 
  ** rolling back a journal file, the IO error code is returned.
  */
  int sqlite3PagerSharedLock(Pager *pPager){
    int rc = SQLITE_OK;                /* Return code */
  
    /* This routine is only called from b-tree and only when there are no
    ** outstanding pages. This implies that the pager state should either
    ** be OPEN or READER. READER is only possible if the pager is or was in 
    ** exclusive access mode.  */
    assert( sqlite3PcacheRefCount(pPager->pPCache)==0 );
    assert( assert_pager_state(pPager) );
    assert( pPager->eState==PAGER_OPEN || pPager->eState==PAGER_READER );
    assert( pPager->errCode==SQLITE_OK );
  
    if( !pagerUseWal(pPager) && pPager->eState==PAGER_OPEN ){
      int bHotJournal = 1;          /* True if there exists a hot journal-file */
  
      assert( !MEMDB );
      assert( pPager->tempFile==0 || pPager->eLock==EXCLUSIVE_LOCK );
  
      rc = pager_wait_on_lock(pPager, SHARED_LOCK);
      if( rc!=SQLITE_OK ){
        assert( pPager->eLock==NO_LOCK || pPager->eLock==UNKNOWN_LOCK );
        goto failed;
      }
  
      /* If a journal file exists, and there is no RESERVED lock on the
      ** database file, then it either needs to be played back or deleted.
      */
      if( pPager->eLock<=SHARED_LOCK ){
        rc = hasHotJournal(pPager, &bHotJournal);
      }
      if( rc!=SQLITE_OK ){
        goto failed;
      }
      if( bHotJournal ){
        if( pPager->readOnly ){
          rc = SQLITE_READONLY_ROLLBACK;
          goto failed;
        }
  
        /* Get an EXCLUSIVE lock on the database file. At this point it is
        ** important that a RESERVED lock is not obtained on the way to the
        ** EXCLUSIVE lock. If it were, another process might open the
        ** database file, detect the RESERVED lock, and conclude that the
        ** database is safe to read while this process is still rolling the 
        ** hot-journal back.
        ** 
        ** Because the intermediate RESERVED lock is not requested, any
        ** other process attempting to access the database file will get to 
        ** this point in the code and fail to obtain its own EXCLUSIVE lock 
        ** on the database file.
        **
        ** Unless the pager is in locking_mode=exclusive mode, the lock is
        ** downgraded to SHARED_LOCK before this function returns.
        */
        rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
        if( rc!=SQLITE_OK ){
          goto failed;
        }
   
        /* If it is not already open and the file exists on disk, open the 
        ** journal for read/write access. Write access is required because 
        ** in exclusive-access mode the file descriptor will be kept open 
        ** and possibly used for a transaction later on. Also, write-access 
        ** is usually required to finalize the journal in journal_mode=persist 
        ** mode (and also for journal_mode=truncate on some systems).
        **
        ** If the journal does not exist, it usually means that some 
        ** other connection managed to get in and roll it back before 
        ** this connection obtained the exclusive lock above. Or, it 
        ** may mean that the pager was in the error-state when this
        ** function was called and the journal file does not exist.
        */
        if( !isOpen(pPager->jfd) ){
          sqlite3_vfs * const pVfs = pPager->pVfs;
          int bExists;              /* True if journal file exists */
          rc = sqlite3OsAccess(
              pVfs, pPager->zJournal, SQLITE_ACCESS_EXISTS, &bExists);
          if( rc==SQLITE_OK && bExists ){
            int fout = 0;
            int f = SQLITE_OPEN_READWRITE|SQLITE_OPEN_MAIN_JOURNAL;
            assert( !pPager->tempFile );
            rc = sqlite3OsOpen(pVfs, pPager->zJournal, pPager->jfd, f, &fout);
            assert( rc!=SQLITE_OK || isOpen(pPager->jfd) );
            if( rc==SQLITE_OK && fout&SQLITE_OPEN_READONLY ){
              rc = SQLITE_CANTOPEN_BKPT;
              sqlite3OsClose(pPager->jfd);
            }
          }
        }
   
        /* Playback and delete the journal.  Drop the database write
        ** lock and reacquire the read lock. Purge the cache before
        ** playing back the hot-journal so that we don't end up with
        ** an inconsistent cache.  Sync the hot journal before playing
        ** it back since the process that crashed and left the hot journal
        ** probably did not sync it and we are required to always sync
        ** the journal before playing it back.
        */
        if( isOpen(pPager->jfd) ){
          assert( rc==SQLITE_OK );
          rc = pagerSyncHotJournal(pPager);
          if( rc==SQLITE_OK ){
            rc = pager_playback(pPager, !pPager->tempFile);
            pPager->eState = PAGER_OPEN;
          }
        }else if( !pPager->exclusiveMode ){
          pagerUnlockDb(pPager, SHARED_LOCK);
        }
  
        if( rc!=SQLITE_OK ){
          /* This branch is taken if an error occurs while trying to open
          ** or roll back a hot-journal while holding an EXCLUSIVE lock. The
          ** pager_unlock() routine will be called before returning to unlock
          ** the file. If the unlock attempt fails, then Pager.eLock must be
          ** set to UNKNOWN_LOCK (see the comment above the #define for 
          ** UNKNOWN_LOCK above for an explanation). 
          **
          ** In order to get pager_unlock() to do this, set Pager.eState to
          ** PAGER_ERROR now. This is not actually counted as a transition
          ** to ERROR state in the state diagram at the top of this file,
          ** since we know that the same call to pager_unlock() will very
          ** shortly transition the pager object to the OPEN state. Calling
          ** assert_pager_state() would fail now, as it should not be possible
          ** to be in ERROR state when there are zero outstanding page 
          ** references.
          */
          pager_error(pPager, rc);
          goto failed;
        }
  
        assert( pPager->eState==PAGER_OPEN );
        assert( (pPager->eLock==SHARED_LOCK)
             || (pPager->exclusiveMode && pPager->eLock>SHARED_LOCK)
        );
      }
  
      if( !pPager->tempFile && pPager->hasHeldSharedLock ){
        /* The shared-lock has just been acquired then check to
        ** see if the database has been modified.  If the database has changed,
        ** flush the cache.  The hasHeldSharedLock flag prevents this from
        ** occurring on the very first access to a file, in order to save a
        ** single unnecessary sqlite3OsRead() call at the start-up.
        **
        ** Database changes are detected by looking at 15 bytes beginning
        ** at offset 24 into the file.  The first 4 of these 16 bytes are
        ** a 32-bit counter that is incremented with each change.  The
        ** other bytes change randomly with each file change when
        ** a codec is in use.
        ** 
        ** There is a vanishingly small chance that a change will not be 
        ** detected.  The chance of an undetected change is so small that
        ** it can be neglected.
        */
        Pgno nPage = 0;
        char dbFileVers[sizeof(pPager->dbFileVers)];
  
        rc = pagerPagecount(pPager, &nPage);
        if( rc ) goto failed;
  
        if( nPage>0 ){
          IOTRACE(("CKVERS %p %d\n", pPager, sizeof(dbFileVers)));
          rc = sqlite3OsRead(pPager->fd, &dbFileVers, sizeof(dbFileVers), 24);
          if( rc!=SQLITE_OK && rc!=SQLITE_IOERR_SHORT_READ ){
            goto failed;
          }
        }else{
          memset(dbFileVers, 0, sizeof(dbFileVers));
        }
  
        if( memcmp(pPager->dbFileVers, dbFileVers, sizeof(dbFileVers))!=0 ){
          pager_reset(pPager);
  
          /* Unmap the database file. It is possible that external processes
          ** may have truncated the database file and then extended it back
          ** to its original size while this process was not holding a lock.
          ** In this case there may exist a Pager.pMap mapping that appears
          ** to be the right size but is not actually valid. Avoid this
          ** possibility by unmapping the db here. */
          if( USEFETCH(pPager) ){
            sqlite3OsUnfetch(pPager->fd, 0, 0);
          }
        }
      }
  
      /* If there is a WAL file in the file-system, open this database in WAL
      ** mode. Otherwise, the following function call is a no-op.
      */
      rc = pagerOpenWalIfPresent(pPager);
  #ifndef SQLITE_OMIT_WAL
      assert( pPager->pWal==0 || rc==SQLITE_OK );
  #endif
    }
  
    if( pagerUseWal(pPager) ){
      assert( rc==SQLITE_OK );
      rc = pagerBeginReadTransaction(pPager);
    }
  
    if( pPager->tempFile==0 && pPager->eState==PAGER_OPEN && rc==SQLITE_OK ){
      rc = pagerPagecount(pPager, &pPager->dbSize);
    }
  
   failed:
    if( rc!=SQLITE_OK ){
      assert( !MEMDB );
      pager_unlock(pPager);
      assert( pPager->eState==PAGER_OPEN );
    }else{
      pPager->eState = PAGER_READER;
      pPager->hasHeldSharedLock = 1;
    }
    return rc;
  }
  
  /*
  ** If the reference count has reached zero, rollback any active
  ** transaction and unlock the pager.
  **
  ** Except, in locking_mode=EXCLUSIVE when there is nothing to in
  ** the rollback journal, the unlock is not performed and there is
  ** nothing to rollback, so this routine is a no-op.
  */ 
  static void pagerUnlockIfUnused(Pager *pPager){
    if( pPager->nMmapOut==0 && (sqlite3PcacheRefCount(pPager->pPCache)==0) ){
      pagerUnlockAndRollback(pPager);
    }
  }
  
  /*
  ** Acquire a reference to page number pgno in pager pPager (a page
  ** reference has type DbPage*). If the requested reference is 
  ** successfully obtained, it is copied to *ppPage and SQLITE_OK returned.
  **
  ** If the requested page is already in the cache, it is returned. 
  ** Otherwise, a new page object is allocated and populated with data
  ** read from the database file. In some cases, the pcache module may
  ** choose not to allocate a new page object and may reuse an existing
  ** object with no outstanding references.
  **
  ** The extra data appended to a page is always initialized to zeros the 
  ** first time a page is loaded into memory. If the page requested is 
  ** already in the cache when this function is called, then the extra
  ** data is left as it was when the page object was last used.
  **
  ** If the database image is smaller than the requested page or if a 
  ** non-zero value is passed as the noContent parameter and the 
  ** requested page is not already stored in the cache, then no 
  ** actual disk read occurs. In this case the memory image of the 
  ** page is initialized to all zeros. 
  **
  ** If noContent is true, it means that we do not care about the contents
  ** of the page. This occurs in two scenarios:
  **
  **   a) When reading a free-list leaf page from the database, and
  **
  **   b) When a savepoint is being rolled back and we need to load
  **      a new page into the cache to be filled with the data read
  **      from the savepoint journal.
  **
  ** If noContent is true, then the data returned is zeroed instead of
  ** being read from the database. Additionally, the bits corresponding
  ** to pgno in Pager.pInJournal (bitvec of pages already written to the
  ** journal file) and the PagerSavepoint.pInSavepoint bitvecs of any open
  ** savepoints are set. This means if the page is made writable at any
  ** point in the future, using a call to sqlite3PagerWrite(), its contents
  ** will not be journaled. This saves IO.
  **
  ** The acquisition might fail for several reasons.  In all cases,
  ** an appropriate error code is returned and *ppPage is set to NULL.
  **
  ** See also sqlite3PagerLookup().  Both this routine and Lookup() attempt
  ** to find a page in the in-memory cache first.  If the page is not already
  ** in memory, this routine goes to disk to read it in whereas Lookup()
  ** just returns 0.  This routine acquires a read-lock the first time it
  ** has to go to disk, and could also playback an old journal if necessary.
  ** Since Lookup() never goes to disk, it never has to deal with locks
  ** or journal files.
  */
  int sqlite3PagerGet(
    Pager *pPager,      /* The pager open on the database file */
    Pgno pgno,          /* Page number to fetch */
    DbPage **ppPage,    /* Write a pointer to the page here */
    int flags           /* PAGER_GET_XXX flags */
  ){
    int rc = SQLITE_OK;
    PgHdr *pPg = 0;
    u32 iFrame = 0;                 /* Frame to read from WAL file */
    const int noContent = (flags & PAGER_GET_NOCONTENT);
  
