text: /*
  ** 2010 February 1
  **
  ** 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 file contains the implementation of a write-ahead log (WAL) used in 
  ** "journal_mode=WAL" mode.
  **
  ** WRITE-AHEAD LOG (WAL) FILE FORMAT
  **
  ** A WAL file consists of a header followed by zero or more "frames".
  ** Each frame records the revised content of a single page from the
  ** database file.  All changes to the database are recorded by writing
  ** frames into the WAL.  Transactions commit when a frame is written that
  ** contains a commit marker.  A single WAL can and usually does record 
  ** multiple transactions.  Periodically, the content of the WAL is
  ** transferred back into the database file in an operation called a
  ** "checkpoint".
  **
  ** A single WAL file can be used multiple times.  In other words, the
  ** WAL can fill up with frames and then be checkpointed and then new
  ** frames can overwrite the old ones.  A WAL always grows from beginning
  ** toward the end.  Checksums and counters attached to each frame are
  ** used to determine which frames within the WAL are valid and which
  ** are leftovers from prior checkpoints.
  **
  ** The WAL header is 32 bytes in size and consists of the following eight
  ** big-endian 32-bit unsigned integer values:
  **
  **     0: Magic number.  0x377f0682 or 0x377f0683
  **     4: File format version.  Currently 3007000
  **     8: Database page size.  Example: 1024
  **    12: Checkpoint sequence number
  **    16: Salt-1, random integer incremented with each checkpoint
  **    20: Salt-2, a different random integer changing with each ckpt
  **    24: Checksum-1 (first part of checksum for first 24 bytes of header).
  **    28: Checksum-2 (second part of checksum for first 24 bytes of header).
  **
  ** Immediately following the wal-header are zero or more frames. Each
  ** frame consists of a 24-byte frame-header followed by a <page-size> bytes
  ** of page data. The frame-header is six big-endian 32-bit unsigned 
  ** integer values, as follows:
  **
  **     0: Page number.
  **     4: For commit records, the size of the database image in pages 
  **        after the commit. For all other records, zero.
  **     8: Salt-1 (copied from the header)
  **    12: Salt-2 (copied from the header)
  **    16: Checksum-1.
  **    20: Checksum-2.
  **
  ** A frame is considered valid if and only if the following conditions are
  ** true:
  **
  **    (1) The salt-1 and salt-2 values in the frame-header match
  **        salt values in the wal-header
  **
  **    (2) The checksum values in the final 8 bytes of the frame-header
  **        exactly match the checksum computed consecutively on the
  **        WAL header and the first 8 bytes and the content of all frames
  **        up to and including the current frame.
  **
  ** The checksum is computed using 32-bit big-endian integers if the
  ** magic number in the first 4 bytes of the WAL is 0x377f0683 and it
  ** is computed using little-endian if the magic number is 0x377f0682.
  ** The checksum values are always stored in the frame header in a
  ** big-endian format regardless of which byte order is used to compute
  ** the checksum.  The checksum is computed by interpreting the input as
  ** an even number of unsigned 32-bit integers: x[0] through x[N].  The
  ** algorithm used for the checksum is as follows:
  ** 
  **   for i from 0 to n-1 step 2:
  **     s0 += x[i] + s1;
  **     s1 += x[i+1] + s0;
  **   endfor
  **
  ** Note that s0 and s1 are both weighted checksums using fibonacci weights
  ** in reverse order (the largest fibonacci weight occurs on the first element
  ** of the sequence being summed.)  The s1 value spans all 32-bit 
  ** terms of the sequence whereas s0 omits the final term.
  **
  ** On a checkpoint, the WAL is first VFS.xSync-ed, then valid content of the
  ** WAL is transferred into the database, then the database is VFS.xSync-ed.
  ** The VFS.xSync operations serve as write barriers - all writes launched
  ** before the xSync must complete before any write that launches after the
  ** xSync begins.
  **
  ** After each checkpoint, the salt-1 value is incremented and the salt-2
  ** value is randomized.  This prevents old and new frames in the WAL from
  ** being considered valid at the same time and being checkpointing together
  ** following a crash.
  **
  ** READER ALGORITHM
  **
  ** To read a page from the database (call it page number P), a reader
  ** first checks the WAL to see if it contains page P.  If so, then the
  ** last valid instance of page P that is a followed by a commit frame
  ** or is a commit frame itself becomes the value read.  If the WAL
  ** contains no copies of page P that are valid and which are a commit
  ** frame or are followed by a commit frame, then page P is read from
  ** the database file.
  **
  ** To start a read transaction, the reader records the index of the last
  ** valid frame in the WAL.  The reader uses this recorded "mxFrame" value
  ** for all subsequent read operations.  New transactions can be appended
  ** to the WAL, but as long as the reader uses its original mxFrame value
  ** and ignores the newly appended content, it will see a consistent snapshot
  ** of the database from a single point in time.  This technique allows
  ** multiple concurrent readers to view different versions of the database
  ** content simultaneously.
  **
  ** The reader algorithm in the previous paragraphs works correctly, but 
  ** because frames for page P can appear anywhere within the WAL, the
  ** reader has to scan the entire WAL looking for page P frames.  If the
  ** WAL is large (multiple megabytes is typical) that scan can be slow,
  ** and read performance suffers.  To overcome this problem, a separate
  ** data structure called the wal-index is maintained to expedite the
  ** search for frames of a particular page.
  ** 
  ** WAL-INDEX FORMAT
  **
  ** Conceptually, the wal-index is shared memory, though VFS implementations
  ** might choose to implement the wal-index using a mmapped file.  Because
  ** the wal-index is shared memory, SQLite does not support journal_mode=WAL 
  ** on a network filesystem.  All users of the database must be able to
  ** share memory.
  **
  ** In the default unix and windows implementation, the wal-index is a mmapped
  ** file whose name is the database name with a "-shm" suffix added.  For that
  ** reason, the wal-index is sometimes called the "shm" file.
  **
  ** The wal-index is transient.  After a crash, the wal-index can (and should
  ** be) reconstructed from the original WAL file.  In fact, the VFS is required
  ** to either truncate or zero the header of the wal-index when the last
  ** connection to it closes.  Because the wal-index is transient, it can
  ** use an architecture-specific format; it does not have to be cross-platform.
  ** Hence, unlike the database and WAL file formats which store all values
  ** as big endian, the wal-index can store multi-byte values in the native
  ** byte order of the host computer.
  **
  ** The purpose of the wal-index is to answer this question quickly:  Given
  ** a page number P and a maximum frame index M, return the index of the 
  ** last frame in the wal before frame M for page P in the WAL, or return
  ** NULL if there are no frames for page P in the WAL prior to M.
  **
  ** The wal-index consists of a header region, followed by an one or
  ** more index blocks.  
  **
  ** The wal-index header contains the total number of frames within the WAL
  ** in the mxFrame field.
  **
  ** Each index block except for the first contains information on 
  ** HASHTABLE_NPAGE frames. The first index block contains information on
  ** HASHTABLE_NPAGE_ONE frames. The values of HASHTABLE_NPAGE_ONE and 
  ** HASHTABLE_NPAGE are selected so that together the wal-index header and
  ** first index block are the same size as all other index blocks in the
  ** wal-index.
  **
  ** Each index block contains two sections, a page-mapping that contains the
  ** database page number associated with each wal frame, and a hash-table 
  ** that allows readers to query an index block for a specific page number.
  ** The page-mapping is an array of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE
  ** for the first index block) 32-bit page numbers. The first entry in the 
  ** first index-block contains the database page number corresponding to the
  ** first frame in the WAL file. The first entry in the second index block
  ** in the WAL file corresponds to the (HASHTABLE_NPAGE_ONE+1)th frame in
  ** the log, and so on.
  **
  ** The last index block in a wal-index usually contains less than the full
  ** complement of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE) page-numbers,
  ** depending on the contents of the WAL file. This does not change the
  ** allocated size of the page-mapping array - the page-mapping array merely
  ** contains unused entries.
  **
  ** Even without using the hash table, the last frame for page P
  ** can be found by scanning the page-mapping sections of each index block
  ** starting with the last index block and moving toward the first, and
  ** within each index block, starting at the end and moving toward the
  ** beginning.  The first entry that equals P corresponds to the frame
  ** holding the content for that page.
  **
  ** The hash table consists of HASHTABLE_NSLOT 16-bit unsigned integers.
  ** HASHTABLE_NSLOT = 2*HASHTABLE_NPAGE, and there is one entry in the
  ** hash table for each page number in the mapping section, so the hash 
  ** table is never more than half full.  The expected number of collisions 
  ** prior to finding a match is 1.  Each entry of the hash table is an
  ** 1-based index of an entry in the mapping section of the same
  ** index block.   Let K be the 1-based index of the largest entry in
  ** the mapping section.  (For index blocks other than the last, K will
  ** always be exactly HASHTABLE_NPAGE (4096) and for the last index block
  ** K will be (mxFrame%HASHTABLE_NPAGE).)  Unused slots of the hash table
  ** contain a value of 0.
  **
  ** To look for page P in the hash table, first compute a hash iKey on
  ** P as follows:
  **
  **      iKey = (P * 383) % HASHTABLE_NSLOT
  **
  ** Then start scanning entries of the hash table, starting with iKey
  ** (wrapping around to the beginning when the end of the hash table is
  ** reached) until an unused hash slot is found. Let the first unused slot
  ** be at index iUnused.  (iUnused might be less than iKey if there was
  ** wrap-around.) Because the hash table is never more than half full,
  ** the search is guaranteed to eventually hit an unused entry.  Let 
  ** iMax be the value between iKey and iUnused, closest to iUnused,
  ** where aHash[iMax]==P.  If there is no iMax entry (if there exists
  ** no hash slot such that aHash[i]==p) then page P is not in the
  ** current index block.  Otherwise the iMax-th mapping entry of the
  ** current index block corresponds to the last entry that references 
  ** page P.
  **
  ** A hash search begins with the last index block and moves toward the
  ** first index block, looking for entries corresponding to page P.  On
  ** average, only two or three slots in each index block need to be
  ** examined in order to either find the last entry for page P, or to
  ** establish that no such entry exists in the block.  Each index block
  ** holds over 4000 entries.  So two or three index blocks are sufficient
  ** to cover a typical 10 megabyte WAL file, assuming 1K pages.  8 or 10
  ** comparisons (on average) suffice to either locate a frame in the
  ** WAL or to establish that the frame does not exist in the WAL.  This
  ** is much faster than scanning the entire 10MB WAL.
  **
  ** Note that entries are added in order of increasing K.  Hence, one
  ** reader might be using some value K0 and a second reader that started
  ** at a later time (after additional transactions were added to the WAL
  ** and to the wal-index) might be using a different value K1, where K1>K0.
  ** Both readers can use the same hash table and mapping section to get
  ** the correct result.  There may be entries in the hash table with
  ** K>K0 but to the first reader, those entries will appear to be unused
  ** slots in the hash table and so the first reader will get an answer as
  ** if no values greater than K0 had ever been inserted into the hash table
  ** in the first place - which is what reader one wants.  Meanwhile, the
  ** second reader using K1 will see additional values that were inserted
  ** later, which is exactly what reader two wants.  
  **
  ** When a rollback occurs, the value of K is decreased. Hash table entries
  ** that correspond to frames greater than the new K value are removed
  ** from the hash table at this point.
  */
  #ifndef SQLITE_OMIT_WAL
  
  #include "wal.h"
  
  /*
  ** Trace output macros
  */
  #if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  int sqlite3WalTrace = 0;
  # define WALTRACE(X)  if(sqlite3WalTrace) sqlite3DebugPrintf X
  #else
  # define WALTRACE(X)
  #endif
  
  /*
  ** WAL mode depends on atomic aligned 32-bit loads and stores in a few
  ** places.  The following macros try to make this explicit.
  */
  #if GCC_VESRION>=5004000
  # define AtomicLoad(PTR)       __atomic_load_n((PTR),__ATOMIC_RELAXED)
  # define AtomicStore(PTR,VAL)  __atomic_store_n((PTR),(VAL),__ATOMIC_RELAXED)
  #else
  # define AtomicLoad(PTR)       (*(PTR))
  # define AtomicStore(PTR,VAL)  (*(PTR) = (VAL))
  #endif
  