    /* It is acceptable to use a read-only (mmap) page for any page except
    ** page 1 if there is no write-transaction open or the ACQUIRE_READONLY
    ** flag was specified by the caller. And so long as the db is not a 
    ** temporary or in-memory database.  */
    const int bMmapOk = (pgno>1 && USEFETCH(pPager)
     && (pPager->eState==PAGER_READER || (flags & PAGER_GET_READONLY))
  #ifdef SQLITE_HAS_CODEC
     && pPager->xCodec==0
  #endif
    );
  
    /* Optimization note:  Adding the "pgno<=1" term before "pgno==0" here
    ** allows the compiler optimizer to reuse the results of the "pgno>1"
    ** test in the previous statement, and avoid testing pgno==0 in the
    ** common case where pgno is large. */
    if( pgno<=1 && pgno==0 ){
      return SQLITE_CORRUPT_BKPT;
    }
    assert( pPager->eState>=PAGER_READER );
    assert( assert_pager_state(pPager) );
    assert( noContent==0 || bMmapOk==0 );
  
    assert( pPager->hasHeldSharedLock==1 );
  
    /* If the pager is in the error state, return an error immediately. 
    ** Otherwise, request the page from the PCache layer. */
    if( pPager->errCode!=SQLITE_OK ){
      rc = pPager->errCode;
    }else{
      if( bMmapOk && pagerUseWal(pPager) ){
        rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iFrame);
        if( rc!=SQLITE_OK ) goto pager_acquire_err;
      }
  
      if( bMmapOk && iFrame==0 ){
        void *pData = 0;
  
        rc = sqlite3OsFetch(pPager->fd, 
            (i64)(pgno-1) * pPager->pageSize, pPager->pageSize, &pData
        );
  
        if( rc==SQLITE_OK && pData ){
          if( pPager->eState>PAGER_READER || pPager->tempFile ){
            pPg = sqlite3PagerLookup(pPager, pgno);
          }
          if( pPg==0 ){
            rc = pagerAcquireMapPage(pPager, pgno, pData, &pPg);
          }else{
            sqlite3OsUnfetch(pPager->fd, (i64)(pgno-1)*pPager->pageSize, pData);
          }
          if( pPg ){
            assert( rc==SQLITE_OK );
            *ppPage = pPg;
            return SQLITE_OK;
          }
        }
        if( rc!=SQLITE_OK ){
          goto pager_acquire_err;
        }
      }
  
      {
        sqlite3_pcache_page *pBase;
        pBase = sqlite3PcacheFetch(pPager->pPCache, pgno, 3);
        if( pBase==0 ){
          rc = sqlite3PcacheFetchStress(pPager->pPCache, pgno, &pBase);
          if( rc!=SQLITE_OK ) goto pager_acquire_err;
          if( pBase==0 ){
            pPg = *ppPage = 0;
            rc = SQLITE_NOMEM_BKPT;
            goto pager_acquire_err;
          }
        }
        pPg = *ppPage = sqlite3PcacheFetchFinish(pPager->pPCache, pgno, pBase);
        assert( pPg!=0 );
      }
    }
  
    if( rc!=SQLITE_OK ){
      /* Either the call to sqlite3PcacheFetch() returned an error or the
      ** pager was already in the error-state when this function was called.
      ** Set pPg to 0 and jump to the exception handler.  */
      pPg = 0;
      goto pager_acquire_err;
    }
    assert( pPg==(*ppPage) );
    assert( pPg->pgno==pgno );
    assert( pPg->pPager==pPager || pPg->pPager==0 );
  
    if( pPg->pPager && !noContent ){
      /* In this case the pcache already contains an initialized copy of
      ** the page. Return without further ado.  */
      assert( pgno<=PAGER_MAX_PGNO && pgno!=PAGER_MJ_PGNO(pPager) );
      pPager->aStat[PAGER_STAT_HIT]++;
      return SQLITE_OK;
  
    }else{
      /* The pager cache has created a new page. Its content needs to 
      ** be initialized.  */
  
      pPg->pPager = pPager;
  
      /* The maximum page number is 2^31. Return SQLITE_CORRUPT if a page
      ** number greater than this, or the unused locking-page, is requested. */
      if( pgno>PAGER_MAX_PGNO || pgno==PAGER_MJ_PGNO(pPager) ){
        rc = SQLITE_CORRUPT_BKPT;
        goto pager_acquire_err;
      }
  
      assert( !isOpen(pPager->fd) || !MEMDB );
      if( !isOpen(pPager->fd) || pPager->dbSize<pgno || noContent ){
        if( pgno>pPager->mxPgno ){
          rc = SQLITE_FULL;
          goto pager_acquire_err;
        }
        if( noContent ){
          /* Failure to set the bits in the InJournal bit-vectors is benign.
          ** It merely means that we might do some extra work to journal a 
          ** page that does not need to be journaled.  Nevertheless, be sure 
          ** to test the case where a malloc error occurs while trying to set 
          ** a bit in a bit vector.
          */
          sqlite3BeginBenignMalloc();
          if( pgno<=pPager->dbOrigSize ){
            TESTONLY( rc = ) sqlite3BitvecSet(pPager->pInJournal, pgno);
            testcase( rc==SQLITE_NOMEM );
          }
          TESTONLY( rc = ) addToSavepointBitvecs(pPager, pgno);
          testcase( rc==SQLITE_NOMEM );
          sqlite3EndBenignMalloc();
        }
        memset(pPg->pData, 0, pPager->pageSize);
        IOTRACE(("ZERO %p %d\n", pPager, pgno));
      }else{
        if( pagerUseWal(pPager) && bMmapOk==0 ){
          rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iFrame);
          if( rc!=SQLITE_OK ) goto pager_acquire_err;
        }
        assert( pPg->pPager==pPager );
        pPager->aStat[PAGER_STAT_MISS]++;
        rc = readDbPage(pPg, iFrame);
        if( rc!=SQLITE_OK ){
          goto pager_acquire_err;
        }
      }
      pager_set_pagehash(pPg);
    }
  
    return SQLITE_OK;
  
  pager_acquire_err:
    assert( rc!=SQLITE_OK );
    if( pPg ){
      sqlite3PcacheDrop(pPg);
    }
    pagerUnlockIfUnused(pPager);
  
    *ppPage = 0;
    return rc;
  }
  
  /*
  ** Acquire a page if it is already in the in-memory cache.  Do
  ** not read the page from disk.  Return a pointer to the page,
  ** or 0 if the page is not in cache. 
  **
  ** See also sqlite3PagerGet().  The difference between this routine
  ** and sqlite3PagerGet() is that _get() will go to the disk and read
  ** in the page if the page is not already in cache.  This routine
  ** returns NULL if the page is not in cache or if a disk I/O error 
  ** has ever happened.
  */
  DbPage *sqlite3PagerLookup(Pager *pPager, Pgno pgno){
    sqlite3_pcache_page *pPage;
    assert( pPager!=0 );
    assert( pgno!=0 );
    assert( pPager->pPCache!=0 );
    pPage = sqlite3PcacheFetch(pPager->pPCache, pgno, 0);
    assert( pPage==0 || pPager->hasHeldSharedLock );
    if( pPage==0 ) return 0;
    return sqlite3PcacheFetchFinish(pPager->pPCache, pgno, pPage);
  }
  
  /*
  ** Release a page reference.
  **
  ** If the number of references to the page drop to zero, then the
  ** page is added to the LRU list.  When all references to all pages
  ** are released, a rollback occurs and the lock on the database is
  ** removed.
  */
  void sqlite3PagerUnrefNotNull(DbPage *pPg){
    Pager *pPager;
    assert( pPg!=0 );
    pPager = pPg->pPager;
    if( pPg->flags & PGHDR_MMAP ){
      pagerReleaseMapPage(pPg);
    }else{
      sqlite3PcacheRelease(pPg);
    }
    pagerUnlockIfUnused(pPager);
  }
  void sqlite3PagerUnref(DbPage *pPg){
    if( pPg ) sqlite3PagerUnrefNotNull(pPg);
  }
  
  /*
  ** This function is called at the start of every write transaction.
  ** There must already be a RESERVED or EXCLUSIVE lock on the database 
  ** file when this routine is called.
  **
  ** Open the journal file for pager pPager and write a journal header
  ** to the start of it. If there are active savepoints, open the sub-journal
  ** as well. This function is only used when the journal file is being 
  ** opened to write a rollback log for a transaction. It is not used 
  ** when opening a hot journal file to roll it back.
  **
  ** If the journal file is already open (as it may be in exclusive mode),
  ** then this function just writes a journal header to the start of the
  ** already open file. 
  **
  ** Whether or not the journal file is opened by this function, the
  ** Pager.pInJournal bitvec structure is allocated.
  **
  ** Return SQLITE_OK if everything is successful. Otherwise, return 
  ** SQLITE_NOMEM if the attempt to allocate Pager.pInJournal fails, or 
  ** an IO error code if opening or writing the journal file fails.
  */
  static int pager_open_journal(Pager *pPager){
    int rc = SQLITE_OK;                        /* Return code */
    sqlite3_vfs * const pVfs = pPager->pVfs;   /* Local cache of vfs pointer */
  
    assert( pPager->eState==PAGER_WRITER_LOCKED );
    assert( assert_pager_state(pPager) );
    assert( pPager->pInJournal==0 );
    
    /* If already in the error state, this function is a no-op.  But on
    ** the other hand, this routine is never called if we are already in
    ** an error state. */
    if( NEVER(pPager->errCode) ) return pPager->errCode;
  
    if( !pagerUseWal(pPager) && pPager->journalMode!=PAGER_JOURNALMODE_OFF ){
      pPager->pInJournal = sqlite3BitvecCreate(pPager->dbSize);
      if( pPager->pInJournal==0 ){
        return SQLITE_NOMEM_BKPT;
      }
    
      /* Open the journal file if it is not already open. */
      if( !isOpen(pPager->jfd) ){
        if( pPager->journalMode==PAGER_JOURNALMODE_MEMORY ){
          sqlite3MemJournalOpen(pPager->jfd);
        }else{
          int flags = SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE;
          int nSpill;
  
          if( pPager->tempFile ){
            flags |= (SQLITE_OPEN_DELETEONCLOSE|SQLITE_OPEN_TEMP_JOURNAL);
            nSpill = sqlite3Config.nStmtSpill;
          }else{
            flags |= SQLITE_OPEN_MAIN_JOURNAL;
            nSpill = jrnlBufferSize(pPager);
          }
            
          /* Verify that the database still has the same name as it did when
          ** it was originally opened. */
          rc = databaseIsUnmoved(pPager);
          if( rc==SQLITE_OK ){
            rc = sqlite3JournalOpen (
                pVfs, pPager->zJournal, pPager->jfd, flags, nSpill
            );
          }
        }
        assert( rc!=SQLITE_OK || isOpen(pPager->jfd) );
      }
    
    
      /* Write the first journal header to the journal file and open 
      ** the sub-journal if necessary.
      */
      if( rc==SQLITE_OK ){
        /* TODO: Check if all of these are really required. */
        pPager->nRec = 0;
        pPager->journalOff = 0;
        pPager->setMaster = 0;
        pPager->journalHdr = 0;
        rc = writeJournalHdr(pPager);
      }
    }
  
    if( rc!=SQLITE_OK ){
      sqlite3BitvecDestroy(pPager->pInJournal);
      pPager->pInJournal = 0;
    }else{
      assert( pPager->eState==PAGER_WRITER_LOCKED );
      pPager->eState = PAGER_WRITER_CACHEMOD;
    }
  
    return rc;
  }
  
  /*
  ** Begin a write-transaction on the specified pager object. If a 
  ** write-transaction has already been opened, this function is a no-op.
  **
  ** If the exFlag argument is false, then acquire at least a RESERVED
  ** lock on the database file. If exFlag is true, then acquire at least
  ** an EXCLUSIVE lock. If such a lock is already held, no locking 
  ** functions need be called.
  **
  ** If the subjInMemory argument is non-zero, then any sub-journal opened
  ** within this transaction will be opened as an in-memory file. This
  ** has no effect if the sub-journal is already opened (as it may be when
  ** running in exclusive mode) or if the transaction does not require a
  ** sub-journal. If the subjInMemory argument is zero, then any required
  ** sub-journal is implemented in-memory if pPager is an in-memory database, 
  ** or using a temporary file otherwise.
  */
  int sqlite3PagerBegin(Pager *pPager, int exFlag, int subjInMemory){
    int rc = SQLITE_OK;
  
    if( pPager->errCode ) return pPager->errCode;
    assert( pPager->eState>=PAGER_READER && pPager->eState<PAGER_ERROR );
    pPager->subjInMemory = (u8)subjInMemory;
  
    if( ALWAYS(pPager->eState==PAGER_READER) ){
      assert( pPager->pInJournal==0 );
  
      if( pagerUseWal(pPager) ){
        /* If the pager is configured to use locking_mode=exclusive, and an
        ** exclusive lock on the database is not already held, obtain it now.
        */
        if( pPager->exclusiveMode && sqlite3WalExclusiveMode(pPager->pWal, -1) ){
          rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
          if( rc!=SQLITE_OK ){
            return rc;
          }
          (void)sqlite3WalExclusiveMode(pPager->pWal, 1);
        }
  
        /* Grab the write lock on the log file. If successful, upgrade to
        ** PAGER_RESERVED state. Otherwise, return an error code to the caller.
        ** The busy-handler is not invoked if another connection already
        ** holds the write-lock. If possible, the upper layer will call it.
        */
        rc = sqlite3WalBeginWriteTransaction(pPager->pWal);
      }else{
        /* Obtain a RESERVED lock on the database file. If the exFlag parameter
        ** is true, then immediately upgrade this to an EXCLUSIVE lock. The
        ** busy-handler callback can be used when upgrading to the EXCLUSIVE
        ** lock, but not when obtaining the RESERVED lock.
        */
        rc = pagerLockDb(pPager, RESERVED_LOCK);
        if( rc==SQLITE_OK && exFlag ){
          rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
        }
      }
  
      if( rc==SQLITE_OK ){
        /* Change to WRITER_LOCKED state.
        **
        ** WAL mode sets Pager.eState to PAGER_WRITER_LOCKED or CACHEMOD
        ** when it has an open transaction, but never to DBMOD or FINISHED.
        ** This is because in those states the code to roll back savepoint 
        ** transactions may copy data from the sub-journal into the database 
        ** file as well as into the page cache. Which would be incorrect in 
        ** WAL mode.
        */
        pPager->eState = PAGER_WRITER_LOCKED;
        pPager->dbHintSize = pPager->dbSize;
        pPager->dbFileSize = pPager->dbSize;
        pPager->dbOrigSize = pPager->dbSize;
        pPager->journalOff = 0;
      }
  
      assert( rc==SQLITE_OK || pPager->eState==PAGER_READER );
      assert( rc!=SQLITE_OK || pPager->eState==PAGER_WRITER_LOCKED );
      assert( assert_pager_state(pPager) );
    }
  
    PAGERTRACE(("TRANSACTION %d\n", PAGERID(pPager)));
    return rc;
  }
  
  /*
  ** Write page pPg onto the end of the rollback journal.
  */
  static SQLITE_NOINLINE int pagerAddPageToRollbackJournal(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    int rc;
    u32 cksum;
    char *pData2;
    i64 iOff = pPager->journalOff;
  
    /* We should never write to the journal file the page that
    ** contains the database locks.  The following assert verifies
    ** that we do not. */
    assert( pPg->pgno!=PAGER_MJ_PGNO(pPager) );
  
    assert( pPager->journalHdr<=pPager->journalOff );
    CODEC2(pPager, pPg->pData, pPg->pgno, 7, return SQLITE_NOMEM_BKPT, pData2);
    cksum = pager_cksum(pPager, (u8*)pData2);
  
    /* Even if an IO or diskfull error occurs while journalling the
    ** page in the block above, set the need-sync flag for the page.
    ** Otherwise, when the transaction is rolled back, the logic in
    ** playback_one_page() will think that the page needs to be restored
    ** in the database file. And if an IO error occurs while doing so,
    ** then corruption may follow.
    */
    pPg->flags |= PGHDR_NEED_SYNC;
  
    rc = write32bits(pPager->jfd, iOff, pPg->pgno);
    if( rc!=SQLITE_OK ) return rc;
    rc = sqlite3OsWrite(pPager->jfd, pData2, pPager->pageSize, iOff+4);
    if( rc!=SQLITE_OK ) return rc;
    rc = write32bits(pPager->jfd, iOff+pPager->pageSize+4, cksum);
    if( rc!=SQLITE_OK ) return rc;
  
    IOTRACE(("JOUT %p %d %lld %d\n", pPager, pPg->pgno, 
             pPager->journalOff, pPager->pageSize));
    PAGER_INCR(sqlite3_pager_writej_count);
    PAGERTRACE(("JOURNAL %d page %d needSync=%d hash(%08x)\n",
         PAGERID(pPager), pPg->pgno, 
         ((pPg->flags&PGHDR_NEED_SYNC)?1:0), pager_pagehash(pPg)));
  
    pPager->journalOff += 8 + pPager->pageSize;
    pPager->nRec++;
    assert( pPager->pInJournal!=0 );
    rc = sqlite3BitvecSet(pPager->pInJournal, pPg->pgno);
    testcase( rc==SQLITE_NOMEM );
    assert( rc==SQLITE_OK || rc==SQLITE_NOMEM );
    rc |= addToSavepointBitvecs(pPager, pPg->pgno);
    assert( rc==SQLITE_OK || rc==SQLITE_NOMEM );
    return rc;
  }
  
  /*
  ** Mark a single data page as writeable. The page is written into the 
  ** main journal or sub-journal as required. If the page is written into
  ** one of the journals, the corresponding bit is set in the 
  ** Pager.pInJournal bitvec and the PagerSavepoint.pInSavepoint bitvecs
  ** of any open savepoints as appropriate.
  */
  static int pager_write(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    int rc = SQLITE_OK;
  
    /* This routine is not called unless a write-transaction has already 
    ** been started. The journal file may or may not be open at this point.
    ** It is never called in the ERROR state.
    */
    assert( pPager->eState==PAGER_WRITER_LOCKED
         || pPager->eState==PAGER_WRITER_CACHEMOD
         || pPager->eState==PAGER_WRITER_DBMOD
    );
    assert( assert_pager_state(pPager) );
    assert( pPager->errCode==0 );
    assert( pPager->readOnly==0 );
    CHECK_PAGE(pPg);
  
    /* The journal file needs to be opened. Higher level routines have already
    ** obtained the necessary locks to begin the write-transaction, but the
    ** rollback journal might not yet be open. Open it now if this is the case.
    **
    ** This is done before calling sqlite3PcacheMakeDirty() on the page. 
    ** Otherwise, if it were done after calling sqlite3PcacheMakeDirty(), then
    ** an error might occur and the pager would end up in WRITER_LOCKED state
    ** with pages marked as dirty in the cache.
    */
    if( pPager->eState==PAGER_WRITER_LOCKED ){
      rc = pager_open_journal(pPager);
      if( rc!=SQLITE_OK ) return rc;
    }
    assert( pPager->eState>=PAGER_WRITER_CACHEMOD );
    assert( assert_pager_state(pPager) );
  
    /* Mark the page that is about to be modified as dirty. */
    sqlite3PcacheMakeDirty(pPg);
  
    /* If a rollback journal is in use, them make sure the page that is about
    ** to change is in the rollback journal, or if the page is a new page off
    ** then end of the file, make sure it is marked as PGHDR_NEED_SYNC.
    */
    assert( (pPager->pInJournal!=0) == isOpen(pPager->jfd) );
    if( pPager->pInJournal!=0
     && sqlite3BitvecTestNotNull(pPager->pInJournal, pPg->pgno)==0
    ){
      assert( pagerUseWal(pPager)==0 );
      if( pPg->pgno<=pPager->dbOrigSize ){
        rc = pagerAddPageToRollbackJournal(pPg);
        if( rc!=SQLITE_OK ){
          return rc;
        }
      }else{
        if( pPager->eState!=PAGER_WRITER_DBMOD ){
          pPg->flags |= PGHDR_NEED_SYNC;
        }
        PAGERTRACE(("APPEND %d page %d needSync=%d\n",
                PAGERID(pPager), pPg->pgno,
               ((pPg->flags&PGHDR_NEED_SYNC)?1:0)));
      }
    }
  
    /* The PGHDR_DIRTY bit is set above when the page was added to the dirty-list
    ** and before writing the page into the rollback journal.  Wait until now,
    ** after the page has been successfully journalled, before setting the
    ** PGHDR_WRITEABLE bit that indicates that the page can be safely modified.
    */
    pPg->flags |= PGHDR_WRITEABLE;
    
    /* If the statement journal is open and the page is not in it,
    ** then write the page into the statement journal.
    */
    if( pPager->nSavepoint>0 ){
      rc = subjournalPageIfRequired(pPg);
    }
  
    /* Update the database size and return. */
    if( pPager->dbSize<pPg->pgno ){
      pPager->dbSize = pPg->pgno;
    }
    return rc;
  }
  
  /*
  ** This is a variant of sqlite3PagerWrite() that runs when the sector size
  ** is larger than the page size.  SQLite makes the (reasonable) assumption that
  ** all bytes of a sector are written together by hardware.  Hence, all bytes of
  ** a sector need to be journalled in case of a power loss in the middle of
  ** a write.
  **
  ** Usually, the sector size is less than or equal to the page size, in which
  ** case pages can be individually written.  This routine only runs in the
  ** exceptional case where the page size is smaller than the sector size.
  */
  static SQLITE_NOINLINE int pagerWriteLargeSector(PgHdr *pPg){
    int rc = SQLITE_OK;          /* Return code */
    Pgno nPageCount;             /* Total number of pages in database file */
    Pgno pg1;                    /* First page of the sector pPg is located on. */
    int nPage = 0;               /* Number of pages starting at pg1 to journal */
    int ii;                      /* Loop counter */
    int needSync = 0;            /* True if any page has PGHDR_NEED_SYNC */
    Pager *pPager = pPg->pPager; /* The pager that owns pPg */
    Pgno nPagePerSector = (pPager->sectorSize/pPager->pageSize);
  
    /* Set the doNotSpill NOSYNC bit to 1. This is because we cannot allow
    ** a journal header to be written between the pages journaled by
    ** this function.
    */
    assert( !MEMDB );
    assert( (pPager->doNotSpill & SPILLFLAG_NOSYNC)==0 );
    pPager->doNotSpill |= SPILLFLAG_NOSYNC;
  
    /* This trick assumes that both the page-size and sector-size are
    ** an integer power of 2. It sets variable pg1 to the identifier
    ** of the first page of the sector pPg is located on.
    */
    pg1 = ((pPg->pgno-1) & ~(nPagePerSector-1)) + 1;
  
    nPageCount = pPager->dbSize;
    if( pPg->pgno>nPageCount ){
      nPage = (pPg->pgno - pg1)+1;
    }else if( (pg1+nPagePerSector-1)>nPageCount ){
      nPage = nPageCount+1-pg1;
    }else{
      nPage = nPagePerSector;
    }
    assert(nPage>0);
    assert(pg1<=pPg->pgno);
    assert((pg1+nPage)>pPg->pgno);
  
    for(ii=0; ii<nPage && rc==SQLITE_OK; ii++){
      Pgno pg = pg1+ii;
      PgHdr *pPage;
      if( pg==pPg->pgno || !sqlite3BitvecTest(pPager->pInJournal, pg) ){
        if( pg!=PAGER_MJ_PGNO(pPager) ){
          rc = sqlite3PagerGet(pPager, pg, &pPage, 0);
          if( rc==SQLITE_OK ){
            rc = pager_write(pPage);
            if( pPage->flags&PGHDR_NEED_SYNC ){
              needSync = 1;
            }
            sqlite3PagerUnrefNotNull(pPage);
          }
        }
      }else if( (pPage = sqlite3PagerLookup(pPager, pg))!=0 ){
        if( pPage->flags&PGHDR_NEED_SYNC ){
          needSync = 1;
        }
        sqlite3PagerUnrefNotNull(pPage);
      }
    }
  