  /*
  ** The maximum (and only) versions of the wal and wal-index formats
  ** that may be interpreted by this version of SQLite.
  **
  ** If a client begins recovering a WAL file and finds that (a) the checksum
  ** values in the wal-header are correct and (b) the version field is not
  ** WAL_MAX_VERSION, recovery fails and SQLite returns SQLITE_CANTOPEN.
  **
  ** Similarly, if a client successfully reads a wal-index header (i.e. the 
  ** checksum test is successful) and finds that the version field is not
  ** WALINDEX_MAX_VERSION, then no read-transaction is opened and SQLite
  ** returns SQLITE_CANTOPEN.
  */
  #define WAL_MAX_VERSION      3007000
  #define WALINDEX_MAX_VERSION 3007000
  
  /*
  ** Index numbers for various locking bytes.   WAL_NREADER is the number
  ** of available reader locks and should be at least 3.  The default
  ** is SQLITE_SHM_NLOCK==8 and  WAL_NREADER==5.
  **
  ** Technically, the various VFSes are free to implement these locks however
  ** they see fit.  However, compatibility is encouraged so that VFSes can
  ** interoperate.  The standard implemention used on both unix and windows
  ** is for the index number to indicate a byte offset into the
  ** WalCkptInfo.aLock[] array in the wal-index header.  In other words, all
  ** locks are on the shm file.  The WALINDEX_LOCK_OFFSET constant (which
  ** should be 120) is the location in the shm file for the first locking
  ** byte.
  */
  #define WAL_WRITE_LOCK         0
  #define WAL_ALL_BUT_WRITE      1
  #define WAL_CKPT_LOCK          1
  #define WAL_RECOVER_LOCK       2
  #define WAL_READ_LOCK(I)       (3+(I))
  #define WAL_NREADER            (SQLITE_SHM_NLOCK-3)
  
  
  /* Object declarations */
  typedef struct WalIndexHdr WalIndexHdr;
  typedef struct WalIterator WalIterator;
  typedef struct WalCkptInfo WalCkptInfo;
  
  
  /*
  ** The following object holds a copy of the wal-index header content.
  **
  ** The actual header in the wal-index consists of two copies of this
  ** object followed by one instance of the WalCkptInfo object.
  ** For all versions of SQLite through 3.10.0 and probably beyond,
  ** the locking bytes (WalCkptInfo.aLock) start at offset 120 and
  ** the total header size is 136 bytes.
  **
  ** The szPage value can be any power of 2 between 512 and 32768, inclusive.
  ** Or it can be 1 to represent a 65536-byte page.  The latter case was
  ** added in 3.7.1 when support for 64K pages was added.  
  */
  struct WalIndexHdr {
    u32 iVersion;                   /* Wal-index version */
    u32 unused;                     /* Unused (padding) field */
    u32 iChange;                    /* Counter incremented each transaction */
    u8 isInit;                      /* 1 when initialized */
    u8 bigEndCksum;                 /* True if checksums in WAL are big-endian */
    u16 szPage;                     /* Database page size in bytes. 1==64K */
    u32 mxFrame;                    /* Index of last valid frame in the WAL */
    u32 nPage;                      /* Size of database in pages */
    u32 aFrameCksum[2];             /* Checksum of last frame in log */
    u32 aSalt[2];                   /* Two salt values copied from WAL header */
    u32 aCksum[2];                  /* Checksum over all prior fields */
  };
  
  /*
  ** A copy of the following object occurs in the wal-index immediately
  ** following the second copy of the WalIndexHdr.  This object stores
  ** information used by checkpoint.
  **
  ** nBackfill is the number of frames in the WAL that have been written
  ** back into the database. (We call the act of moving content from WAL to
  ** database "backfilling".)  The nBackfill number is never greater than
  ** WalIndexHdr.mxFrame.  nBackfill can only be increased by threads
  ** holding the WAL_CKPT_LOCK lock (which includes a recovery thread).
  ** However, a WAL_WRITE_LOCK thread can move the value of nBackfill from
  ** mxFrame back to zero when the WAL is reset.
  **
  ** nBackfillAttempted is the largest value of nBackfill that a checkpoint
  ** has attempted to achieve.  Normally nBackfill==nBackfillAtempted, however
  ** the nBackfillAttempted is set before any backfilling is done and the
  ** nBackfill is only set after all backfilling completes.  So if a checkpoint
  ** crashes, nBackfillAttempted might be larger than nBackfill.  The
  ** WalIndexHdr.mxFrame must never be less than nBackfillAttempted.
  **
  ** The aLock[] field is a set of bytes used for locking.  These bytes should
  ** never be read or written.
  **
  ** There is one entry in aReadMark[] for each reader lock.  If a reader
  ** holds read-lock K, then the value in aReadMark[K] is no greater than
  ** the mxFrame for that reader.  The value READMARK_NOT_USED (0xffffffff)
  ** for any aReadMark[] means that entry is unused.  aReadMark[0] is 
  ** a special case; its value is never used and it exists as a place-holder
  ** to avoid having to offset aReadMark[] indexs by one.  Readers holding
  ** WAL_READ_LOCK(0) always ignore the entire WAL and read all content
  ** directly from the database.
  **
  ** The value of aReadMark[K] may only be changed by a thread that
  ** is holding an exclusive lock on WAL_READ_LOCK(K).  Thus, the value of
  ** aReadMark[K] cannot changed while there is a reader is using that mark
  ** since the reader will be holding a shared lock on WAL_READ_LOCK(K).
  **
  ** The checkpointer may only transfer frames from WAL to database where
  ** the frame numbers are less than or equal to every aReadMark[] that is
  ** in use (that is, every aReadMark[j] for which there is a corresponding
  ** WAL_READ_LOCK(j)).  New readers (usually) pick the aReadMark[] with the
  ** largest value and will increase an unused aReadMark[] to mxFrame if there
  ** is not already an aReadMark[] equal to mxFrame.  The exception to the
  ** previous sentence is when nBackfill equals mxFrame (meaning that everything
  ** in the WAL has been backfilled into the database) then new readers
  ** will choose aReadMark[0] which has value 0 and hence such reader will
  ** get all their all content directly from the database file and ignore 
  ** the WAL.
  **
  ** Writers normally append new frames to the end of the WAL.  However,
  ** if nBackfill equals mxFrame (meaning that all WAL content has been
  ** written back into the database) and if no readers are using the WAL
  ** (in other words, if there are no WAL_READ_LOCK(i) where i>0) then
  ** the writer will first "reset" the WAL back to the beginning and start
  ** writing new content beginning at frame 1.
  **
  ** We assume that 32-bit loads are atomic and so no locks are needed in
  ** order to read from any aReadMark[] entries.
  */
  struct WalCkptInfo {
    u32 nBackfill;                  /* Number of WAL frames backfilled into DB */
    u32 aReadMark[WAL_NREADER];     /* Reader marks */
    u8 aLock[SQLITE_SHM_NLOCK];     /* Reserved space for locks */
    u32 nBackfillAttempted;         /* WAL frames perhaps written, or maybe not */
    u32 notUsed0;                   /* Available for future enhancements */
  };
  #define READMARK_NOT_USED  0xffffffff
  
  
  /* A block of WALINDEX_LOCK_RESERVED bytes beginning at
  ** WALINDEX_LOCK_OFFSET is reserved for locks. Since some systems
  ** only support mandatory file-locks, we do not read or write data
  ** from the region of the file on which locks are applied.
  */
  #define WALINDEX_LOCK_OFFSET (sizeof(WalIndexHdr)*2+offsetof(WalCkptInfo,aLock))
  #define WALINDEX_HDR_SIZE    (sizeof(WalIndexHdr)*2+sizeof(WalCkptInfo))
  
  /* Size of header before each frame in wal */
  #define WAL_FRAME_HDRSIZE 24
  
  /* Size of write ahead log header, including checksum. */
  #define WAL_HDRSIZE 32
  
  /* WAL magic value. Either this value, or the same value with the least
  ** significant bit also set (WAL_MAGIC | 0x00000001) is stored in 32-bit
  ** big-endian format in the first 4 bytes of a WAL file.
  **
  ** If the LSB is set, then the checksums for each frame within the WAL
  ** file are calculated by treating all data as an array of 32-bit 
  ** big-endian words. Otherwise, they are calculated by interpreting 
  ** all data as 32-bit little-endian words.
  */
  #define WAL_MAGIC 0x377f0682
  
  /*
  ** Return the offset of frame iFrame in the write-ahead log file, 
  ** assuming a database page size of szPage bytes. The offset returned
  ** is to the start of the write-ahead log frame-header.
  */
  #define walFrameOffset(iFrame, szPage) (                               \
    WAL_HDRSIZE + ((iFrame)-1)*(i64)((szPage)+WAL_FRAME_HDRSIZE)         \
  )
  
  /*
  ** An open write-ahead log file is represented by an instance of the
  ** following object.
  */
  struct Wal {
    sqlite3_vfs *pVfs;         /* The VFS used to create pDbFd */
    sqlite3_file *pDbFd;       /* File handle for the database file */
    sqlite3_file *pWalFd;      /* File handle for WAL file */
    u32 iCallback;             /* Value to pass to log callback (or 0) */
    i64 mxWalSize;             /* Truncate WAL to this size upon reset */
    int nWiData;               /* Size of array apWiData */
    int szFirstBlock;          /* Size of first block written to WAL file */
    volatile u32 **apWiData;   /* Pointer to wal-index content in memory */
    u32 szPage;                /* Database page size */
    i16 readLock;              /* Which read lock is being held.  -1 for none */
    u8 syncFlags;              /* Flags to use to sync header writes */
    u8 exclusiveMode;          /* Non-zero if connection is in exclusive mode */
    u8 writeLock;              /* True if in a write transaction */
    u8 ckptLock;               /* True if holding a checkpoint lock */
    u8 readOnly;               /* WAL_RDWR, WAL_RDONLY, or WAL_SHM_RDONLY */
    u8 truncateOnCommit;       /* True to truncate WAL file on commit */
    u8 syncHeader;             /* Fsync the WAL header if true */
    u8 padToSectorBoundary;    /* Pad transactions out to the next sector */
    u8 bShmUnreliable;         /* SHM content is read-only and unreliable */
    WalIndexHdr hdr;           /* Wal-index header for current transaction */
    u32 minFrame;              /* Ignore wal frames before this one */
    u32 iReCksum;              /* On commit, recalculate checksums from here */
    const char *zWalName;      /* Name of WAL file */
    u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
  #ifdef SQLITE_DEBUG
    u8 lockError;              /* True if a locking error has occurred */
  #endif
  #ifdef SQLITE_ENABLE_SNAPSHOT
    WalIndexHdr *pSnapshot;    /* Start transaction here if not NULL */
  #endif
  };
  
  /*
  ** Candidate values for Wal.exclusiveMode.
  */
  #define WAL_NORMAL_MODE     0
  #define WAL_EXCLUSIVE_MODE  1     
  #define WAL_HEAPMEMORY_MODE 2
  
  /*
  ** Possible values for WAL.readOnly
  */
  #define WAL_RDWR        0    /* Normal read/write connection */
  #define WAL_RDONLY      1    /* The WAL file is readonly */
  #define WAL_SHM_RDONLY  2    /* The SHM file is readonly */
  
  /*
  ** Each page of the wal-index mapping contains a hash-table made up of
  ** an array of HASHTABLE_NSLOT elements of the following type.
  */
  typedef u16 ht_slot;
  
  /*
  ** This structure is used to implement an iterator that loops through
  ** all frames in the WAL in database page order. Where two or more frames
  ** correspond to the same database page, the iterator visits only the 
  ** frame most recently written to the WAL (in other words, the frame with
  ** the largest index).
  **
  ** The internals of this structure are only accessed by:
  **
  **   walIteratorInit() - Create a new iterator,
  **   walIteratorNext() - Step an iterator,
  **   walIteratorFree() - Free an iterator.
  **
  ** This functionality is used by the checkpoint code (see walCheckpoint()).
  */
  struct WalIterator {
    int iPrior;                     /* Last result returned from the iterator */
    int nSegment;                   /* Number of entries in aSegment[] */
    struct WalSegment {
      int iNext;                    /* Next slot in aIndex[] not yet returned */
      ht_slot *aIndex;              /* i0, i1, i2... such that aPgno[iN] ascend */
      u32 *aPgno;                   /* Array of page numbers. */
      int nEntry;                   /* Nr. of entries in aPgno[] and aIndex[] */
      int iZero;                    /* Frame number associated with aPgno[0] */
    } aSegment[1];                  /* One for every 32KB page in the wal-index */
  };
  