    /* If the PGHDR_NEED_SYNC flag is set for any of the nPage pages 
    ** starting at pg1, then it needs to be set for all of them. Because
    ** writing to any of these nPage pages may damage the others, the
    ** journal file must contain sync()ed copies of all of them
    ** before any of them can be written out to the database file.
    */
    if( rc==SQLITE_OK && needSync ){
      assert( !MEMDB );
      for(ii=0; ii<nPage; ii++){
        PgHdr *pPage = sqlite3PagerLookup(pPager, pg1+ii);
        if( pPage ){
          pPage->flags |= PGHDR_NEED_SYNC;
          sqlite3PagerUnrefNotNull(pPage);
        }
      }
    }
  
    assert( (pPager->doNotSpill & SPILLFLAG_NOSYNC)!=0 );
    pPager->doNotSpill &= ~SPILLFLAG_NOSYNC;
    return rc;
  }
  
  /*
  ** Mark a data page as writeable. This routine must be called before 
  ** making changes to a page. The caller must check the return value 
  ** of this function and be careful not to change any page data unless 
  ** this routine returns SQLITE_OK.
  **
  ** The difference between this function and pager_write() is that this
  ** function also deals with the special case where 2 or more pages
  ** fit on a single disk sector. In this case all co-resident pages
  ** must have been written to the journal file before returning.
  **
  ** If an error occurs, SQLITE_NOMEM or an IO error code is returned
  ** as appropriate. Otherwise, SQLITE_OK.
  */
  int sqlite3PagerWrite(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    assert( (pPg->flags & PGHDR_MMAP)==0 );
    assert( pPager->eState>=PAGER_WRITER_LOCKED );
    assert( assert_pager_state(pPager) );
    if( pPager->errCode ){
      return pPager->errCode;
    }else if( (pPg->flags & PGHDR_WRITEABLE)!=0 && pPager->dbSize>=pPg->pgno ){
      if( pPager->nSavepoint ) return subjournalPageIfRequired(pPg);
      return SQLITE_OK;
    }else if( pPager->sectorSize > (u32)pPager->pageSize ){
      assert( pPager->tempFile==0 );
      return pagerWriteLargeSector(pPg);
    }else{
      return pager_write(pPg);
    }
  }
  
  /*
  ** Return TRUE if the page given in the argument was previously passed
  ** to sqlite3PagerWrite().  In other words, return TRUE if it is ok
  ** to change the content of the page.
  */
  #ifndef NDEBUG
  int sqlite3PagerIswriteable(DbPage *pPg){
    return pPg->flags & PGHDR_WRITEABLE;
  }
  #endif
  
  /*
  ** A call to this routine tells the pager that it is not necessary to
  ** write the information on page pPg back to the disk, even though
  ** that page might be marked as dirty.  This happens, for example, when
  ** the page has been added as a leaf of the freelist and so its
  ** content no longer matters.
  **
  ** The overlying software layer calls this routine when all of the data
  ** on the given page is unused. The pager marks the page as clean so
  ** that it does not get written to disk.
  **
  ** Tests show that this optimization can quadruple the speed of large 
  ** DELETE operations.
  **
  ** This optimization cannot be used with a temp-file, as the page may
  ** have been dirty at the start of the transaction. In that case, if
  ** memory pressure forces page pPg out of the cache, the data does need 
  ** to be written out to disk so that it may be read back in if the 
  ** current transaction is rolled back.
  */
  void sqlite3PagerDontWrite(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    if( !pPager->tempFile && (pPg->flags&PGHDR_DIRTY) && pPager->nSavepoint==0 ){
      PAGERTRACE(("DONT_WRITE page %d of %d\n", pPg->pgno, PAGERID(pPager)));
      IOTRACE(("CLEAN %p %d\n", pPager, pPg->pgno))
      pPg->flags |= PGHDR_DONT_WRITE;
      pPg->flags &= ~PGHDR_WRITEABLE;
      testcase( pPg->flags & PGHDR_NEED_SYNC );
      pager_set_pagehash(pPg);
    }
  }
  
  /*
  ** This routine is called to increment the value of the database file 
  ** change-counter, stored as a 4-byte big-endian integer starting at 
  ** byte offset 24 of the pager file.  The secondary change counter at
  ** 92 is also updated, as is the SQLite version number at offset 96.
  **
  ** But this only happens if the pPager->changeCountDone flag is false.
  ** To avoid excess churning of page 1, the update only happens once.
  ** See also the pager_write_changecounter() routine that does an 
  ** unconditional update of the change counters.
  **
  ** If the isDirectMode flag is zero, then this is done by calling 
  ** sqlite3PagerWrite() on page 1, then modifying the contents of the
  ** page data. In this case the file will be updated when the current
  ** transaction is committed.
  **
  ** The isDirectMode flag may only be non-zero if the library was compiled
  ** with the SQLITE_ENABLE_ATOMIC_WRITE macro defined. In this case,
  ** if isDirect is non-zero, then the database file is updated directly
  ** by writing an updated version of page 1 using a call to the 
  ** sqlite3OsWrite() function.
  */
  static int pager_incr_changecounter(Pager *pPager, int isDirectMode){
    int rc = SQLITE_OK;
  
    assert( pPager->eState==PAGER_WRITER_CACHEMOD
         || pPager->eState==PAGER_WRITER_DBMOD
    );
    assert( assert_pager_state(pPager) );
  
    /* Declare and initialize constant integer 'isDirect'. If the
    ** atomic-write optimization is enabled in this build, then isDirect
    ** is initialized to the value passed as the isDirectMode parameter
    ** to this function. Otherwise, it is always set to zero.
    **
    ** The idea is that if the atomic-write optimization is not
    ** enabled at compile time, the compiler can omit the tests of
    ** 'isDirect' below, as well as the block enclosed in the
    ** "if( isDirect )" condition.
    */
  #ifndef SQLITE_ENABLE_ATOMIC_WRITE
  # define DIRECT_MODE 0
    assert( isDirectMode==0 );
    UNUSED_PARAMETER(isDirectMode);
  #else
  # define DIRECT_MODE isDirectMode
  #endif
  
    if( !pPager->changeCountDone && ALWAYS(pPager->dbSize>0) ){
      PgHdr *pPgHdr;                /* Reference to page 1 */
  
      assert( !pPager->tempFile && isOpen(pPager->fd) );
  
      /* Open page 1 of the file for writing. */
      rc = sqlite3PagerGet(pPager, 1, &pPgHdr, 0);
      assert( pPgHdr==0 || rc==SQLITE_OK );
  
      /* If page one was fetched successfully, and this function is not
      ** operating in direct-mode, make page 1 writable.  When not in 
      ** direct mode, page 1 is always held in cache and hence the PagerGet()
      ** above is always successful - hence the ALWAYS on rc==SQLITE_OK.
      */
      if( !DIRECT_MODE && ALWAYS(rc==SQLITE_OK) ){
        rc = sqlite3PagerWrite(pPgHdr);
      }
  
      if( rc==SQLITE_OK ){
        /* Actually do the update of the change counter */
        pager_write_changecounter(pPgHdr);
  
        /* If running in direct mode, write the contents of page 1 to the file. */
        if( DIRECT_MODE ){
          const void *zBuf;
          assert( pPager->dbFileSize>0 );
          CODEC2(pPager, pPgHdr->pData, 1, 6, rc=SQLITE_NOMEM_BKPT, zBuf);
          if( rc==SQLITE_OK ){
            rc = sqlite3OsWrite(pPager->fd, zBuf, pPager->pageSize, 0);
            pPager->aStat[PAGER_STAT_WRITE]++;
          }
          if( rc==SQLITE_OK ){
            /* Update the pager's copy of the change-counter. Otherwise, the
            ** next time a read transaction is opened the cache will be
            ** flushed (as the change-counter values will not match).  */
            const void *pCopy = (const void *)&((const char *)zBuf)[24];
            memcpy(&pPager->dbFileVers, pCopy, sizeof(pPager->dbFileVers));
            pPager->changeCountDone = 1;
          }
        }else{
          pPager->changeCountDone = 1;
        }
      }
  
      /* Release the page reference. */
      sqlite3PagerUnref(pPgHdr);
    }
    return rc;
  }
  
  /*
  ** Sync the database file to disk. This is a no-op for in-memory databases
  ** or pages with the Pager.noSync flag set.
  **
  ** If successful, or if called on a pager for which it is a no-op, this
  ** function returns SQLITE_OK. Otherwise, an IO error code is returned.
  */
  int sqlite3PagerSync(Pager *pPager, const char *zMaster){
    int rc = SQLITE_OK;
  
    if( isOpen(pPager->fd) ){
      void *pArg = (void*)zMaster;
      rc = sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_SYNC, pArg);
      if( rc==SQLITE_NOTFOUND ) rc = SQLITE_OK;
    }
    if( rc==SQLITE_OK && !pPager->noSync ){
      assert( !MEMDB );
      rc = sqlite3OsSync(pPager->fd, pPager->syncFlags);
    }
    return rc;
  }
  
  /*
  ** This function may only be called while a write-transaction is active in
  ** rollback. If the connection is in WAL mode, this call is a no-op. 
  ** Otherwise, if the connection does not already have an EXCLUSIVE lock on 
  ** the database file, an attempt is made to obtain one.
  **
  ** If the EXCLUSIVE lock is already held or the attempt to obtain it is
  ** successful, or the connection is in WAL mode, SQLITE_OK is returned.
  ** Otherwise, either SQLITE_BUSY or an SQLITE_IOERR_XXX error code is 
  ** returned.
  */
  int sqlite3PagerExclusiveLock(Pager *pPager){
    int rc = pPager->errCode;
    assert( assert_pager_state(pPager) );
    if( rc==SQLITE_OK ){
      assert( pPager->eState==PAGER_WRITER_CACHEMOD 
           || pPager->eState==PAGER_WRITER_DBMOD 
           || pPager->eState==PAGER_WRITER_LOCKED 
      );
      assert( assert_pager_state(pPager) );
      if( 0==pagerUseWal(pPager) ){
        rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
      }
    }
    return rc;
  }
  
  /*
  ** Sync the database file for the pager pPager. zMaster points to the name
  ** of a master journal file that should be written into the individual
  ** journal file. zMaster may be NULL, which is interpreted as no master
  ** journal (a single database transaction).
  **
  ** This routine ensures that:
  **
  **   * The database file change-counter is updated,
  **   * the journal is synced (unless the atomic-write optimization is used),
  **   * all dirty pages are written to the database file, 
  **   * the database file is truncated (if required), and
  **   * the database file synced. 
  **
  ** The only thing that remains to commit the transaction is to finalize 
  ** (delete, truncate or zero the first part of) the journal file (or 
  ** delete the master journal file if specified).
  **
  ** Note that if zMaster==NULL, this does not overwrite a previous value
  ** passed to an sqlite3PagerCommitPhaseOne() call.
  **
  ** If the final parameter - noSync - is true, then the database file itself
  ** is not synced. The caller must call sqlite3PagerSync() directly to
  ** sync the database file before calling CommitPhaseTwo() to delete the
  ** journal file in this case.
  */
  int sqlite3PagerCommitPhaseOne(
    Pager *pPager,                  /* Pager object */
    const char *zMaster,            /* If not NULL, the master journal name */
    int noSync                      /* True to omit the xSync on the db file */
  ){
    int rc = SQLITE_OK;             /* Return code */
  
    assert( pPager->eState==PAGER_WRITER_LOCKED
         || pPager->eState==PAGER_WRITER_CACHEMOD
         || pPager->eState==PAGER_WRITER_DBMOD
         || pPager->eState==PAGER_ERROR
    );
    assert( assert_pager_state(pPager) );
  
    /* If a prior error occurred, report that error again. */
    if( NEVER(pPager->errCode) ) return pPager->errCode;
  
    /* Provide the ability to easily simulate an I/O error during testing */
    if( sqlite3FaultSim(400) ) return SQLITE_IOERR;
  
    PAGERTRACE(("DATABASE SYNC: File=%s zMaster=%s nSize=%d\n", 
        pPager->zFilename, zMaster, pPager->dbSize));
  
    /* If no database changes have been made, return early. */
    if( pPager->eState<PAGER_WRITER_CACHEMOD ) return SQLITE_OK;
  
    assert( MEMDB==0 || pPager->tempFile );
    assert( isOpen(pPager->fd) || pPager->tempFile );
    if( 0==pagerFlushOnCommit(pPager, 1) ){
      /* If this is an in-memory db, or no pages have been written to, or this
      ** function has already been called, it is mostly a no-op.  However, any
      ** backup in progress needs to be restarted.  */
      sqlite3BackupRestart(pPager->pBackup);
    }else{
      if( pagerUseWal(pPager) ){
        PgHdr *pList = sqlite3PcacheDirtyList(pPager->pPCache);
        PgHdr *pPageOne = 0;
        if( pList==0 ){
          /* Must have at least one page for the WAL commit flag.
          ** Ticket [2d1a5c67dfc2363e44f29d9bbd57f] 2011-05-18 */
          rc = sqlite3PagerGet(pPager, 1, &pPageOne, 0);
          pList = pPageOne;
          pList->pDirty = 0;
        }
        assert( rc==SQLITE_OK );
        if( ALWAYS(pList) ){
          rc = pagerWalFrames(pPager, pList, pPager->dbSize, 1);
        }
        sqlite3PagerUnref(pPageOne);
        if( rc==SQLITE_OK ){
          sqlite3PcacheCleanAll(pPager->pPCache);
        }
      }else{
        /* The following block updates the change-counter. Exactly how it
        ** does this depends on whether or not the atomic-update optimization
        ** was enabled at compile time, and if this transaction meets the 
        ** runtime criteria to use the operation: 
        **
        **    * The file-system supports the atomic-write property for
        **      blocks of size page-size, and 
        **    * This commit is not part of a multi-file transaction, and
        **    * Exactly one page has been modified and store in the journal file.
        **
        ** If the optimization was not enabled at compile time, then the
        ** pager_incr_changecounter() function is called to update the change
        ** counter in 'indirect-mode'. If the optimization is compiled in but
        ** is not applicable to this transaction, call sqlite3JournalCreate()
        ** to make sure the journal file has actually been created, then call
        ** pager_incr_changecounter() to update the change-counter in indirect
        ** mode. 
        **
        ** Otherwise, if the optimization is both enabled and applicable,
        ** then call pager_incr_changecounter() to update the change-counter
        ** in 'direct' mode. In this case the journal file will never be
        ** created for this transaction.
        */
    #ifdef SQLITE_ENABLE_ATOMIC_WRITE
        PgHdr *pPg;
        assert( isOpen(pPager->jfd) 
             || pPager->journalMode==PAGER_JOURNALMODE_OFF 
             || pPager->journalMode==PAGER_JOURNALMODE_WAL 
        );
        if( !zMaster && isOpen(pPager->jfd) 
         && pPager->journalOff==jrnlBufferSize(pPager) 
         && pPager->dbSize>=pPager->dbOrigSize
         && (0==(pPg = sqlite3PcacheDirtyList(pPager->pPCache)) || 0==pPg->pDirty)
        ){
          /* Update the db file change counter via the direct-write method. The 
          ** following call will modify the in-memory representation of page 1 
          ** to include the updated change counter and then write page 1 
          ** directly to the database file. Because of the atomic-write 
          ** property of the host file-system, this is safe.
          */
          rc = pager_incr_changecounter(pPager, 1);
        }else{
          rc = sqlite3JournalCreate(pPager->jfd);
          if( rc==SQLITE_OK ){
            rc = pager_incr_changecounter(pPager, 0);
          }
        }
    #else
        rc = pager_incr_changecounter(pPager, 0);
    #endif
        if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
    
        /* Write the master journal name into the journal file. If a master 
        ** journal file name has already been written to the journal file, 
        ** or if zMaster is NULL (no master journal), then this call is a no-op.
        */
        rc = writeMasterJournal(pPager, zMaster);
        if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
    
        /* Sync the journal file and write all dirty pages to the database.
        ** If the atomic-update optimization is being used, this sync will not 
        ** create the journal file or perform any real IO.
        **
        ** Because the change-counter page was just modified, unless the
        ** atomic-update optimization is used it is almost certain that the
        ** journal requires a sync here. However, in locking_mode=exclusive
        ** on a system under memory pressure it is just possible that this is 
        ** not the case. In this case it is likely enough that the redundant
        ** xSync() call will be changed to a no-op by the OS anyhow. 
        */
        rc = syncJournal(pPager, 0);
        if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
    
        rc = pager_write_pagelist(pPager,sqlite3PcacheDirtyList(pPager->pPCache));
        if( rc!=SQLITE_OK ){
          assert( rc!=SQLITE_IOERR_BLOCKED );
          goto commit_phase_one_exit;
        }
        sqlite3PcacheCleanAll(pPager->pPCache);
  
        /* If the file on disk is smaller than the database image, use 
        ** pager_truncate to grow the file here. This can happen if the database
        ** image was extended as part of the current transaction and then the
        ** last page in the db image moved to the free-list. In this case the
        ** last page is never written out to disk, leaving the database file
        ** undersized. Fix this now if it is the case.  */
        if( pPager->dbSize>pPager->dbFileSize ){
          Pgno nNew = pPager->dbSize - (pPager->dbSize==PAGER_MJ_PGNO(pPager));
          assert( pPager->eState==PAGER_WRITER_DBMOD );
          rc = pager_truncate(pPager, nNew);
          if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
        }
    
        /* Finally, sync the database file. */
        if( !noSync ){
          rc = sqlite3PagerSync(pPager, zMaster);
        }
        IOTRACE(("DBSYNC %p\n", pPager))
      }
    }
  
  commit_phase_one_exit:
    if( rc==SQLITE_OK && !pagerUseWal(pPager) ){
      pPager->eState = PAGER_WRITER_FINISHED;
    }
    return rc;
  }
  
  
  /*
  ** When this function is called, the database file has been completely
  ** updated to reflect the changes made by the current transaction and
  ** synced to disk. The journal file still exists in the file-system 
  ** though, and if a failure occurs at this point it will eventually
  ** be used as a hot-journal and the current transaction rolled back.
  **
  ** This function finalizes the journal file, either by deleting, 
  ** truncating or partially zeroing it, so that it cannot be used 
  ** for hot-journal rollback. Once this is done the transaction is
  ** irrevocably committed.
  **
  ** If an error occurs, an IO error code is returned and the pager
  ** moves into the error state. Otherwise, SQLITE_OK is returned.
  */
  int sqlite3PagerCommitPhaseTwo(Pager *pPager){
    int rc = SQLITE_OK;                  /* Return code */
  
    /* This routine should not be called if a prior error has occurred.
    ** But if (due to a coding error elsewhere in the system) it does get
    ** called, just return the same error code without doing anything. */
    if( NEVER(pPager->errCode) ) return pPager->errCode;
  
    assert( pPager->eState==PAGER_WRITER_LOCKED
         || pPager->eState==PAGER_WRITER_FINISHED
         || (pagerUseWal(pPager) && pPager->eState==PAGER_WRITER_CACHEMOD)
    );
    assert( assert_pager_state(pPager) );
  
    /* An optimization. If the database was not actually modified during
    ** this transaction, the pager is running in exclusive-mode and is
    ** using persistent journals, then this function is a no-op.
    **
    ** The start of the journal file currently contains a single journal 
    ** header with the nRec field set to 0. If such a journal is used as
    ** a hot-journal during hot-journal rollback, 0 changes will be made
    ** to the database file. So there is no need to zero the journal 
    ** header. Since the pager is in exclusive mode, there is no need
    ** to drop any locks either.
    */
    if( pPager->eState==PAGER_WRITER_LOCKED 
     && pPager->exclusiveMode 
     && pPager->journalMode==PAGER_JOURNALMODE_PERSIST
    ){
      assert( pPager->journalOff==JOURNAL_HDR_SZ(pPager) || !pPager->journalOff );
      pPager->eState = PAGER_READER;
      return SQLITE_OK;
    }
  
    PAGERTRACE(("COMMIT %d\n", PAGERID(pPager)));
    pPager->iDataVersion++;
    rc = pager_end_transaction(pPager, pPager->setMaster, 1);
    return pager_error(pPager, rc);
  }
  
  /*
  ** If a write transaction is open, then all changes made within the 
  ** transaction are reverted and the current write-transaction is closed.
  ** The pager falls back to PAGER_READER state if successful, or PAGER_ERROR
  ** state if an error occurs.
  **
  ** If the pager is already in PAGER_ERROR state when this function is called,
  ** it returns Pager.errCode immediately. No work is performed in this case.
  **
  ** Otherwise, in rollback mode, this function performs two functions:
  **
  **   1) It rolls back the journal file, restoring all database file and 
  **      in-memory cache pages to the state they were in when the transaction
  **      was opened, and
  **
  **   2) It finalizes the journal file, so that it is not used for hot
  **      rollback at any point in the future.
  **
  ** Finalization of the journal file (task 2) is only performed if the 
  ** rollback is successful.
  **
  ** In WAL mode, all cache-entries containing data modified within the
  ** current transaction are either expelled from the cache or reverted to
  ** their pre-transaction state by re-reading data from the database or
  ** WAL files. The WAL transaction is then closed.
  */
  int sqlite3PagerRollback(Pager *pPager){
    int rc = SQLITE_OK;                  /* Return code */
    PAGERTRACE(("ROLLBACK %d\n", PAGERID(pPager)));
  
    /* PagerRollback() is a no-op if called in READER or OPEN state. If
    ** the pager is already in the ERROR state, the rollback is not 
    ** attempted here. Instead, the error code is returned to the caller.
    */
    assert( assert_pager_state(pPager) );
    if( pPager->eState==PAGER_ERROR ) return pPager->errCode;
    if( pPager->eState<=PAGER_READER ) return SQLITE_OK;
  
    if( pagerUseWal(pPager) ){
      int rc2;
      rc = sqlite3PagerSavepoint(pPager, SAVEPOINT_ROLLBACK, -1);
      rc2 = pager_end_transaction(pPager, pPager->setMaster, 0);
      if( rc==SQLITE_OK ) rc = rc2;
    }else if( !isOpen(pPager->jfd) || pPager->eState==PAGER_WRITER_LOCKED ){
      int eState = pPager->eState;
      rc = pager_end_transaction(pPager, 0, 0);
      if( !MEMDB && eState>PAGER_WRITER_LOCKED ){
        /* This can happen using journal_mode=off. Move the pager to the error 
        ** state to indicate that the contents of the cache may not be trusted.
        ** Any active readers will get SQLITE_ABORT.
        */
        pPager->errCode = SQLITE_ABORT;
        pPager->eState = PAGER_ERROR;
        return rc;
      }
    }else{
      rc = pager_playback(pPager, 0);
    }
  
    assert( pPager->eState==PAGER_READER || rc!=SQLITE_OK );
    assert( rc==SQLITE_OK || rc==SQLITE_FULL || rc==SQLITE_CORRUPT
            || rc==SQLITE_NOMEM || (rc&0xFF)==SQLITE_IOERR 
            || rc==SQLITE_CANTOPEN
    );
  
    /* If an error occurs during a ROLLBACK, we can no longer trust the pager
    ** cache. So call pager_error() on the way out to make any error persistent.
    */
    return pager_error(pPager, rc);
  }
  
  /*
  ** Return TRUE if the database file is opened read-only.  Return FALSE
  ** if the database is (in theory) writable.
  */
  u8 sqlite3PagerIsreadonly(Pager *pPager){
    return pPager->readOnly;
  }
  
  #ifdef SQLITE_DEBUG
  /*
  ** Return the sum of the reference counts for all pages held by pPager.
  */
  int sqlite3PagerRefcount(Pager *pPager){
    return sqlite3PcacheRefCount(pPager->pPCache);
  }
  #endif
  
  /*
  ** Return the approximate number of bytes of memory currently
  ** used by the pager and its associated cache.
  */
  int sqlite3PagerMemUsed(Pager *pPager){
    int perPageSize = pPager->pageSize + pPager->nExtra + sizeof(PgHdr)
                                       + 5*sizeof(void*);
    return perPageSize*sqlite3PcachePagecount(pPager->pPCache)
             + sqlite3MallocSize(pPager)
             + pPager->pageSize;
  }
  
  /*
  ** Return the number of references to the specified page.
  */
  int sqlite3PagerPageRefcount(DbPage *pPage){
    return sqlite3PcachePageRefcount(pPage);
  }
  