  /*
  ** Define the parameters of the hash tables in the wal-index file. There
  ** is a hash-table following every HASHTABLE_NPAGE page numbers in the
  ** wal-index.
  **
  ** Changing any of these constants will alter the wal-index format and
  ** create incompatibilities.
  */
  #define HASHTABLE_NPAGE      4096                 /* Must be power of 2 */
  #define HASHTABLE_HASH_1     383                  /* Should be prime */
  #define HASHTABLE_NSLOT      (HASHTABLE_NPAGE*2)  /* Must be a power of 2 */
  
  /* 
  ** The block of page numbers associated with the first hash-table in a
  ** wal-index is smaller than usual. This is so that there is a complete
  ** hash-table on each aligned 32KB page of the wal-index.
  */
  #define HASHTABLE_NPAGE_ONE  (HASHTABLE_NPAGE - (WALINDEX_HDR_SIZE/sizeof(u32)))
  
  /* The wal-index is divided into pages of WALINDEX_PGSZ bytes each. */
  #define WALINDEX_PGSZ   (                                         \
      sizeof(ht_slot)*HASHTABLE_NSLOT + HASHTABLE_NPAGE*sizeof(u32) \
  )
  
  /*
  ** Obtain a pointer to the iPage'th page of the wal-index. The wal-index
  ** is broken into pages of WALINDEX_PGSZ bytes. Wal-index pages are
  ** numbered from zero.
  **
  ** If the wal-index is currently smaller the iPage pages then the size
  ** of the wal-index might be increased, but only if it is safe to do
  ** so.  It is safe to enlarge the wal-index if pWal->writeLock is true
  ** or pWal->exclusiveMode==WAL_HEAPMEMORY_MODE.
  **
  ** If this call is successful, *ppPage is set to point to the wal-index
  ** page and SQLITE_OK is returned. If an error (an OOM or VFS error) occurs,
  ** then an SQLite error code is returned and *ppPage is set to 0.
  */
  static SQLITE_NOINLINE int walIndexPageRealloc(
    Wal *pWal,               /* The WAL context */
    int iPage,               /* The page we seek */
    volatile u32 **ppPage    /* Write the page pointer here */
  ){
    int rc = SQLITE_OK;
  
    /* Enlarge the pWal->apWiData[] array if required */
    if( pWal->nWiData<=iPage ){
      sqlite3_int64 nByte = sizeof(u32*)*(iPage+1);
      volatile u32 **apNew;
      apNew = (volatile u32 **)sqlite3_realloc64((void *)pWal->apWiData, nByte);
      if( !apNew ){
        *ppPage = 0;
        return SQLITE_NOMEM_BKPT;
      }
      memset((void*)&apNew[pWal->nWiData], 0,
             sizeof(u32*)*(iPage+1-pWal->nWiData));
      pWal->apWiData = apNew;
      pWal->nWiData = iPage+1;
    }
  
    /* Request a pointer to the required page from the VFS */
    assert( pWal->apWiData[iPage]==0 );
    if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
      pWal->apWiData[iPage] = (u32 volatile *)sqlite3MallocZero(WALINDEX_PGSZ);
      if( !pWal->apWiData[iPage] ) rc = SQLITE_NOMEM_BKPT;
    }else{
      rc = sqlite3OsShmMap(pWal->pDbFd, iPage, WALINDEX_PGSZ, 
          pWal->writeLock, (void volatile **)&pWal->apWiData[iPage]
      );
      assert( pWal->apWiData[iPage]!=0 || rc!=SQLITE_OK || pWal->writeLock==0 );
      testcase( pWal->apWiData[iPage]==0 && rc==SQLITE_OK );
      if( (rc&0xff)==SQLITE_READONLY ){
        pWal->readOnly |= WAL_SHM_RDONLY;
        if( rc==SQLITE_READONLY ){
          rc = SQLITE_OK;
        }
      }
    }
  
    *ppPage = pWal->apWiData[iPage];
    assert( iPage==0 || *ppPage || rc!=SQLITE_OK );
    return rc;
  }
  static int walIndexPage(
    Wal *pWal,               /* The WAL context */
    int iPage,               /* The page we seek */
    volatile u32 **ppPage    /* Write the page pointer here */
  ){
    if( pWal->nWiData<=iPage || (*ppPage = pWal->apWiData[iPage])==0 ){
      return walIndexPageRealloc(pWal, iPage, ppPage);
    }
    return SQLITE_OK;
  }
  
  /*
  ** Return a pointer to the WalCkptInfo structure in the wal-index.
  */
  static volatile WalCkptInfo *walCkptInfo(Wal *pWal){
    assert( pWal->nWiData>0 && pWal->apWiData[0] );
    return (volatile WalCkptInfo*)&(pWal->apWiData[0][sizeof(WalIndexHdr)/2]);
  }
  
  /*
  ** Return a pointer to the WalIndexHdr structure in the wal-index.
  */
  static volatile WalIndexHdr *walIndexHdr(Wal *pWal){
    assert( pWal->nWiData>0 && pWal->apWiData[0] );
    return (volatile WalIndexHdr*)pWal->apWiData[0];
  }
  
  /*
  ** The argument to this macro must be of type u32. On a little-endian
  ** architecture, it returns the u32 value that results from interpreting
  ** the 4 bytes as a big-endian value. On a big-endian architecture, it
  ** returns the value that would be produced by interpreting the 4 bytes
  ** of the input value as a little-endian integer.
  */
  #define BYTESWAP32(x) ( \
      (((x)&0x000000FF)<<24) + (((x)&0x0000FF00)<<8)  \
    + (((x)&0x00FF0000)>>8)  + (((x)&0xFF000000)>>24) \
  )
  
  /*
  ** Generate or extend an 8 byte checksum based on the data in 
  ** array aByte[] and the initial values of aIn[0] and aIn[1] (or
  ** initial values of 0 and 0 if aIn==NULL).
  **
  ** The checksum is written back into aOut[] before returning.
  **
  ** nByte must be a positive multiple of 8.
  */
  static void walChecksumBytes(
    int nativeCksum, /* True for native byte-order, false for non-native */
    u8 *a,           /* Content to be checksummed */
    int nByte,       /* Bytes of content in a[].  Must be a multiple of 8. */
    const u32 *aIn,  /* Initial checksum value input */
    u32 *aOut        /* OUT: Final checksum value output */
  ){
    u32 s1, s2;
    u32 *aData = (u32 *)a;
    u32 *aEnd = (u32 *)&a[nByte];
  
    if( aIn ){
      s1 = aIn[0];
      s2 = aIn[1];
    }else{
      s1 = s2 = 0;
    }
  
    assert( nByte>=8 );
    assert( (nByte&0x00000007)==0 );
    assert( nByte<=65536 );
  
    if( nativeCksum ){
      do {
        s1 += *aData++ + s2;
        s2 += *aData++ + s1;
      }while( aData<aEnd );
    }else{
      do {
        s1 += BYTESWAP32(aData[0]) + s2;
        s2 += BYTESWAP32(aData[1]) + s1;
        aData += 2;
      }while( aData<aEnd );
    }
  
    aOut[0] = s1;
    aOut[1] = s2;
  }
  
  static void walShmBarrier(Wal *pWal){
    if( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE ){
      sqlite3OsShmBarrier(pWal->pDbFd);
    }
  }
  
  /*
  ** Write the header information in pWal->hdr into the wal-index.
  **
  ** The checksum on pWal->hdr is updated before it is written.
  */
  static void walIndexWriteHdr(Wal *pWal){
    volatile WalIndexHdr *aHdr = walIndexHdr(pWal);
    const int nCksum = offsetof(WalIndexHdr, aCksum);
  
    assert( pWal->writeLock );
    pWal->hdr.isInit = 1;
    pWal->hdr.iVersion = WALINDEX_MAX_VERSION;
    walChecksumBytes(1, (u8*)&pWal->hdr, nCksum, 0, pWal->hdr.aCksum);
    memcpy((void*)&aHdr[1], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
    walShmBarrier(pWal);
    memcpy((void*)&aHdr[0], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
  }
  
  /*
  ** This function encodes a single frame header and writes it to a buffer
  ** supplied by the caller. A frame-header is made up of a series of 
  ** 4-byte big-endian integers, as follows:
  **
  **     0: Page number.
  **     4: For commit records, the size of the database image in pages 
  **        after the commit. For all other records, zero.
  **     8: Salt-1 (copied from the wal-header)
  **    12: Salt-2 (copied from the wal-header)
  **    16: Checksum-1.
  **    20: Checksum-2.
  */
  static void walEncodeFrame(
    Wal *pWal,                      /* The write-ahead log */
    u32 iPage,                      /* Database page number for frame */
    u32 nTruncate,                  /* New db size (or 0 for non-commit frames) */
    u8 *aData,                      /* Pointer to page data */
    u8 *aFrame                      /* OUT: Write encoded frame here */
  ){
    int nativeCksum;                /* True for native byte-order checksums */
    u32 *aCksum = pWal->hdr.aFrameCksum;
    assert( WAL_FRAME_HDRSIZE==24 );
    sqlite3Put4byte(&aFrame[0], iPage);
    sqlite3Put4byte(&aFrame[4], nTruncate);
    if( pWal->iReCksum==0 ){
      memcpy(&aFrame[8], pWal->hdr.aSalt, 8);
  
      nativeCksum = (pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN);
      walChecksumBytes(nativeCksum, aFrame, 8, aCksum, aCksum);
      walChecksumBytes(nativeCksum, aData, pWal->szPage, aCksum, aCksum);
  
      sqlite3Put4byte(&aFrame[16], aCksum[0]);
      sqlite3Put4byte(&aFrame[20], aCksum[1]);
    }else{
      memset(&aFrame[8], 0, 16);
    }
  }
  
  /*
  ** Check to see if the frame with header in aFrame[] and content
  ** in aData[] is valid.  If it is a valid frame, fill *piPage and
  ** *pnTruncate and return true.  Return if the frame is not valid.
  */
  static int walDecodeFrame(
    Wal *pWal,                      /* The write-ahead log */
    u32 *piPage,                    /* OUT: Database page number for frame */
    u32 *pnTruncate,                /* OUT: New db size (or 0 if not commit) */
    u8 *aData,                      /* Pointer to page data (for checksum) */
    u8 *aFrame                      /* Frame data */
  ){
    int nativeCksum;                /* True for native byte-order checksums */
    u32 *aCksum = pWal->hdr.aFrameCksum;
    u32 pgno;                       /* Page number of the frame */
    assert( WAL_FRAME_HDRSIZE==24 );
  
    /* A frame is only valid if the salt values in the frame-header
    ** match the salt values in the wal-header. 
    */
    if( memcmp(&pWal->hdr.aSalt, &aFrame[8], 8)!=0 ){
      return 0;
    }
  
    /* A frame is only valid if the page number is creater than zero.
    */
    pgno = sqlite3Get4byte(&aFrame[0]);
    if( pgno==0 ){
      return 0;
    }
  
    /* A frame is only valid if a checksum of the WAL header,
    ** all prior frams, the first 16 bytes of this frame-header, 
    ** and the frame-data matches the checksum in the last 8 
    ** bytes of this frame-header.
    */
    nativeCksum = (pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN);
    walChecksumBytes(nativeCksum, aFrame, 8, aCksum, aCksum);
    walChecksumBytes(nativeCksum, aData, pWal->szPage, aCksum, aCksum);
    if( aCksum[0]!=sqlite3Get4byte(&aFrame[16]) 
     || aCksum[1]!=sqlite3Get4byte(&aFrame[20]) 
    ){
      /* Checksum failed. */
      return 0;
    }
  
    /* If we reach this point, the frame is valid.  Return the page number
    ** and the new database size.
    */
    *piPage = pgno;
    *pnTruncate = sqlite3Get4byte(&aFrame[4]);
    return 1;
  }
  