  #ifdef SQLITE_TEST
  /*
  ** This routine is used for testing and analysis only.
  */
  int *sqlite3PagerStats(Pager *pPager){
    static int a[11];
    a[0] = sqlite3PcacheRefCount(pPager->pPCache);
    a[1] = sqlite3PcachePagecount(pPager->pPCache);
    a[2] = sqlite3PcacheGetCachesize(pPager->pPCache);
    a[3] = pPager->eState==PAGER_OPEN ? -1 : (int) pPager->dbSize;
    a[4] = pPager->eState;
    a[5] = pPager->errCode;
    a[6] = pPager->aStat[PAGER_STAT_HIT];
    a[7] = pPager->aStat[PAGER_STAT_MISS];
    a[8] = 0;  /* Used to be pPager->nOvfl */
    a[9] = pPager->nRead;
    a[10] = pPager->aStat[PAGER_STAT_WRITE];
    return a;
  }
  #endif
  
  /*
  ** Parameter eStat must be either SQLITE_DBSTATUS_CACHE_HIT or
  ** SQLITE_DBSTATUS_CACHE_MISS. Before returning, *pnVal is incremented by the
  ** current cache hit or miss count, according to the value of eStat. If the 
  ** reset parameter is non-zero, the cache hit or miss count is zeroed before 
  ** returning.
  */
  void sqlite3PagerCacheStat(Pager *pPager, int eStat, int reset, int *pnVal){
  
    assert( eStat==SQLITE_DBSTATUS_CACHE_HIT
         || eStat==SQLITE_DBSTATUS_CACHE_MISS
         || eStat==SQLITE_DBSTATUS_CACHE_WRITE
    );
  
    assert( SQLITE_DBSTATUS_CACHE_HIT+1==SQLITE_DBSTATUS_CACHE_MISS );
    assert( SQLITE_DBSTATUS_CACHE_HIT+2==SQLITE_DBSTATUS_CACHE_WRITE );
    assert( PAGER_STAT_HIT==0 && PAGER_STAT_MISS==1 && PAGER_STAT_WRITE==2 );
  
    *pnVal += pPager->aStat[eStat - SQLITE_DBSTATUS_CACHE_HIT];
    if( reset ){
      pPager->aStat[eStat - SQLITE_DBSTATUS_CACHE_HIT] = 0;
    }
  }
  
  /*
  ** Return true if this is an in-memory or temp-file backed pager.
  */
  int sqlite3PagerIsMemdb(Pager *pPager){
    return pPager->tempFile;
  }
  
  /*
  ** Check that there are at least nSavepoint savepoints open. If there are
  ** currently less than nSavepoints open, then open one or more savepoints
  ** to make up the difference. If the number of savepoints is already
  ** equal to nSavepoint, then this function is a no-op.
  **
  ** If a memory allocation fails, SQLITE_NOMEM is returned. If an error 
  ** occurs while opening the sub-journal file, then an IO error code is
  ** returned. Otherwise, SQLITE_OK.
  */
  static SQLITE_NOINLINE int pagerOpenSavepoint(Pager *pPager, int nSavepoint){
    int rc = SQLITE_OK;                       /* Return code */
    int nCurrent = pPager->nSavepoint;        /* Current number of savepoints */
    int ii;                                   /* Iterator variable */
    PagerSavepoint *aNew;                     /* New Pager.aSavepoint array */
  
    assert( pPager->eState>=PAGER_WRITER_LOCKED );
    assert( assert_pager_state(pPager) );
    assert( nSavepoint>nCurrent && pPager->useJournal );
  
    /* Grow the Pager.aSavepoint array using realloc(). Return SQLITE_NOMEM
    ** if the allocation fails. Otherwise, zero the new portion in case a 
    ** malloc failure occurs while populating it in the for(...) loop below.
    */
    aNew = (PagerSavepoint *)sqlite3Realloc(
        pPager->aSavepoint, sizeof(PagerSavepoint)*nSavepoint
    );
    if( !aNew ){
      return SQLITE_NOMEM_BKPT;
    }
    memset(&aNew[nCurrent], 0, (nSavepoint-nCurrent) * sizeof(PagerSavepoint));
    pPager->aSavepoint = aNew;
  
    /* Populate the PagerSavepoint structures just allocated. */
    for(ii=nCurrent; ii<nSavepoint; ii++){
      aNew[ii].nOrig = pPager->dbSize;
      if( isOpen(pPager->jfd) && pPager->journalOff>0 ){
        aNew[ii].iOffset = pPager->journalOff;
      }else{
        aNew[ii].iOffset = JOURNAL_HDR_SZ(pPager);
      }
      aNew[ii].iSubRec = pPager->nSubRec;
      aNew[ii].pInSavepoint = sqlite3BitvecCreate(pPager->dbSize);
      if( !aNew[ii].pInSavepoint ){
        return SQLITE_NOMEM_BKPT;
      }
      if( pagerUseWal(pPager) ){
        sqlite3WalSavepoint(pPager->pWal, aNew[ii].aWalData);
      }
      pPager->nSavepoint = ii+1;
    }
    assert( pPager->nSavepoint==nSavepoint );
    assertTruncateConstraint(pPager);
    return rc;
  }
  int sqlite3PagerOpenSavepoint(Pager *pPager, int nSavepoint){
    assert( pPager->eState>=PAGER_WRITER_LOCKED );
    assert( assert_pager_state(pPager) );
  
    if( nSavepoint>pPager->nSavepoint && pPager->useJournal ){
      return pagerOpenSavepoint(pPager, nSavepoint);
    }else{
      return SQLITE_OK;
    }
  }
  
  
  /*
  ** This function is called to rollback or release (commit) a savepoint.
  ** The savepoint to release or rollback need not be the most recently 
  ** created savepoint.
  **
  ** Parameter op is always either SAVEPOINT_ROLLBACK or SAVEPOINT_RELEASE.
  ** If it is SAVEPOINT_RELEASE, then release and destroy the savepoint with
  ** index iSavepoint. If it is SAVEPOINT_ROLLBACK, then rollback all changes
  ** that have occurred since the specified savepoint was created.
  **
  ** The savepoint to rollback or release is identified by parameter 
  ** iSavepoint. A value of 0 means to operate on the outermost savepoint
  ** (the first created). A value of (Pager.nSavepoint-1) means operate
  ** on the most recently created savepoint. If iSavepoint is greater than
  ** (Pager.nSavepoint-1), then this function is a no-op.
  **
  ** If a negative value is passed to this function, then the current
  ** transaction is rolled back. This is different to calling 
  ** sqlite3PagerRollback() because this function does not terminate
  ** the transaction or unlock the database, it just restores the 
  ** contents of the database to its original state. 
  **
  ** In any case, all savepoints with an index greater than iSavepoint 
  ** are destroyed. If this is a release operation (op==SAVEPOINT_RELEASE),
  ** then savepoint iSavepoint is also destroyed.
  **
  ** This function may return SQLITE_NOMEM if a memory allocation fails,
  ** or an IO error code if an IO error occurs while rolling back a 
  ** savepoint. If no errors occur, SQLITE_OK is returned.
  */ 
  int sqlite3PagerSavepoint(Pager *pPager, int op, int iSavepoint){
    int rc = pPager->errCode;       /* Return code */
  
    assert( op==SAVEPOINT_RELEASE || op==SAVEPOINT_ROLLBACK );
    assert( iSavepoint>=0 || op==SAVEPOINT_ROLLBACK );
  
    if( rc==SQLITE_OK && iSavepoint<pPager->nSavepoint ){
      int ii;            /* Iterator variable */
      int nNew;          /* Number of remaining savepoints after this op. */
  
      /* Figure out how many savepoints will still be active after this
      ** operation. Store this value in nNew. Then free resources associated 
      ** with any savepoints that are destroyed by this operation.
      */
      nNew = iSavepoint + (( op==SAVEPOINT_RELEASE ) ? 0 : 1);
      for(ii=nNew; ii<pPager->nSavepoint; ii++){
        sqlite3BitvecDestroy(pPager->aSavepoint[ii].pInSavepoint);
      }
      pPager->nSavepoint = nNew;
  
      /* If this is a release of the outermost savepoint, truncate 
      ** the sub-journal to zero bytes in size. */
      if( op==SAVEPOINT_RELEASE ){
        if( nNew==0 && isOpen(pPager->sjfd) ){
          /* Only truncate if it is an in-memory sub-journal. */
          if( sqlite3JournalIsInMemory(pPager->sjfd) ){
            rc = sqlite3OsTruncate(pPager->sjfd, 0);
            assert( rc==SQLITE_OK );
          }
          pPager->nSubRec = 0;
        }
      }
      /* Else this is a rollback operation, playback the specified savepoint.
      ** If this is a temp-file, it is possible that the journal file has
      ** not yet been opened. In this case there have been no changes to
      ** the database file, so the playback operation can be skipped.
      */
      else if( pagerUseWal(pPager) || isOpen(pPager->jfd) ){
        PagerSavepoint *pSavepoint = (nNew==0)?0:&pPager->aSavepoint[nNew-1];
        rc = pagerPlaybackSavepoint(pPager, pSavepoint);
        assert(rc!=SQLITE_DONE);
      }
    }
  
    return rc;
  }
  
  /*
  ** Return the full pathname of the database file.
  **
  ** Except, if the pager is in-memory only, then return an empty string if
  ** nullIfMemDb is true.  This routine is called with nullIfMemDb==1 when
  ** used to report the filename to the user, for compatibility with legacy
  ** behavior.  But when the Btree needs to know the filename for matching to
  ** shared cache, it uses nullIfMemDb==0 so that in-memory databases can
  ** participate in shared-cache.
  */
  const char *sqlite3PagerFilename(Pager *pPager, int nullIfMemDb){
    return (nullIfMemDb && pPager->memDb) ? "" : pPager->zFilename;
  }
  
  /*
  ** Return the VFS structure for the pager.
  */
  sqlite3_vfs *sqlite3PagerVfs(Pager *pPager){
    return pPager->pVfs;
  }
  
  /*
  ** Return the file handle for the database file associated
  ** with the pager.  This might return NULL if the file has
  ** not yet been opened.
  */
  sqlite3_file *sqlite3PagerFile(Pager *pPager){
    return pPager->fd;
  }
  
  /*
  ** Return the file handle for the journal file (if it exists).
  ** This will be either the rollback journal or the WAL file.
  */
  sqlite3_file *sqlite3PagerJrnlFile(Pager *pPager){
  #if SQLITE_OMIT_WAL
    return pPager->jfd;
  #else
    return pPager->pWal ? sqlite3WalFile(pPager->pWal) : pPager->jfd;
  #endif
  }
  
  /*
  ** Return the full pathname of the journal file.
  */
  const char *sqlite3PagerJournalname(Pager *pPager){
    return pPager->zJournal;
  }
  
  #ifdef SQLITE_HAS_CODEC
  /*
  ** Set or retrieve the codec for this pager
  */
  void sqlite3PagerSetCodec(
    Pager *pPager,
    void *(*xCodec)(void*,void*,Pgno,int),
    void (*xCodecSizeChng)(void*,int,int),
    void (*xCodecFree)(void*),
    void *pCodec
  ){
    if( pPager->xCodecFree ) pPager->xCodecFree(pPager->pCodec);
    pPager->xCodec = pPager->memDb ? 0 : xCodec;
    pPager->xCodecSizeChng = xCodecSizeChng;
    pPager->xCodecFree = xCodecFree;
    pPager->pCodec = pCodec;
    pagerReportSize(pPager);
  }
  void *sqlite3PagerGetCodec(Pager *pPager){
    return pPager->pCodec;
  }
  
  /*
  ** This function is called by the wal module when writing page content
  ** into the log file.
  **
  ** This function returns a pointer to a buffer containing the encrypted
  ** page content. If a malloc fails, this function may return NULL.
  */
  void *sqlite3PagerCodec(PgHdr *pPg){
    void *aData = 0;
    CODEC2(pPg->pPager, pPg->pData, pPg->pgno, 6, return 0, aData);
    return aData;
  }
  
  /*
  ** Return the current pager state
  */
  int sqlite3PagerState(Pager *pPager){
    return pPager->eState;
  }
  #endif /* SQLITE_HAS_CODEC */
  