  
  #if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  /*
  ** Names of locks.  This routine is used to provide debugging output and is not
  ** a part of an ordinary build.
  */
  static const char *walLockName(int lockIdx){
    if( lockIdx==WAL_WRITE_LOCK ){
      return "WRITE-LOCK";
    }else if( lockIdx==WAL_CKPT_LOCK ){
      return "CKPT-LOCK";
    }else if( lockIdx==WAL_RECOVER_LOCK ){
      return "RECOVER-LOCK";
    }else{
      static char zName[15];
      sqlite3_snprintf(sizeof(zName), zName, "READ-LOCK[%d]",
                       lockIdx-WAL_READ_LOCK(0));
      return zName;
    }
  }
  #endif /*defined(SQLITE_TEST) || defined(SQLITE_DEBUG) */
      
  
  /*
  ** Set or release locks on the WAL.  Locks are either shared or exclusive.
  ** A lock cannot be moved directly between shared and exclusive - it must go
  ** through the unlocked state first.
  **
  ** In locking_mode=EXCLUSIVE, all of these routines become no-ops.
  */
  static int walLockShared(Wal *pWal, int lockIdx){
    int rc;
    if( pWal->exclusiveMode ) return SQLITE_OK;
    rc = sqlite3OsShmLock(pWal->pDbFd, lockIdx, 1,
                          SQLITE_SHM_LOCK | SQLITE_SHM_SHARED);
    WALTRACE(("WAL%p: acquire SHARED-%s %s\n", pWal,
              walLockName(lockIdx), rc ? "failed" : "ok"));
    VVA_ONLY( pWal->lockError = (u8)(rc!=SQLITE_OK && rc!=SQLITE_BUSY); )
    return rc;
  }
  static void walUnlockShared(Wal *pWal, int lockIdx){
    if( pWal->exclusiveMode ) return;
    (void)sqlite3OsShmLock(pWal->pDbFd, lockIdx, 1,
                           SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED);
    WALTRACE(("WAL%p: release SHARED-%s\n", pWal, walLockName(lockIdx)));
  }
  static int walLockExclusive(Wal *pWal, int lockIdx, int n){
    int rc;
    if( pWal->exclusiveMode ) return SQLITE_OK;
    rc = sqlite3OsShmLock(pWal->pDbFd, lockIdx, n,
                          SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE);
    WALTRACE(("WAL%p: acquire EXCLUSIVE-%s cnt=%d %s\n", pWal,
              walLockName(lockIdx), n, rc ? "failed" : "ok"));
    VVA_ONLY( pWal->lockError = (u8)(rc!=SQLITE_OK && rc!=SQLITE_BUSY); )
    return rc;
  }
  static void walUnlockExclusive(Wal *pWal, int lockIdx, int n){
    if( pWal->exclusiveMode ) return;
    (void)sqlite3OsShmLock(pWal->pDbFd, lockIdx, n,
                           SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE);
    WALTRACE(("WAL%p: release EXCLUSIVE-%s cnt=%d\n", pWal,
               walLockName(lockIdx), n));
  }
  
  /*
  ** Compute a hash on a page number.  The resulting hash value must land
  ** between 0 and (HASHTABLE_NSLOT-1).  The walHashNext() function advances
  ** the hash to the next value in the event of a collision.
  */
  static int walHash(u32 iPage){
    assert( iPage>0 );
    assert( (HASHTABLE_NSLOT & (HASHTABLE_NSLOT-1))==0 );
    return (iPage*HASHTABLE_HASH_1) & (HASHTABLE_NSLOT-1);
  }
  static int walNextHash(int iPriorHash){
    return (iPriorHash+1)&(HASHTABLE_NSLOT-1);
  }
  
  /*
  ** An instance of the WalHashLoc object is used to describe the location
  ** of a page hash table in the wal-index.  This becomes the return value
  ** from walHashGet().
  */
  typedef struct WalHashLoc WalHashLoc;
  struct WalHashLoc {
    volatile ht_slot *aHash;  /* Start of the wal-index hash table */
    volatile u32 *aPgno;      /* aPgno[1] is the page of first frame indexed */
    u32 iZero;                /* One less than the frame number of first indexed*/
  };
  
  /* 
  ** Return pointers to the hash table and page number array stored on
  ** page iHash of the wal-index. The wal-index is broken into 32KB pages
  ** numbered starting from 0.
  **
  ** Set output variable pLoc->aHash to point to the start of the hash table
  ** in the wal-index file. Set pLoc->iZero to one less than the frame 
  ** number of the first frame indexed by this hash table. If a
  ** slot in the hash table is set to N, it refers to frame number 
  ** (pLoc->iZero+N) in the log.
  **
  ** Finally, set pLoc->aPgno so that pLoc->aPgno[1] is the page number of the
  ** first frame indexed by the hash table, frame (pLoc->iZero+1).
  */
  static int walHashGet(
    Wal *pWal,                      /* WAL handle */
    int iHash,                      /* Find the iHash'th table */
    WalHashLoc *pLoc                /* OUT: Hash table location */
  ){
    int rc;                         /* Return code */
  
    rc = walIndexPage(pWal, iHash, &pLoc->aPgno);
    assert( rc==SQLITE_OK || iHash>0 );
  
    if( rc==SQLITE_OK ){
      pLoc->aHash = (volatile ht_slot *)&pLoc->aPgno[HASHTABLE_NPAGE];
      if( iHash==0 ){
        pLoc->aPgno = &pLoc->aPgno[WALINDEX_HDR_SIZE/sizeof(u32)];
        pLoc->iZero = 0;
      }else{
        pLoc->iZero = HASHTABLE_NPAGE_ONE + (iHash-1)*HASHTABLE_NPAGE;
      }
      pLoc->aPgno = &pLoc->aPgno[-1];
    }
    return rc;
  }
  
  /*
  ** Return the number of the wal-index page that contains the hash-table
  ** and page-number array that contain entries corresponding to WAL frame
  ** iFrame. The wal-index is broken up into 32KB pages. Wal-index pages 
  ** are numbered starting from 0.
  */
  static int walFramePage(u32 iFrame){
    int iHash = (iFrame+HASHTABLE_NPAGE-HASHTABLE_NPAGE_ONE-1) / HASHTABLE_NPAGE;
    assert( (iHash==0 || iFrame>HASHTABLE_NPAGE_ONE)
         && (iHash>=1 || iFrame<=HASHTABLE_NPAGE_ONE)
         && (iHash<=1 || iFrame>(HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE))
         && (iHash>=2 || iFrame<=HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE)
         && (iHash<=2 || iFrame>(HASHTABLE_NPAGE_ONE+2*HASHTABLE_NPAGE))
    );
    return iHash;
  }
  
  /*
  ** Return the page number associated with frame iFrame in this WAL.
  */
  static u32 walFramePgno(Wal *pWal, u32 iFrame){
    int iHash = walFramePage(iFrame);
    if( iHash==0 ){
      return pWal->apWiData[0][WALINDEX_HDR_SIZE/sizeof(u32) + iFrame - 1];
    }
    return pWal->apWiData[iHash][(iFrame-1-HASHTABLE_NPAGE_ONE)%HASHTABLE_NPAGE];
  }
  
  /*
  ** Remove entries from the hash table that point to WAL slots greater
  ** than pWal->hdr.mxFrame.
  **
  ** This function is called whenever pWal->hdr.mxFrame is decreased due
  ** to a rollback or savepoint.
  **
  ** At most only the hash table containing pWal->hdr.mxFrame needs to be
  ** updated.  Any later hash tables will be automatically cleared when
  ** pWal->hdr.mxFrame advances to the point where those hash tables are
  ** actually needed.
  */
  static void walCleanupHash(Wal *pWal){
    WalHashLoc sLoc;                /* Hash table location */
    int iLimit = 0;                 /* Zero values greater than this */
    int nByte;                      /* Number of bytes to zero in aPgno[] */
    int i;                          /* Used to iterate through aHash[] */
    int rc;                         /* Return code form walHashGet() */
  
    assert( pWal->writeLock );
    testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE-1 );
    testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE );
    testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE+1 );
  
    if( pWal->hdr.mxFrame==0 ) return;
  
    /* Obtain pointers to the hash-table and page-number array containing 
    ** the entry that corresponds to frame pWal->hdr.mxFrame. It is guaranteed
    ** that the page said hash-table and array reside on is already mapped.(1)
    */
    assert( pWal->nWiData>walFramePage(pWal->hdr.mxFrame) );
    assert( pWal->apWiData[walFramePage(pWal->hdr.mxFrame)] );
    rc = walHashGet(pWal, walFramePage(pWal->hdr.mxFrame), &sLoc);
    if( NEVER(rc) ) return; /* Defense-in-depth, in case (1) above is wrong */
  
    /* Zero all hash-table entries that correspond to frame numbers greater
    ** than pWal->hdr.mxFrame.
    */
    iLimit = pWal->hdr.mxFrame - sLoc.iZero;
    assert( iLimit>0 );
    for(i=0; i<HASHTABLE_NSLOT; i++){
      if( sLoc.aHash[i]>iLimit ){
        sLoc.aHash[i] = 0;
      }
    }
    
    /* Zero the entries in the aPgno array that correspond to frames with
    ** frame numbers greater than pWal->hdr.mxFrame. 
    */
    nByte = (int)((char *)sLoc.aHash - (char *)&sLoc.aPgno[iLimit+1]);
    memset((void *)&sLoc.aPgno[iLimit+1], 0, nByte);
  
  #ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
    /* Verify that the every entry in the mapping region is still reachable
    ** via the hash table even after the cleanup.
    */
    if( iLimit ){
      int j;           /* Loop counter */
      int iKey;        /* Hash key */
      for(j=1; j<=iLimit; j++){
        for(iKey=walHash(sLoc.aPgno[j]);sLoc.aHash[iKey];iKey=walNextHash(iKey)){
          if( sLoc.aHash[iKey]==j ) break;
        }
        assert( sLoc.aHash[iKey]==j );
      }
    }
  #endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
  }
  
  
  /*
  ** Set an entry in the wal-index that will map database page number
  ** pPage into WAL frame iFrame.
  */
  static int walIndexAppend(Wal *pWal, u32 iFrame, u32 iPage){
    int rc;                         /* Return code */
    WalHashLoc sLoc;                /* Wal-index hash table location */
  
    rc = walHashGet(pWal, walFramePage(iFrame), &sLoc);
  
    /* Assuming the wal-index file was successfully mapped, populate the
    ** page number array and hash table entry.
    */
    if( rc==SQLITE_OK ){
      int iKey;                     /* Hash table key */
      int idx;                      /* Value to write to hash-table slot */
      int nCollide;                 /* Number of hash collisions */
  
      idx = iFrame - sLoc.iZero;
      assert( idx <= HASHTABLE_NSLOT/2 + 1 );
      
      /* If this is the first entry to be added to this hash-table, zero the
      ** entire hash table and aPgno[] array before proceeding. 
      */
      if( idx==1 ){
        int nByte = (int)((u8 *)&sLoc.aHash[HASHTABLE_NSLOT]
                                 - (u8 *)&sLoc.aPgno[1]);
        memset((void*)&sLoc.aPgno[1], 0, nByte);
      }
  
      /* If the entry in aPgno[] is already set, then the previous writer
      ** must have exited unexpectedly in the middle of a transaction (after
      ** writing one or more dirty pages to the WAL to free up memory). 
      ** Remove the remnants of that writers uncommitted transaction from 
      ** the hash-table before writing any new entries.
      */
      if( sLoc.aPgno[idx] ){
        walCleanupHash(pWal);
        assert( !sLoc.aPgno[idx] );
      }
  
      /* Write the aPgno[] array entry and the hash-table slot. */
      nCollide = idx;
      for(iKey=walHash(iPage); sLoc.aHash[iKey]; iKey=walNextHash(iKey)){
        if( (nCollide--)==0 ) return SQLITE_CORRUPT_BKPT;
      }
      sLoc.aPgno[idx] = iPage;
      sLoc.aHash[iKey] = (ht_slot)idx;
  
  #ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
      /* Verify that the number of entries in the hash table exactly equals
      ** the number of entries in the mapping region.
      */
      {
        int i;           /* Loop counter */
        int nEntry = 0;  /* Number of entries in the hash table */
        for(i=0; i<HASHTABLE_NSLOT; i++){ if( sLoc.aHash[i] ) nEntry++; }
        assert( nEntry==idx );
      }
  
      /* Verify that the every entry in the mapping region is reachable
      ** via the hash table.  This turns out to be a really, really expensive
      ** thing to check, so only do this occasionally - not on every
      ** iteration.
      */
      if( (idx&0x3ff)==0 ){
        int i;           /* Loop counter */
        for(i=1; i<=idx; i++){
          for(iKey=walHash(sLoc.aPgno[i]);
              sLoc.aHash[iKey];
              iKey=walNextHash(iKey)){
            if( sLoc.aHash[iKey]==i ) break;
          }
          assert( sLoc.aHash[iKey]==i );
        }
      }
  #endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
    }
  
  
    return rc;
  }
  
  
  /*
  ** Recover the wal-index by reading the write-ahead log file. 
  **
  ** This routine first tries to establish an exclusive lock on the
  ** wal-index to prevent other threads/processes from doing anything
  ** with the WAL or wal-index while recovery is running.  The
  ** WAL_RECOVER_LOCK is also held so that other threads will know
  ** that this thread is running recovery.  If unable to establish
  ** the necessary locks, this routine returns SQLITE_BUSY.
  */
  static int walIndexRecover(Wal *pWal){
    int rc;                         /* Return Code */
    i64 nSize;                      /* Size of log file */
    u32 aFrameCksum[2] = {0, 0};
    int iLock;                      /* Lock offset to lock for checkpoint */
  
    /* Obtain an exclusive lock on all byte in the locking range not already
    ** locked by the caller. The caller is guaranteed to have locked the
    ** WAL_WRITE_LOCK byte, and may have also locked the WAL_CKPT_LOCK byte.
    ** If successful, the same bytes that are locked here are unlocked before
    ** this function returns.
    */
    assert( pWal->ckptLock==1 || pWal->ckptLock==0 );
    assert( WAL_ALL_BUT_WRITE==WAL_WRITE_LOCK+1 );
    assert( WAL_CKPT_LOCK==WAL_ALL_BUT_WRITE );
    assert( pWal->writeLock );
    iLock = WAL_ALL_BUT_WRITE + pWal->ckptLock;
    rc = walLockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
    if( rc==SQLITE_OK ){
      rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      if( rc!=SQLITE_OK ){
        walUnlockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
      }
    }
    if( rc ){
      return rc;
    }
  
    WALTRACE(("WAL%p: recovery begin...\n", pWal));
  
    memset(&pWal->hdr, 0, sizeof(WalIndexHdr));
  
    rc = sqlite3OsFileSize(pWal->pWalFd, &nSize);
    if( rc!=SQLITE_OK ){
      goto recovery_error;
    }
  
    if( nSize>WAL_HDRSIZE ){
      u8 aBuf[WAL_HDRSIZE];         /* Buffer to load WAL header into */
      u8 *aFrame = 0;               /* Malloc'd buffer to load entire frame */
      int szFrame;                  /* Number of bytes in buffer aFrame[] */
      u8 *aData;                    /* Pointer to data part of aFrame buffer */
      int iFrame;                   /* Index of last frame read */
      i64 iOffset;                  /* Next offset to read from log file */
      int szPage;                   /* Page size according to the log */
      u32 magic;                    /* Magic value read from WAL header */
      u32 version;                  /* Magic value read from WAL header */
      int isValid;                  /* True if this frame is valid */
  
      /* Read in the WAL header. */
      rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0);
      if( rc!=SQLITE_OK ){
        goto recovery_error;
      }
  
      /* If the database page size is not a power of two, or is greater than
      ** SQLITE_MAX_PAGE_SIZE, conclude that the WAL file contains no valid 
      ** data. Similarly, if the 'magic' value is invalid, ignore the whole
      ** WAL file.
      */
      magic = sqlite3Get4byte(&aBuf[0]);
      szPage = sqlite3Get4byte(&aBuf[8]);
      if( (magic&0xFFFFFFFE)!=WAL_MAGIC 
       || szPage&(szPage-1) 
       || szPage>SQLITE_MAX_PAGE_SIZE 
       || szPage<512 
      ){
        goto finished;
      }
      pWal->hdr.bigEndCksum = (u8)(magic&0x00000001);
      pWal->szPage = szPage;
      pWal->nCkpt = sqlite3Get4byte(&aBuf[12]);
      memcpy(&pWal->hdr.aSalt, &aBuf[16], 8);
  
      /* Verify that the WAL header checksum is correct */
      walChecksumBytes(pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN, 
          aBuf, WAL_HDRSIZE-2*4, 0, pWal->hdr.aFrameCksum
      );
      if( pWal->hdr.aFrameCksum[0]!=sqlite3Get4byte(&aBuf[24])
       || pWal->hdr.aFrameCksum[1]!=sqlite3Get4byte(&aBuf[28])
      ){
        goto finished;
      }
  
      /* Verify that the version number on the WAL format is one that
      ** are able to understand */
      version = sqlite3Get4byte(&aBuf[4]);
      if( version!=WAL_MAX_VERSION ){
        rc = SQLITE_CANTOPEN_BKPT;
        goto finished;
      }
  
      /* Malloc a buffer to read frames into. */
      szFrame = szPage + WAL_FRAME_HDRSIZE;
      aFrame = (u8 *)sqlite3_malloc64(szFrame);
      if( !aFrame ){
        rc = SQLITE_NOMEM_BKPT;
        goto recovery_error;
      }
      aData = &aFrame[WAL_FRAME_HDRSIZE];
  
      /* Read all frames from the log file. */
      iFrame = 0;
      for(iOffset=WAL_HDRSIZE; (iOffset+szFrame)<=nSize; iOffset+=szFrame){
        u32 pgno;                   /* Database page number for frame */
        u32 nTruncate;              /* dbsize field from frame header */
  
        /* Read and decode the next log frame. */
        iFrame++;
        rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset);
        if( rc!=SQLITE_OK ) break;
        isValid = walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame);
        if( !isValid ) break;
        rc = walIndexAppend(pWal, iFrame, pgno);
        if( rc!=SQLITE_OK ) break;
  
        /* If nTruncate is non-zero, this is a commit record. */
        if( nTruncate ){
          pWal->hdr.mxFrame = iFrame;
          pWal->hdr.nPage = nTruncate;
          pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
          testcase( szPage<=32768 );
          testcase( szPage>=65536 );
          aFrameCksum[0] = pWal->hdr.aFrameCksum[0];
          aFrameCksum[1] = pWal->hdr.aFrameCksum[1];
        }
      }
  
      sqlite3_free(aFrame);
    }
  
  finished:
    if( rc==SQLITE_OK ){
      volatile WalCkptInfo *pInfo;
      int i;
      pWal->hdr.aFrameCksum[0] = aFrameCksum[0];
      pWal->hdr.aFrameCksum[1] = aFrameCksum[1];
      walIndexWriteHdr(pWal);
  
      /* Reset the checkpoint-header. This is safe because this thread is 
      ** currently holding locks that exclude all other readers, writers and
      ** checkpointers.
      */
      pInfo = walCkptInfo(pWal);
      pInfo->nBackfill = 0;
      pInfo->nBackfillAttempted = pWal->hdr.mxFrame;
      pInfo->aReadMark[0] = 0;
      for(i=1; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
      if( pWal->hdr.mxFrame ) pInfo->aReadMark[1] = pWal->hdr.mxFrame;
  
      /* If more than one frame was recovered from the log file, report an
      ** event via sqlite3_log(). This is to help with identifying performance
      ** problems caused by applications routinely shutting down without
      ** checkpointing the log file.
      */
      if( pWal->hdr.nPage ){
        sqlite3_log(SQLITE_NOTICE_RECOVER_WAL,
            "recovered %d frames from WAL file %s",
            pWal->hdr.mxFrame, pWal->zWalName
        );
      }
    }
  
  recovery_error:
    WALTRACE(("WAL%p: recovery %s\n", pWal, rc ? "failed" : "ok"));
    walUnlockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
    walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
    return rc;
  }
  
  /*
  ** Close an open wal-index.
  */
  static void walIndexClose(Wal *pWal, int isDelete){
    if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE || pWal->bShmUnreliable ){
      int i;
      for(i=0; i<pWal->nWiData; i++){
        sqlite3_free((void *)pWal->apWiData[i]);
        pWal->apWiData[i] = 0;
      }
    }
    if( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE ){
      sqlite3OsShmUnmap(pWal->pDbFd, isDelete);
    }
  }
  
  /* 
  ** Open a connection to the WAL file zWalName. The database file must 
  ** already be opened on connection pDbFd. The buffer that zWalName points
  ** to must remain valid for the lifetime of the returned Wal* handle.
  **
  ** A SHARED lock should be held on the database file when this function
  ** is called. The purpose of this SHARED lock is to prevent any other
  ** client from unlinking the WAL or wal-index file. If another process
  ** were to do this just after this client opened one of these files, the
  ** system would be badly broken.
  **
  ** If the log file is successfully opened, SQLITE_OK is returned and 
  ** *ppWal is set to point to a new WAL handle. If an error occurs,
  ** an SQLite error code is returned and *ppWal is left unmodified.
  */
  int sqlite3WalOpen(
    sqlite3_vfs *pVfs,              /* vfs module to open wal and wal-index */
    sqlite3_file *pDbFd,            /* The open database file */
    const char *zWalName,           /* Name of the WAL file */
    int bNoShm,                     /* True to run in heap-memory mode */
    i64 mxWalSize,                  /* Truncate WAL to this size on reset */
    Wal **ppWal                     /* OUT: Allocated Wal handle */
  ){
    int rc;                         /* Return Code */
    Wal *pRet;                      /* Object to allocate and return */
    int flags;                      /* Flags passed to OsOpen() */
  
    assert( zWalName && zWalName[0] );
    assert( pDbFd );
  
    /* In the amalgamation, the os_unix.c and os_win.c source files come before
    ** this source file.  Verify that the #defines of the locking byte offsets
    ** in os_unix.c and os_win.c agree with the WALINDEX_LOCK_OFFSET value.
    ** For that matter, if the lock offset ever changes from its initial design
    ** value of 120, we need to know that so there is an assert() to check it.
    */
    assert( 120==WALINDEX_LOCK_OFFSET );
    assert( 136==WALINDEX_HDR_SIZE );
  #ifdef WIN_SHM_BASE
    assert( WIN_SHM_BASE==WALINDEX_LOCK_OFFSET );
  #endif
  #ifdef UNIX_SHM_BASE
    assert( UNIX_SHM_BASE==WALINDEX_LOCK_OFFSET );
  #endif
  
  
    /* Allocate an instance of struct Wal to return. */
    *ppWal = 0;
    pRet = (Wal*)sqlite3MallocZero(sizeof(Wal) + pVfs->szOsFile);
    if( !pRet ){
      return SQLITE_NOMEM_BKPT;
    }
  
    pRet->pVfs = pVfs;
    pRet->pWalFd = (sqlite3_file *)&pRet[1];
    pRet->pDbFd = pDbFd;
    pRet->readLock = -1;
    pRet->mxWalSize = mxWalSize;
    pRet->zWalName = zWalName;
    pRet->syncHeader = 1;
    pRet->padToSectorBoundary = 1;
    pRet->exclusiveMode = (bNoShm ? WAL_HEAPMEMORY_MODE: WAL_NORMAL_MODE);
  
    /* Open file handle on the write-ahead log file. */
    flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
    rc = sqlite3OsOpen(pVfs, zWalName, pRet->pWalFd, flags, &flags);
    if( rc==SQLITE_OK && flags&SQLITE_OPEN_READONLY ){
      pRet->readOnly = WAL_RDONLY;
    }
  
    if( rc!=SQLITE_OK ){
      walIndexClose(pRet, 0);
      sqlite3OsClose(pRet->pWalFd);
      sqlite3_free(pRet);
    }else{
      int iDC = sqlite3OsDeviceCharacteristics(pDbFd);
      if( iDC & SQLITE_IOCAP_SEQUENTIAL ){ pRet->syncHeader = 0; }
      if( iDC & SQLITE_IOCAP_POWERSAFE_OVERWRITE ){
        pRet->padToSectorBoundary = 0;
      }
      *ppWal = pRet;
      WALTRACE(("WAL%d: opened\n", pRet));
    }
    return rc;
  }
  
  /*
  ** Change the size to which the WAL file is trucated on each reset.
  */
  void sqlite3WalLimit(Wal *pWal, i64 iLimit){
    if( pWal ) pWal->mxWalSize = iLimit;
  }
  