  #ifndef SQLITE_OMIT_AUTOVACUUM
  /*
  ** Move the page pPg to location pgno in the file.
  **
  ** There must be no references to the page previously located at
  ** pgno (which we call pPgOld) though that page is allowed to be
  ** in cache.  If the page previously located at pgno is not already
  ** in the rollback journal, it is not put there by by this routine.
  **
  ** References to the page pPg remain valid. Updating any
  ** meta-data associated with pPg (i.e. data stored in the nExtra bytes
  ** allocated along with the page) is the responsibility of the caller.
  **
  ** A transaction must be active when this routine is called. It used to be
  ** required that a statement transaction was not active, but this restriction
  ** has been removed (CREATE INDEX needs to move a page when a statement
  ** transaction is active).
  **
  ** If the fourth argument, isCommit, is non-zero, then this page is being
  ** moved as part of a database reorganization just before the transaction 
  ** is being committed. In this case, it is guaranteed that the database page 
  ** pPg refers to will not be written to again within this transaction.
  **
  ** This function may return SQLITE_NOMEM or an IO error code if an error
  ** occurs. Otherwise, it returns SQLITE_OK.
  */
  int sqlite3PagerMovepage(Pager *pPager, DbPage *pPg, Pgno pgno, int isCommit){
    PgHdr *pPgOld;               /* The page being overwritten. */
    Pgno needSyncPgno = 0;       /* Old value of pPg->pgno, if sync is required */
    int rc;                      /* Return code */
    Pgno origPgno;               /* The original page number */
  
    assert( pPg->nRef>0 );
    assert( pPager->eState==PAGER_WRITER_CACHEMOD
         || pPager->eState==PAGER_WRITER_DBMOD
    );
    assert( assert_pager_state(pPager) );
  
    /* In order to be able to rollback, an in-memory database must journal
    ** the page we are moving from.
    */
    assert( pPager->tempFile || !MEMDB );
    if( pPager->tempFile ){
      rc = sqlite3PagerWrite(pPg);
      if( rc ) return rc;
    }
  
    /* If the page being moved is dirty and has not been saved by the latest
    ** savepoint, then save the current contents of the page into the 
    ** sub-journal now. This is required to handle the following scenario:
    **
    **   BEGIN;
    **     <journal page X, then modify it in memory>
    **     SAVEPOINT one;
    **       <Move page X to location Y>
    **     ROLLBACK TO one;
    **
    ** If page X were not written to the sub-journal here, it would not
    ** be possible to restore its contents when the "ROLLBACK TO one"
    ** statement were is processed.
    **
    ** subjournalPage() may need to allocate space to store pPg->pgno into
    ** one or more savepoint bitvecs. This is the reason this function
    ** may return SQLITE_NOMEM.
    */
    if( (pPg->flags & PGHDR_DIRTY)!=0
     && SQLITE_OK!=(rc = subjournalPageIfRequired(pPg))
    ){
      return rc;
    }
  
    PAGERTRACE(("MOVE %d page %d (needSync=%d) moves to %d\n", 
        PAGERID(pPager), pPg->pgno, (pPg->flags&PGHDR_NEED_SYNC)?1:0, pgno));
    IOTRACE(("MOVE %p %d %d\n", pPager, pPg->pgno, pgno))
  
    /* If the journal needs to be sync()ed before page pPg->pgno can
    ** be written to, store pPg->pgno in local variable needSyncPgno.
    **
    ** If the isCommit flag is set, there is no need to remember that
    ** the journal needs to be sync()ed before database page pPg->pgno 
    ** can be written to. The caller has already promised not to write to it.
    */
    if( (pPg->flags&PGHDR_NEED_SYNC) && !isCommit ){
      needSyncPgno = pPg->pgno;
      assert( pPager->journalMode==PAGER_JOURNALMODE_OFF ||
              pageInJournal(pPager, pPg) || pPg->pgno>pPager->dbOrigSize );
      assert( pPg->flags&PGHDR_DIRTY );
    }
  
    /* If the cache contains a page with page-number pgno, remove it
    ** from its hash chain. Also, if the PGHDR_NEED_SYNC flag was set for 
    ** page pgno before the 'move' operation, it needs to be retained 
    ** for the page moved there.
    */
    pPg->flags &= ~PGHDR_NEED_SYNC;
    pPgOld = sqlite3PagerLookup(pPager, pgno);
    assert( !pPgOld || pPgOld->nRef==1 );
    if( pPgOld ){
      pPg->flags |= (pPgOld->flags&PGHDR_NEED_SYNC);
      if( pPager->tempFile ){
        /* Do not discard pages from an in-memory database since we might
        ** need to rollback later.  Just move the page out of the way. */
        sqlite3PcacheMove(pPgOld, pPager->dbSize+1);
      }else{
        sqlite3PcacheDrop(pPgOld);
      }
    }
  
    origPgno = pPg->pgno;
    sqlite3PcacheMove(pPg, pgno);
    sqlite3PcacheMakeDirty(pPg);
  
    /* For an in-memory database, make sure the original page continues
    ** to exist, in case the transaction needs to roll back.  Use pPgOld
    ** as the original page since it has already been allocated.
    */
    if( pPager->tempFile && pPgOld ){
      sqlite3PcacheMove(pPgOld, origPgno);
      sqlite3PagerUnrefNotNull(pPgOld);
    }
  
    if( needSyncPgno ){
      /* If needSyncPgno is non-zero, then the journal file needs to be 
      ** sync()ed before any data is written to database file page needSyncPgno.
      ** Currently, no such page exists in the page-cache and the 
      ** "is journaled" bitvec flag has been set. This needs to be remedied by
      ** loading the page into the pager-cache and setting the PGHDR_NEED_SYNC
      ** flag.
      **
      ** If the attempt to load the page into the page-cache fails, (due
      ** to a malloc() or IO failure), clear the bit in the pInJournal[]
      ** array. Otherwise, if the page is loaded and written again in
      ** this transaction, it may be written to the database file before
      ** it is synced into the journal file. This way, it may end up in
      ** the journal file twice, but that is not a problem.
      */
      PgHdr *pPgHdr;
      rc = sqlite3PagerGet(pPager, needSyncPgno, &pPgHdr, 0);
      if( rc!=SQLITE_OK ){
        if( needSyncPgno<=pPager->dbOrigSize ){
          assert( pPager->pTmpSpace!=0 );
          sqlite3BitvecClear(pPager->pInJournal, needSyncPgno, pPager->pTmpSpace);
        }
        return rc;
      }
      pPgHdr->flags |= PGHDR_NEED_SYNC;
      sqlite3PcacheMakeDirty(pPgHdr);
      sqlite3PagerUnrefNotNull(pPgHdr);
    }
  
    return SQLITE_OK;
  }
  #endif
  
  /*
  ** The page handle passed as the first argument refers to a dirty page 
  ** with a page number other than iNew. This function changes the page's 
  ** page number to iNew and sets the value of the PgHdr.flags field to 
  ** the value passed as the third parameter.
  */
  void sqlite3PagerRekey(DbPage *pPg, Pgno iNew, u16 flags){
    assert( pPg->pgno!=iNew );
    pPg->flags = flags;
    sqlite3PcacheMove(pPg, iNew);
  }
  
  /*
  ** Return a pointer to the data for the specified page.
  */
  void *sqlite3PagerGetData(DbPage *pPg){
    assert( pPg->nRef>0 || pPg->pPager->memDb );
    return pPg->pData;
  }
  
  /*
  ** Return a pointer to the Pager.nExtra bytes of "extra" space 
  ** allocated along with the specified page.
  */
  void *sqlite3PagerGetExtra(DbPage *pPg){
    return pPg->pExtra;
  }
  
  /*
  ** Get/set the locking-mode for this pager. Parameter eMode must be one
  ** of PAGER_LOCKINGMODE_QUERY, PAGER_LOCKINGMODE_NORMAL or 
  ** PAGER_LOCKINGMODE_EXCLUSIVE. If the parameter is not _QUERY, then
  ** the locking-mode is set to the value specified.
  **
  ** The returned value is either PAGER_LOCKINGMODE_NORMAL or
  ** PAGER_LOCKINGMODE_EXCLUSIVE, indicating the current (possibly updated)
  ** locking-mode.
  */
  int sqlite3PagerLockingMode(Pager *pPager, int eMode){
    assert( eMode==PAGER_LOCKINGMODE_QUERY
              || eMode==PAGER_LOCKINGMODE_NORMAL
              || eMode==PAGER_LOCKINGMODE_EXCLUSIVE );
    assert( PAGER_LOCKINGMODE_QUERY<0 );
    assert( PAGER_LOCKINGMODE_NORMAL>=0 && PAGER_LOCKINGMODE_EXCLUSIVE>=0 );
    assert( pPager->exclusiveMode || 0==sqlite3WalHeapMemory(pPager->pWal) );
    if( eMode>=0 && !pPager->tempFile && !sqlite3WalHeapMemory(pPager->pWal) ){
      pPager->exclusiveMode = (u8)eMode;
    }
    return (int)pPager->exclusiveMode;
  }
  
  /*
  ** Set the journal-mode for this pager. Parameter eMode must be one of:
  **
  **    PAGER_JOURNALMODE_DELETE
  **    PAGER_JOURNALMODE_TRUNCATE
  **    PAGER_JOURNALMODE_PERSIST
  **    PAGER_JOURNALMODE_OFF
  **    PAGER_JOURNALMODE_MEMORY
  **    PAGER_JOURNALMODE_WAL
  **
  ** The journalmode is set to the value specified if the change is allowed.
  ** The change may be disallowed for the following reasons:
  **
  **   *  An in-memory database can only have its journal_mode set to _OFF
  **      or _MEMORY.
  **
  **   *  Temporary databases cannot have _WAL journalmode.
  **
  ** The returned indicate the current (possibly updated) journal-mode.
  */
  int sqlite3PagerSetJournalMode(Pager *pPager, int eMode){
    u8 eOld = pPager->journalMode;    /* Prior journalmode */
  
  #ifdef SQLITE_DEBUG
    /* The print_pager_state() routine is intended to be used by the debugger
    ** only.  We invoke it once here to suppress a compiler warning. */
    print_pager_state(pPager);
  #endif
  
  
    /* The eMode parameter is always valid */
    assert(      eMode==PAGER_JOURNALMODE_DELETE
              || eMode==PAGER_JOURNALMODE_TRUNCATE
              || eMode==PAGER_JOURNALMODE_PERSIST
              || eMode==PAGER_JOURNALMODE_OFF 
              || eMode==PAGER_JOURNALMODE_WAL 
              || eMode==PAGER_JOURNALMODE_MEMORY );
  
    /* This routine is only called from the OP_JournalMode opcode, and
    ** the logic there will never allow a temporary file to be changed
    ** to WAL mode.
    */
    assert( pPager->tempFile==0 || eMode!=PAGER_JOURNALMODE_WAL );
  
    /* Do allow the journalmode of an in-memory database to be set to
    ** anything other than MEMORY or OFF
    */
    if( MEMDB ){
      assert( eOld==PAGER_JOURNALMODE_MEMORY || eOld==PAGER_JOURNALMODE_OFF );
      if( eMode!=PAGER_JOURNALMODE_MEMORY && eMode!=PAGER_JOURNALMODE_OFF ){
        eMode = eOld;
      }
    }
  
    if( eMode!=eOld ){
  
      /* Change the journal mode. */
      assert( pPager->eState!=PAGER_ERROR );
      pPager->journalMode = (u8)eMode;
  
      /* When transistioning from TRUNCATE or PERSIST to any other journal
      ** mode except WAL, unless the pager is in locking_mode=exclusive mode,
      ** delete the journal file.
      */
      assert( (PAGER_JOURNALMODE_TRUNCATE & 5)==1 );
      assert( (PAGER_JOURNALMODE_PERSIST & 5)==1 );
      assert( (PAGER_JOURNALMODE_DELETE & 5)==0 );
      assert( (PAGER_JOURNALMODE_MEMORY & 5)==4 );
      assert( (PAGER_JOURNALMODE_OFF & 5)==0 );
      assert( (PAGER_JOURNALMODE_WAL & 5)==5 );
  
      assert( isOpen(pPager->fd) || pPager->exclusiveMode );
      if( !pPager->exclusiveMode && (eOld & 5)==1 && (eMode & 1)==0 ){
  