  /*
  ** Find the smallest page number out of all pages held in the WAL that
  ** has not been returned by any prior invocation of this method on the
  ** same WalIterator object.   Write into *piFrame the frame index where
  ** that page was last written into the WAL.  Write into *piPage the page
  ** number.
  **
  ** Return 0 on success.  If there are no pages in the WAL with a page
  ** number larger than *piPage, then return 1.
  */
  static int walIteratorNext(
    WalIterator *p,               /* Iterator */
    u32 *piPage,                  /* OUT: The page number of the next page */
    u32 *piFrame                  /* OUT: Wal frame index of next page */
  ){
    u32 iMin;                     /* Result pgno must be greater than iMin */
    u32 iRet = 0xFFFFFFFF;        /* 0xffffffff is never a valid page number */
    int i;                        /* For looping through segments */
  
    iMin = p->iPrior;
    assert( iMin<0xffffffff );
    for(i=p->nSegment-1; i>=0; i--){
      struct WalSegment *pSegment = &p->aSegment[i];
      while( pSegment->iNext<pSegment->nEntry ){
        u32 iPg = pSegment->aPgno[pSegment->aIndex[pSegment->iNext]];
        if( iPg>iMin ){
          if( iPg<iRet ){
            iRet = iPg;
            *piFrame = pSegment->iZero + pSegment->aIndex[pSegment->iNext];
          }
          break;
        }
        pSegment->iNext++;
      }
    }
  
    *piPage = p->iPrior = iRet;
    return (iRet==0xFFFFFFFF);
  }
  
  /*
  ** This function merges two sorted lists into a single sorted list.
  **
  ** aLeft[] and aRight[] are arrays of indices.  The sort key is
  ** aContent[aLeft[]] and aContent[aRight[]].  Upon entry, the following
  ** is guaranteed for all J<K:
  **
  **        aContent[aLeft[J]] < aContent[aLeft[K]]
  **        aContent[aRight[J]] < aContent[aRight[K]]
  **
  ** This routine overwrites aRight[] with a new (probably longer) sequence
  ** of indices such that the aRight[] contains every index that appears in
  ** either aLeft[] or the old aRight[] and such that the second condition
  ** above is still met.
  **
  ** The aContent[aLeft[X]] values will be unique for all X.  And the
  ** aContent[aRight[X]] values will be unique too.  But there might be
  ** one or more combinations of X and Y such that
  **
  **      aLeft[X]!=aRight[Y]  &&  aContent[aLeft[X]] == aContent[aRight[Y]]
  **
  ** When that happens, omit the aLeft[X] and use the aRight[Y] index.
  */
  static void walMerge(
    const u32 *aContent,            /* Pages in wal - keys for the sort */
    ht_slot *aLeft,                 /* IN: Left hand input list */
    int nLeft,                      /* IN: Elements in array *paLeft */
    ht_slot **paRight,              /* IN/OUT: Right hand input list */
    int *pnRight,                   /* IN/OUT: Elements in *paRight */
    ht_slot *aTmp                   /* Temporary buffer */
  ){
    int iLeft = 0;                  /* Current index in aLeft */
    int iRight = 0;                 /* Current index in aRight */
    int iOut = 0;                   /* Current index in output buffer */
    int nRight = *pnRight;
    ht_slot *aRight = *paRight;
  
    assert( nLeft>0 && nRight>0 );
    while( iRight<nRight || iLeft<nLeft ){
      ht_slot logpage;
      Pgno dbpage;
  
      if( (iLeft<nLeft) 
       && (iRight>=nRight || aContent[aLeft[iLeft]]<aContent[aRight[iRight]])
      ){
        logpage = aLeft[iLeft++];
      }else{
        logpage = aRight[iRight++];
      }
      dbpage = aContent[logpage];
  
      aTmp[iOut++] = logpage;
      if( iLeft<nLeft && aContent[aLeft[iLeft]]==dbpage ) iLeft++;
  
      assert( iLeft>=nLeft || aContent[aLeft[iLeft]]>dbpage );
      assert( iRight>=nRight || aContent[aRight[iRight]]>dbpage );
    }
  
    *paRight = aLeft;
    *pnRight = iOut;
    memcpy(aLeft, aTmp, sizeof(aTmp[0])*iOut);
  }
  
  /*
  ** Sort the elements in list aList using aContent[] as the sort key.
  ** Remove elements with duplicate keys, preferring to keep the
  ** larger aList[] values.
  **
  ** The aList[] entries are indices into aContent[].  The values in
  ** aList[] are to be sorted so that for all J<K:
  **
  **      aContent[aList[J]] < aContent[aList[K]]
  **
  ** For any X and Y such that
  **
  **      aContent[aList[X]] == aContent[aList[Y]]
  **
  ** Keep the larger of the two values aList[X] and aList[Y] and discard
  ** the smaller.
  */
  static void walMergesort(
    const u32 *aContent,            /* Pages in wal */
    ht_slot *aBuffer,               /* Buffer of at least *pnList items to use */
    ht_slot *aList,                 /* IN/OUT: List to sort */
    int *pnList                     /* IN/OUT: Number of elements in aList[] */
  ){
    struct Sublist {
      int nList;                    /* Number of elements in aList */
      ht_slot *aList;               /* Pointer to sub-list content */
    };
  
    const int nList = *pnList;      /* Size of input list */
    int nMerge = 0;                 /* Number of elements in list aMerge */
    ht_slot *aMerge = 0;            /* List to be merged */
    int iList;                      /* Index into input list */
    u32 iSub = 0;                   /* Index into aSub array */
    struct Sublist aSub[13];        /* Array of sub-lists */
  
    memset(aSub, 0, sizeof(aSub));
    assert( nList<=HASHTABLE_NPAGE && nList>0 );
    assert( HASHTABLE_NPAGE==(1<<(ArraySize(aSub)-1)) );
  
    for(iList=0; iList<nList; iList++){
      nMerge = 1;
      aMerge = &aList[iList];
      for(iSub=0; iList & (1<<iSub); iSub++){
        struct Sublist *p;
        assert( iSub<ArraySize(aSub) );
        p = &aSub[iSub];
        assert( p->aList && p->nList<=(1<<iSub) );
        assert( p->aList==&aList[iList&~((2<<iSub)-1)] );
        walMerge(aContent, p->aList, p->nList, &aMerge, &nMerge, aBuffer);
      }
      aSub[iSub].aList = aMerge;
      aSub[iSub].nList = nMerge;
    }
  
    for(iSub++; iSub<ArraySize(aSub); iSub++){
      if( nList & (1<<iSub) ){
        struct Sublist *p;
        assert( iSub<ArraySize(aSub) );
        p = &aSub[iSub];
        assert( p->nList<=(1<<iSub) );
        assert( p->aList==&aList[nList&~((2<<iSub)-1)] );
        walMerge(aContent, p->aList, p->nList, &aMerge, &nMerge, aBuffer);
      }
    }
    assert( aMerge==aList );
    *pnList = nMerge;
  
  #ifdef SQLITE_DEBUG
    {
      int i;
      for(i=1; i<*pnList; i++){
        assert( aContent[aList[i]] > aContent[aList[i-1]] );
      }
    }
  #endif
  }
  
  /* 
  ** Free an iterator allocated by walIteratorInit().
  */
  static void walIteratorFree(WalIterator *p){
    sqlite3_free(p);
  }
  
  /*
  ** Construct a WalInterator object that can be used to loop over all 
  ** pages in the WAL following frame nBackfill in ascending order. Frames
  ** nBackfill or earlier may be included - excluding them is an optimization
  ** only. The caller must hold the checkpoint lock.
  **
  ** On success, make *pp point to the newly allocated WalInterator object
  ** return SQLITE_OK. Otherwise, return an error code. If this routine
  ** returns an error, the value of *pp is undefined.
  **
  ** The calling routine should invoke walIteratorFree() to destroy the
  ** WalIterator object when it has finished with it.
  */
  static int walIteratorInit(Wal *pWal, u32 nBackfill, WalIterator **pp){
    WalIterator *p;                 /* Return value */
    int nSegment;                   /* Number of segments to merge */
    u32 iLast;                      /* Last frame in log */
    sqlite3_int64 nByte;            /* Number of bytes to allocate */
    int i;                          /* Iterator variable */
    ht_slot *aTmp;                  /* Temp space used by merge-sort */
    int rc = SQLITE_OK;             /* Return Code */
  
    /* This routine only runs while holding the checkpoint lock. And
    ** it only runs if there is actually content in the log (mxFrame>0).
    */
    assert( pWal->ckptLock && pWal->hdr.mxFrame>0 );
    iLast = pWal->hdr.mxFrame;
  
    /* Allocate space for the WalIterator object. */
    nSegment = walFramePage(iLast) + 1;
    nByte = sizeof(WalIterator) 
          + (nSegment-1)*sizeof(struct WalSegment)
          + iLast*sizeof(ht_slot);
    p = (WalIterator *)sqlite3_malloc64(nByte);
    if( !p ){
      return SQLITE_NOMEM_BKPT;
    }
    memset(p, 0, nByte);
    p->nSegment = nSegment;
  
    /* Allocate temporary space used by the merge-sort routine. This block
    ** of memory will be freed before this function returns.
    */
    aTmp = (ht_slot *)sqlite3_malloc64(
        sizeof(ht_slot) * (iLast>HASHTABLE_NPAGE?HASHTABLE_NPAGE:iLast)
    );
    if( !aTmp ){
      rc = SQLITE_NOMEM_BKPT;
    }
  
    for(i=walFramePage(nBackfill+1); rc==SQLITE_OK && i<nSegment; i++){
      WalHashLoc sLoc;
  
      rc = walHashGet(pWal, i, &sLoc);
      if( rc==SQLITE_OK ){
        int j;                      /* Counter variable */
        int nEntry;                 /* Number of entries in this segment */
        ht_slot *aIndex;            /* Sorted index for this segment */
  
        sLoc.aPgno++;
        if( (i+1)==nSegment ){
          nEntry = (int)(iLast - sLoc.iZero);
        }else{
          nEntry = (int)((u32*)sLoc.aHash - (u32*)sLoc.aPgno);
        }
        aIndex = &((ht_slot *)&p->aSegment[p->nSegment])[sLoc.iZero];
        sLoc.iZero++;
    
        for(j=0; j<nEntry; j++){
          aIndex[j] = (ht_slot)j;
        }
        walMergesort((u32 *)sLoc.aPgno, aTmp, aIndex, &nEntry);
        p->aSegment[i].iZero = sLoc.iZero;
        p->aSegment[i].nEntry = nEntry;
        p->aSegment[i].aIndex = aIndex;
        p->aSegment[i].aPgno = (u32 *)sLoc.aPgno;
      }
    }
    sqlite3_free(aTmp);
  
    if( rc!=SQLITE_OK ){
      walIteratorFree(p);
      p = 0;
    }
    *pp = p;
    return rc;
  }
  
  /*
  ** Attempt to obtain the exclusive WAL lock defined by parameters lockIdx and
  ** n. If the attempt fails and parameter xBusy is not NULL, then it is a
  ** busy-handler function. Invoke it and retry the lock until either the
  ** lock is successfully obtained or the busy-handler returns 0.
  */
  static int walBusyLock(
    Wal *pWal,                      /* WAL connection */
    int (*xBusy)(void*),            /* Function to call when busy */
    void *pBusyArg,                 /* Context argument for xBusyHandler */
    int lockIdx,                    /* Offset of first byte to lock */
    int n                           /* Number of bytes to lock */
  ){
    int rc;
    do {
      rc = walLockExclusive(pWal, lockIdx, n);
    }while( xBusy && rc==SQLITE_BUSY && xBusy(pBusyArg) );
    return rc;
  }
  
  /*
  ** The cache of the wal-index header must be valid to call this function.
  ** Return the page-size in bytes used by the database.
  */
  static int walPagesize(Wal *pWal){
    return (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
  }
  
  /*
  ** The following is guaranteed when this function is called:
  **
  **   a) the WRITER lock is held,
  **   b) the entire log file has been checkpointed, and
  **   c) any existing readers are reading exclusively from the database
  **      file - there are no readers that may attempt to read a frame from
  **      the log file.
  **
  ** This function updates the shared-memory structures so that the next
  ** client to write to the database (which may be this one) does so by
  ** writing frames into the start of the log file.
  **
  ** The value of parameter salt1 is used as the aSalt[1] value in the 
  ** new wal-index header. It should be passed a pseudo-random value (i.e. 
  ** one obtained from sqlite3_randomness()).
  */
  static void walRestartHdr(Wal *pWal, u32 salt1){
    volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
    int i;                          /* Loop counter */
    u32 *aSalt = pWal->hdr.aSalt;   /* Big-endian salt values */
    pWal->nCkpt++;
    pWal->hdr.mxFrame = 0;
    sqlite3Put4byte((u8*)&aSalt[0], 1 + sqlite3Get4byte((u8*)&aSalt[0]));
    memcpy(&pWal->hdr.aSalt[1], &salt1, 4);
    walIndexWriteHdr(pWal);
    pInfo->nBackfill = 0;
    pInfo->nBackfillAttempted = 0;
    pInfo->aReadMark[1] = 0;
    for(i=2; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
    assert( pInfo->aReadMark[0]==0 );
  }
  
  /*
  ** Copy as much content as we can from the WAL back into the database file
  ** in response to an sqlite3_wal_checkpoint() request or the equivalent.
  **
  ** The amount of information copies from WAL to database might be limited
  ** by active readers.  This routine will never overwrite a database page
  ** that a concurrent reader might be using.
  **
  ** All I/O barrier operations (a.k.a fsyncs) occur in this routine when
  ** SQLite is in WAL-mode in synchronous=NORMAL.  That means that if 
  ** checkpoints are always run by a background thread or background 
  ** process, foreground threads will never block on a lengthy fsync call.
  **
  ** Fsync is called on the WAL before writing content out of the WAL and
  ** into the database.  This ensures that if the new content is persistent
  ** in the WAL and can be recovered following a power-loss or hard reset.
  **
  ** Fsync is also called on the database file if (and only if) the entire
  ** WAL content is copied into the database file.  This second fsync makes
  ** it safe to delete the WAL since the new content will persist in the
  ** database file.
  **
  ** This routine uses and updates the nBackfill field of the wal-index header.
  ** This is the only routine that will increase the value of nBackfill.  
  ** (A WAL reset or recovery will revert nBackfill to zero, but not increase
  ** its value.)
  **
  ** The caller must be holding sufficient locks to ensure that no other
  ** checkpoint is running (in any other thread or process) at the same
  ** time.
  */
  static int walCheckpoint(
    Wal *pWal,                      /* Wal connection */
    sqlite3 *db,                    /* Check for interrupts on this handle */
    int eMode,                      /* One of PASSIVE, FULL or RESTART */
    int (*xBusy)(void*),            /* Function to call when busy */
    void *pBusyArg,                 /* Context argument for xBusyHandler */
    int sync_flags,                 /* Flags for OsSync() (or 0) */
    u8 *zBuf                        /* Temporary buffer to use */
  ){
    int rc = SQLITE_OK;             /* Return code */
    int szPage;                     /* Database page-size */
    WalIterator *pIter = 0;         /* Wal iterator context */
    u32 iDbpage = 0;                /* Next database page to write */
    u32 iFrame = 0;                 /* Wal frame containing data for iDbpage */
    u32 mxSafeFrame;                /* Max frame that can be backfilled */
    u32 mxPage;                     /* Max database page to write */
    int i;                          /* Loop counter */
    volatile WalCkptInfo *pInfo;    /* The checkpoint status information */
  
    szPage = walPagesize(pWal);
    testcase( szPage<=32768 );
    testcase( szPage>=65536 );
    pInfo = walCkptInfo(pWal);
    if( pInfo->nBackfill<pWal->hdr.mxFrame ){
  
      /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
      ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
      assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );
  
      /* Compute in mxSafeFrame the index of the last frame of the WAL that is
      ** safe to write into the database.  Frames beyond mxSafeFrame might
      ** overwrite database pages that are in use by active readers and thus
      ** cannot be backfilled from the WAL.
      */
      mxSafeFrame = pWal->hdr.mxFrame;
      mxPage = pWal->hdr.nPage;
      for(i=1; i<WAL_NREADER; i++){
        /* Thread-sanitizer reports that the following is an unsafe read,
        ** as some other thread may be in the process of updating the value
        ** of the aReadMark[] slot. The assumption here is that if that is
        ** happening, the other client may only be increasing the value,
        ** not decreasing it. So assuming either that either the "old" or
        ** "new" version of the value is read, and not some arbitrary value
        ** that would never be written by a real client, things are still 
        ** safe.  */
        u32 y = pInfo->aReadMark[i];
        if( mxSafeFrame>y ){
          assert( y<=pWal->hdr.mxFrame );
          rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(i), 1);
          if( rc==SQLITE_OK ){
            pInfo->aReadMark[i] = (i==1 ? mxSafeFrame : READMARK_NOT_USED);
            walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          }else if( rc==SQLITE_BUSY ){
            mxSafeFrame = y;
            xBusy = 0;
          }else{
            goto walcheckpoint_out;
          }
        }
      }
  
      /* Allocate the iterator */
      if( pInfo->nBackfill<mxSafeFrame ){
        rc = walIteratorInit(pWal, pInfo->nBackfill, &pIter);
        assert( rc==SQLITE_OK || pIter==0 );
      }
  
      if( pIter
       && (rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(0),1))==SQLITE_OK
      ){
        u32 nBackfill = pInfo->nBackfill;
  
        pInfo->nBackfillAttempted = mxSafeFrame;
  
        /* Sync the WAL to disk */
        rc = sqlite3OsSync(pWal->pWalFd, CKPT_SYNC_FLAGS(sync_flags));
  
        /* If the database may grow as a result of this checkpoint, hint
        ** about the eventual size of the db file to the VFS layer.
        */
        if( rc==SQLITE_OK ){
          i64 nReq = ((i64)mxPage * szPage);
          i64 nSize;                    /* Current size of database file */
          rc = sqlite3OsFileSize(pWal->pDbFd, &nSize);
          if( rc==SQLITE_OK && nSize<nReq ){
            sqlite3OsFileControlHint(pWal->pDbFd, SQLITE_FCNTL_SIZE_HINT, &nReq);
          }
        }
  
  
        /* Iterate through the contents of the WAL, copying data to the db file */
        while( rc==SQLITE_OK && 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){
          i64 iOffset;
          assert( walFramePgno(pWal, iFrame)==iDbpage );
          if( db->u1.isInterrupted ){
            rc = db->mallocFailed ? SQLITE_NOMEM_BKPT : SQLITE_INTERRUPT;
            break;
          }
          if( iFrame<=nBackfill || iFrame>mxSafeFrame || iDbpage>mxPage ){
            continue;
          }
          iOffset = walFrameOffset(iFrame, szPage) + WAL_FRAME_HDRSIZE;
          /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL file */
          rc = sqlite3OsRead(pWal->pWalFd, zBuf, szPage, iOffset);
          if( rc!=SQLITE_OK ) break;
          iOffset = (iDbpage-1)*(i64)szPage;
          testcase( IS_BIG_INT(iOffset) );
          rc = sqlite3OsWrite(pWal->pDbFd, zBuf, szPage, iOffset);
          if( rc!=SQLITE_OK ) break;
        }
  
        /* If work was actually accomplished... */
        if( rc==SQLITE_OK ){
          if( mxSafeFrame==walIndexHdr(pWal)->mxFrame ){
            i64 szDb = pWal->hdr.nPage*(i64)szPage;
            testcase( IS_BIG_INT(szDb) );
            rc = sqlite3OsTruncate(pWal->pDbFd, szDb);
            if( rc==SQLITE_OK ){
              rc = sqlite3OsSync(pWal->pDbFd, CKPT_SYNC_FLAGS(sync_flags));
            }
          }
          if( rc==SQLITE_OK ){
            pInfo->nBackfill = mxSafeFrame;
          }
        }
  
        /* Release the reader lock held while backfilling */
        walUnlockExclusive(pWal, WAL_READ_LOCK(0), 1);
      }
  
      if( rc==SQLITE_BUSY ){
        /* Reset the return code so as not to report a checkpoint failure
        ** just because there are active readers.  */
        rc = SQLITE_OK;
      }
    }
  
    /* If this is an SQLITE_CHECKPOINT_RESTART or TRUNCATE operation, and the
    ** entire wal file has been copied into the database file, then block 
    ** until all readers have finished using the wal file. This ensures that 
    ** the next process to write to the database restarts the wal file.
    */
    if( rc==SQLITE_OK && eMode!=SQLITE_CHECKPOINT_PASSIVE ){
      assert( pWal->writeLock );
      if( pInfo->nBackfill<pWal->hdr.mxFrame ){
        rc = SQLITE_BUSY;
      }else if( eMode>=SQLITE_CHECKPOINT_RESTART ){
        u32 salt1;
        sqlite3_randomness(4, &salt1);
        assert( pInfo->nBackfill==pWal->hdr.mxFrame );
        rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(1), WAL_NREADER-1);
        if( rc==SQLITE_OK ){
          if( eMode==SQLITE_CHECKPOINT_TRUNCATE ){
            /* IMPLEMENTATION-OF: R-44699-57140 This mode works the same way as
            ** SQLITE_CHECKPOINT_RESTART with the addition that it also
            ** truncates the log file to zero bytes just prior to a
            ** successful return.
            **
            ** In theory, it might be safe to do this without updating the
            ** wal-index header in shared memory, as all subsequent reader or
            ** writer clients should see that the entire log file has been
            ** checkpointed and behave accordingly. This seems unsafe though,
            ** as it would leave the system in a state where the contents of
            ** the wal-index header do not match the contents of the 
            ** file-system. To avoid this, update the wal-index header to
            ** indicate that the log file contains zero valid frames.  */
            walRestartHdr(pWal, salt1);
            rc = sqlite3OsTruncate(pWal->pWalFd, 0);
          }
          walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
        }
      }
    }
  
   walcheckpoint_out:
    walIteratorFree(pIter);
    return rc;
  }
  
  /*
  ** If the WAL file is currently larger than nMax bytes in size, truncate
  ** it to exactly nMax bytes. If an error occurs while doing so, ignore it.
  */
  static void walLimitSize(Wal *pWal, i64 nMax){
    i64 sz;
    int rx;
    sqlite3BeginBenignMalloc();
    rx = sqlite3OsFileSize(pWal->pWalFd, &sz);
    if( rx==SQLITE_OK && (sz > nMax ) ){
      rx = sqlite3OsTruncate(pWal->pWalFd, nMax);
    }
    sqlite3EndBenignMalloc();
    if( rx ){
      sqlite3_log(rx, "cannot limit WAL size: %s", pWal->zWalName);
    }
  }
  
  /*
  ** Close a connection to a log file.
  */
  int sqlite3WalClose(
    Wal *pWal,                      /* Wal to close */
    sqlite3 *db,                    /* For interrupt flag */
    int sync_flags,                 /* Flags to pass to OsSync() (or 0) */
    int nBuf,
    u8 *zBuf                        /* Buffer of at least nBuf bytes */
  ){
    int rc = SQLITE_OK;
    if( pWal ){
      int isDelete = 0;             /* True to unlink wal and wal-index files */
  