        /* In this case we would like to delete the journal file. If it is
        ** not possible, then that is not a problem. Deleting the journal file
        ** here is an optimization only.
        **
        ** Before deleting the journal file, obtain a RESERVED lock on the
        ** database file. This ensures that the journal file is not deleted
        ** while it is in use by some other client.
        */
        sqlite3OsClose(pPager->jfd);
        if( pPager->eLock>=RESERVED_LOCK ){
          sqlite3OsDelete(pPager->pVfs, pPager->zJournal, 0);
        }else{
          int rc = SQLITE_OK;
          int state = pPager->eState;
          assert( state==PAGER_OPEN || state==PAGER_READER );
          if( state==PAGER_OPEN ){
            rc = sqlite3PagerSharedLock(pPager);
          }
          if( pPager->eState==PAGER_READER ){
            assert( rc==SQLITE_OK );
            rc = pagerLockDb(pPager, RESERVED_LOCK);
          }
          if( rc==SQLITE_OK ){
            sqlite3OsDelete(pPager->pVfs, pPager->zJournal, 0);
          }
          if( rc==SQLITE_OK && state==PAGER_READER ){
            pagerUnlockDb(pPager, SHARED_LOCK);
          }else if( state==PAGER_OPEN ){
            pager_unlock(pPager);
          }
          assert( state==pPager->eState );
        }
      }else if( eMode==PAGER_JOURNALMODE_OFF ){
        sqlite3OsClose(pPager->jfd);
      }
    }
  
    /* Return the new journal mode */
    return (int)pPager->journalMode;
  }
  
  /*
  ** Return the current journal mode.
  */
  int sqlite3PagerGetJournalMode(Pager *pPager){
    return (int)pPager->journalMode;
  }
  
  /*
  ** Return TRUE if the pager is in a state where it is OK to change the
  ** journalmode.  Journalmode changes can only happen when the database
  ** is unmodified.
  */
  int sqlite3PagerOkToChangeJournalMode(Pager *pPager){
    assert( assert_pager_state(pPager) );
    if( pPager->eState>=PAGER_WRITER_CACHEMOD ) return 0;
    if( NEVER(isOpen(pPager->jfd) && pPager->journalOff>0) ) return 0;
    return 1;
  }
  
  /*
  ** Get/set the size-limit used for persistent journal files.
  **
  ** Setting the size limit to -1 means no limit is enforced.
  ** An attempt to set a limit smaller than -1 is a no-op.
  */
  i64 sqlite3PagerJournalSizeLimit(Pager *pPager, i64 iLimit){
    if( iLimit>=-1 ){
      pPager->journalSizeLimit = iLimit;
      sqlite3WalLimit(pPager->pWal, iLimit);
    }
    return pPager->journalSizeLimit;
  }
  
  /*
  ** Return a pointer to the pPager->pBackup variable. The backup module
  ** in backup.c maintains the content of this variable. This module
  ** uses it opaquely as an argument to sqlite3BackupRestart() and
  ** sqlite3BackupUpdate() only.
  */
  sqlite3_backup **sqlite3PagerBackupPtr(Pager *pPager){
    return &pPager->pBackup;
  }
  
  #ifndef SQLITE_OMIT_VACUUM
  /*
  ** Unless this is an in-memory or temporary database, clear the pager cache.
  */
  void sqlite3PagerClearCache(Pager *pPager){
    assert( MEMDB==0 || pPager->tempFile );
    if( pPager->tempFile==0 ) pager_reset(pPager);
  }
  #endif
  
  
  #ifndef SQLITE_OMIT_WAL
  /*
  ** This function is called when the user invokes "PRAGMA wal_checkpoint",
  ** "PRAGMA wal_blocking_checkpoint" or calls the sqlite3_wal_checkpoint()
  ** or wal_blocking_checkpoint() API functions.
  **
  ** Parameter eMode is one of SQLITE_CHECKPOINT_PASSIVE, FULL or RESTART.
  */
  int sqlite3PagerCheckpoint(Pager *pPager, int eMode, int *pnLog, int *pnCkpt){
    int rc = SQLITE_OK;
    if( pPager->pWal ){
      rc = sqlite3WalCheckpoint(pPager->pWal, eMode,
          (eMode==SQLITE_CHECKPOINT_PASSIVE ? 0 : pPager->xBusyHandler),
          pPager->pBusyHandlerArg,
          pPager->ckptSyncFlags, pPager->pageSize, (u8 *)pPager->pTmpSpace,
          pnLog, pnCkpt
      );
    }
    return rc;
  }
  
  int sqlite3PagerWalCallback(Pager *pPager){
    return sqlite3WalCallback(pPager->pWal);
  }
  
  /*
  ** Return true if the underlying VFS for the given pager supports the
  ** primitives necessary for write-ahead logging.
  */
  int sqlite3PagerWalSupported(Pager *pPager){
    const sqlite3_io_methods *pMethods = pPager->fd->pMethods;
    if( pPager->noLock ) return 0;
    return pPager->exclusiveMode || (pMethods->iVersion>=2 && pMethods->xShmMap);
  }
  
  /*
  ** Attempt to take an exclusive lock on the database file. If a PENDING lock
  ** is obtained instead, immediately release it.
  */
  static int pagerExclusiveLock(Pager *pPager){
    int rc;                         /* Return code */
  
    assert( pPager->eLock==SHARED_LOCK || pPager->eLock==EXCLUSIVE_LOCK );
    rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
    if( rc!=SQLITE_OK ){
      /* If the attempt to grab the exclusive lock failed, release the 
      ** pending lock that may have been obtained instead.  */
      pagerUnlockDb(pPager, SHARED_LOCK);
    }
  
    return rc;
  }
  
  /*
  ** Call sqlite3WalOpen() to open the WAL handle. If the pager is in 
  ** exclusive-locking mode when this function is called, take an EXCLUSIVE
  ** lock on the database file and use heap-memory to store the wal-index
  ** in. Otherwise, use the normal shared-memory.
  */
  static int pagerOpenWal(Pager *pPager){
    int rc = SQLITE_OK;
  
    assert( pPager->pWal==0 && pPager->tempFile==0 );
    assert( pPager->eLock==SHARED_LOCK || pPager->eLock==EXCLUSIVE_LOCK );
  
    /* If the pager is already in exclusive-mode, the WAL module will use 
    ** heap-memory for the wal-index instead of the VFS shared-memory 
    ** implementation. Take the exclusive lock now, before opening the WAL
    ** file, to make sure this is safe.
    */
    if( pPager->exclusiveMode ){
      rc = pagerExclusiveLock(pPager);
    }
  
    /* Open the connection to the log file. If this operation fails, 
    ** (e.g. due to malloc() failure), return an error code.
    */
    if( rc==SQLITE_OK ){
      rc = sqlite3WalOpen(pPager->pVfs,
          pPager->fd, pPager->zWal, pPager->exclusiveMode,
          pPager->journalSizeLimit, &pPager->pWal
      );
    }
    pagerFixMaplimit(pPager);
  
    return rc;
  }
  
  
  /*
  ** The caller must be holding a SHARED lock on the database file to call
  ** this function.
  **
  ** If the pager passed as the first argument is open on a real database
  ** file (not a temp file or an in-memory database), and the WAL file
  ** is not already open, make an attempt to open it now. If successful,
  ** return SQLITE_OK. If an error occurs or the VFS used by the pager does 
  ** not support the xShmXXX() methods, return an error code. *pbOpen is
  ** not modified in either case.
  **
  ** If the pager is open on a temp-file (or in-memory database), or if
  ** the WAL file is already open, set *pbOpen to 1 and return SQLITE_OK
  ** without doing anything.
  */
  int sqlite3PagerOpenWal(
    Pager *pPager,                  /* Pager object */
    int *pbOpen                     /* OUT: Set to true if call is a no-op */
  ){
    int rc = SQLITE_OK;             /* Return code */
  
    assert( assert_pager_state(pPager) );
    assert( pPager->eState==PAGER_OPEN   || pbOpen );
    assert( pPager->eState==PAGER_READER || !pbOpen );
    assert( pbOpen==0 || *pbOpen==0 );
    assert( pbOpen!=0 || (!pPager->tempFile && !pPager->pWal) );
  
    if( !pPager->tempFile && !pPager->pWal ){
      if( !sqlite3PagerWalSupported(pPager) ) return SQLITE_CANTOPEN;
  
      /* Close any rollback journal previously open */
      sqlite3OsClose(pPager->jfd);
  
      rc = pagerOpenWal(pPager);
      if( rc==SQLITE_OK ){
        pPager->journalMode = PAGER_JOURNALMODE_WAL;
        pPager->eState = PAGER_OPEN;
      }
    }else{
      *pbOpen = 1;
    }
  
    return rc;
  }
  
  /*
  ** This function is called to close the connection to the log file prior
  ** to switching from WAL to rollback mode.
  **
  ** Before closing the log file, this function attempts to take an 
  ** EXCLUSIVE lock on the database file. If this cannot be obtained, an
  ** error (SQLITE_BUSY) is returned and the log connection is not closed.
  ** If successful, the EXCLUSIVE lock is not released before returning.
  */
  int sqlite3PagerCloseWal(Pager *pPager){
    int rc = SQLITE_OK;
  
    assert( pPager->journalMode==PAGER_JOURNALMODE_WAL );
  
    /* If the log file is not already open, but does exist in the file-system,
    ** it may need to be checkpointed before the connection can switch to
    ** rollback mode. Open it now so this can happen.
    */
    if( !pPager->pWal ){
      int logexists = 0;
      rc = pagerLockDb(pPager, SHARED_LOCK);
      if( rc==SQLITE_OK ){
        rc = sqlite3OsAccess(
            pPager->pVfs, pPager->zWal, SQLITE_ACCESS_EXISTS, &logexists
        );
      }
      if( rc==SQLITE_OK && logexists ){
        rc = pagerOpenWal(pPager);
      }
    }
      
    /* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
    ** the database file, the log and log-summary files will be deleted.
    */
    if( rc==SQLITE_OK && pPager->pWal ){
      rc = pagerExclusiveLock(pPager);
      if( rc==SQLITE_OK ){
        rc = sqlite3WalClose(pPager->pWal, pPager->ckptSyncFlags,
                             pPager->pageSize, (u8*)pPager->pTmpSpace);
        pPager->pWal = 0;
        pagerFixMaplimit(pPager);
        if( rc && !pPager->exclusiveMode ) pagerUnlockDb(pPager, SHARED_LOCK);
      }
    }
    return rc;
  }
  
  #ifdef SQLITE_ENABLE_SNAPSHOT
  /*
  ** If this is a WAL database, obtain a snapshot handle for the snapshot
  ** currently open. Otherwise, return an error.
  */
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot){
    int rc = SQLITE_ERROR;
    if( pPager->pWal ){
      rc = sqlite3WalSnapshotGet(pPager->pWal, ppSnapshot);
    }
    return rc;
  }
  
  /*
  ** If this is a WAL database, store a pointer to pSnapshot. Next time a
  ** read transaction is opened, attempt to read from the snapshot it 
  ** identifies. If this is not a WAL database, return an error.
  */
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot){
    int rc = SQLITE_OK;
    if( pPager->pWal ){
      sqlite3WalSnapshotOpen(pPager->pWal, pSnapshot);
    }else{
      rc = SQLITE_ERROR;
    }
    return rc;
  }
  #endif /* SQLITE_ENABLE_SNAPSHOT */
  #endif /* !SQLITE_OMIT_WAL */
  
  #ifdef SQLITE_ENABLE_ZIPVFS
  /*
  ** A read-lock must be held on the pager when this function is called. If
  ** the pager is in WAL mode and the WAL file currently contains one or more
  ** frames, return the size in bytes of the page images stored within the
  ** WAL frames. Otherwise, if this is not a WAL database or the WAL file
  ** is empty, return 0.
  */
  int sqlite3PagerWalFramesize(Pager *pPager){
    assert( pPager->eState>=PAGER_READER );
    return sqlite3WalFramesize(pPager->pWal);
  }
  #endif
  
  #endif /* SQLITE_OMIT_DISKIO */
  