      /* If an EXCLUSIVE lock can be obtained on the database file (using the
      ** ordinary, rollback-mode locking methods, this guarantees that the
      ** connection associated with this log file is the only connection to
      ** the database. In this case checkpoint the database and unlink both
      ** the wal and wal-index files.
      **
      ** The EXCLUSIVE lock is not released before returning.
      */
      if( zBuf!=0
       && SQLITE_OK==(rc = sqlite3OsLock(pWal->pDbFd, SQLITE_LOCK_EXCLUSIVE))
      ){
        if( pWal->exclusiveMode==WAL_NORMAL_MODE ){
          pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
        }
        rc = sqlite3WalCheckpoint(pWal, db, 
            SQLITE_CHECKPOINT_PASSIVE, 0, 0, sync_flags, nBuf, zBuf, 0, 0
        );
        if( rc==SQLITE_OK ){
          int bPersist = -1;
          sqlite3OsFileControlHint(
              pWal->pDbFd, SQLITE_FCNTL_PERSIST_WAL, &bPersist
          );
          if( bPersist!=1 ){
            /* Try to delete the WAL file if the checkpoint completed and
            ** fsyned (rc==SQLITE_OK) and if we are not in persistent-wal
            ** mode (!bPersist) */
            isDelete = 1;
          }else if( pWal->mxWalSize>=0 ){
            /* Try to truncate the WAL file to zero bytes if the checkpoint
            ** completed and fsynced (rc==SQLITE_OK) and we are in persistent
            ** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a
            ** non-negative value (pWal->mxWalSize>=0).  Note that we truncate
            ** to zero bytes as truncating to the journal_size_limit might
            ** leave a corrupt WAL file on disk. */
            walLimitSize(pWal, 0);
          }
        }
      }
  
      walIndexClose(pWal, isDelete);
      sqlite3OsClose(pWal->pWalFd);
      if( isDelete ){
        sqlite3BeginBenignMalloc();
        sqlite3OsDelete(pWal->pVfs, pWal->zWalName, 0);
        sqlite3EndBenignMalloc();
      }
      WALTRACE(("WAL%p: closed\n", pWal));
      sqlite3_free((void *)pWal->apWiData);
      sqlite3_free(pWal);
    }
    return rc;
  }
  
  /*
  ** Try to read the wal-index header.  Return 0 on success and 1 if
  ** there is a problem.
  **
  ** The wal-index is in shared memory.  Another thread or process might
  ** be writing the header at the same time this procedure is trying to
  ** read it, which might result in inconsistency.  A dirty read is detected
  ** by verifying that both copies of the header are the same and also by
  ** a checksum on the header.
  **
  ** If and only if the read is consistent and the header is different from
  ** pWal->hdr, then pWal->hdr is updated to the content of the new header
  ** and *pChanged is set to 1.
  **
  ** If the checksum cannot be verified return non-zero. If the header
  ** is read successfully and the checksum verified, return zero.
  */
  static int walIndexTryHdr(Wal *pWal, int *pChanged){
    u32 aCksum[2];                  /* Checksum on the header content */
    WalIndexHdr h1, h2;             /* Two copies of the header content */
    WalIndexHdr volatile *aHdr;     /* Header in shared memory */
  
    /* The first page of the wal-index must be mapped at this point. */
    assert( pWal->nWiData>0 && pWal->apWiData[0] );
  
    /* Read the header. This might happen concurrently with a write to the
    ** same area of shared memory on a different CPU in a SMP,
    ** meaning it is possible that an inconsistent snapshot is read
    ** from the file. If this happens, return non-zero.
    **
    ** There are two copies of the header at the beginning of the wal-index.
    ** When reading, read [0] first then [1].  Writes are in the reverse order.
    ** Memory barriers are used to prevent the compiler or the hardware from
    ** reordering the reads and writes.
    */
    aHdr = walIndexHdr(pWal);
    memcpy(&h1, (void *)&aHdr[0], sizeof(h1));
    walShmBarrier(pWal);
    memcpy(&h2, (void *)&aHdr[1], sizeof(h2));
  
    if( memcmp(&h1, &h2, sizeof(h1))!=0 ){
      return 1;   /* Dirty read */
    }  
    if( h1.isInit==0 ){
      return 1;   /* Malformed header - probably all zeros */
    }
    walChecksumBytes(1, (u8*)&h1, sizeof(h1)-sizeof(h1.aCksum), 0, aCksum);
    if( aCksum[0]!=h1.aCksum[0] || aCksum[1]!=h1.aCksum[1] ){
      return 1;   /* Checksum does not match */
    }
  
    if( memcmp(&pWal->hdr, &h1, sizeof(WalIndexHdr)) ){
      *pChanged = 1;
      memcpy(&pWal->hdr, &h1, sizeof(WalIndexHdr));
      pWal->szPage = (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
      testcase( pWal->szPage<=32768 );
      testcase( pWal->szPage>=65536 );
    }
  
    /* The header was successfully read. Return zero. */
    return 0;
  }
  
  /*
  ** This is the value that walTryBeginRead returns when it needs to
  ** be retried.
  */
  #define WAL_RETRY  (-1)
  
  /*
  ** Read the wal-index header from the wal-index and into pWal->hdr.
  ** If the wal-header appears to be corrupt, try to reconstruct the
  ** wal-index from the WAL before returning.
  **
  ** Set *pChanged to 1 if the wal-index header value in pWal->hdr is
  ** changed by this operation.  If pWal->hdr is unchanged, set *pChanged
  ** to 0.
  **
  ** If the wal-index header is successfully read, return SQLITE_OK. 
  ** Otherwise an SQLite error code.
  */
  static int walIndexReadHdr(Wal *pWal, int *pChanged){
    int rc;                         /* Return code */
    int badHdr;                     /* True if a header read failed */
    volatile u32 *page0;            /* Chunk of wal-index containing header */
  
    /* Ensure that page 0 of the wal-index (the page that contains the 
    ** wal-index header) is mapped. Return early if an error occurs here.
    */
    assert( pChanged );
    rc = walIndexPage(pWal, 0, &page0);
    if( rc!=SQLITE_OK ){
      assert( rc!=SQLITE_READONLY ); /* READONLY changed to OK in walIndexPage */
      if( rc==SQLITE_READONLY_CANTINIT ){
        /* The SQLITE_READONLY_CANTINIT return means that the shared-memory
        ** was openable but is not writable, and this thread is unable to
        ** confirm that another write-capable connection has the shared-memory
        ** open, and hence the content of the shared-memory is unreliable,
        ** since the shared-memory might be inconsistent with the WAL file
        ** and there is no writer on hand to fix it. */
        assert( page0==0 );
        assert( pWal->writeLock==0 );
        assert( pWal->readOnly & WAL_SHM_RDONLY );
        pWal->bShmUnreliable = 1;
        pWal->exclusiveMode = WAL_HEAPMEMORY_MODE;
        *pChanged = 1;
      }else{
        return rc; /* Any other non-OK return is just an error */
      }
    }else{
      /* page0 can be NULL if the SHM is zero bytes in size and pWal->writeLock
      ** is zero, which prevents the SHM from growing */
      testcase( page0!=0 );
    }
    assert( page0!=0 || pWal->writeLock==0 );
  
    /* If the first page of the wal-index has been mapped, try to read the
    ** wal-index header immediately, without holding any lock. This usually
    ** works, but may fail if the wal-index header is corrupt or currently 
    ** being modified by another thread or process.
    */
    badHdr = (page0 ? walIndexTryHdr(pWal, pChanged) : 1);
  
    /* If the first attempt failed, it might have been due to a race
    ** with a writer.  So get a WRITE lock and try again.
    */
    assert( badHdr==0 || pWal->writeLock==0 );
    if( badHdr ){
      if( pWal->bShmUnreliable==0 && (pWal->readOnly & WAL_SHM_RDONLY) ){
        if( SQLITE_OK==(rc = walLockShared(pWal, WAL_WRITE_LOCK)) ){
          walUnlockShared(pWal, WAL_WRITE_LOCK);
          rc = SQLITE_READONLY_RECOVERY;
        }
      }else if( SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) ){
        pWal->writeLock = 1;
        if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
          badHdr = walIndexTryHdr(pWal, pChanged);
          if( badHdr ){
            /* If the wal-index header is still malformed even while holding
            ** a WRITE lock, it can only mean that the header is corrupted and
            ** needs to be reconstructed.  So run recovery to do exactly that.
            */
            rc = walIndexRecover(pWal);
            *pChanged = 1;
          }
        }
        pWal->writeLock = 0;
        walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
      }
    }
  
    /* If the header is read successfully, check the version number to make
    ** sure the wal-index was not constructed with some future format that
    ** this version of SQLite cannot understand.
    */
    if( badHdr==0 && pWal->hdr.iVersion!=WALINDEX_MAX_VERSION ){
      rc = SQLITE_CANTOPEN_BKPT;
    }
    if( pWal->bShmUnreliable ){
      if( rc!=SQLITE_OK ){
        walIndexClose(pWal, 0);
        pWal->bShmUnreliable = 0;
        assert( pWal->nWiData>0 && pWal->apWiData[0]==0 );
        /* walIndexRecover() might have returned SHORT_READ if a concurrent
        ** writer truncated the WAL out from under it.  If that happens, it
        ** indicates that a writer has fixed the SHM file for us, so retry */
        if( rc==SQLITE_IOERR_SHORT_READ ) rc = WAL_RETRY;
      }
      pWal->exclusiveMode = WAL_NORMAL_MODE;
    }
  
    return rc;
  }
  
  /*
  ** Open a transaction in a connection where the shared-memory is read-only
  ** and where we cannot verify that there is a separate write-capable connection
  ** on hand to keep the shared-memory up-to-date with the WAL file.
  **
  ** This can happen, for example, when the shared-memory is implemented by
  ** memory-mapping a *-shm file, where a prior writer has shut down and
  ** left the *-shm file on disk, and now the present connection is trying
  ** to use that database but lacks write permission on the *-shm file.
  ** Other scenarios are also possible, depending on the VFS implementation.
  **
  ** Precondition:
  **
  **    The *-wal file has been read and an appropriate wal-index has been
  **    constructed in pWal->apWiData[] using heap memory instead of shared
  **    memory. 
  **
  ** If this function returns SQLITE_OK, then the read transaction has
  ** been successfully opened. In this case output variable (*pChanged) 
  ** is set to true before returning if the caller should discard the
  ** contents of the page cache before proceeding. Or, if it returns 
  ** WAL_RETRY, then the heap memory wal-index has been discarded and 
  ** the caller should retry opening the read transaction from the 
  ** beginning (including attempting to map the *-shm file). 
  **
  ** If an error occurs, an SQLite error code is returned.
  */
  static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
    i64 szWal;                      /* Size of wal file on disk in bytes */
    i64 iOffset;                    /* Current offset when reading wal file */
    u8 aBuf[WAL_HDRSIZE];           /* Buffer to load WAL header into */
    u8 *aFrame = 0;                 /* Malloc'd buffer to load entire frame */
    int szFrame;                    /* Number of bytes in buffer aFrame[] */
    u8 *aData;                      /* Pointer to data part of aFrame buffer */
    volatile void *pDummy;          /* Dummy argument for xShmMap */
    int rc;                         /* Return code */
    u32 aSaveCksum[2];              /* Saved copy of pWal->hdr.aFrameCksum */
  
    assert( pWal->bShmUnreliable );
    assert( pWal->readOnly & WAL_SHM_RDONLY );
    assert( pWal->nWiData>0 && pWal->apWiData[0] );
  
    /* Take WAL_READ_LOCK(0). This has the effect of preventing any
    ** writers from running a checkpoint, but does not stop them
    ** from running recovery.  */
    rc = walLockShared(pWal, WAL_READ_LOCK(0));
    if( rc!=SQLITE_OK ){
      if( rc==SQLITE_BUSY ) rc = WAL_RETRY;
      goto begin_unreliable_shm_out;
    }
    pWal->readLock = 0;
  
    /* Check to see if a separate writer has attached to the shared-memory area,
    ** thus making the shared-memory "reliable" again.  Do this by invoking
    ** the xShmMap() routine of the VFS and looking to see if the return
    ** is SQLITE_READONLY instead of SQLITE_READONLY_CANTINIT.
    **
    ** If the shared-memory is now "reliable" return WAL_RETRY, which will
    ** cause the heap-memory WAL-index to be discarded and the actual
    ** shared memory to be used in its place.
    **
    ** This step is important because, even though this connection is holding
    ** the WAL_READ_LOCK(0) which prevents a checkpoint, a writer might
    ** have already checkpointed the WAL file and, while the current
    ** is active, wrap the WAL and start overwriting frames that this
    ** process wants to use.
    **
    ** Once sqlite3OsShmMap() has been called for an sqlite3_file and has
    ** returned any SQLITE_READONLY value, it must return only SQLITE_READONLY
    ** or SQLITE_READONLY_CANTINIT or some error for all subsequent invocations,
    ** even if some external agent does a "chmod" to make the shared-memory
    ** writable by us, until sqlite3OsShmUnmap() has been called.
    ** This is a requirement on the VFS implementation.
     */
    rc = sqlite3OsShmMap(pWal->pDbFd, 0, WALINDEX_PGSZ, 0, &pDummy);
    assert( rc!=SQLITE_OK ); /* SQLITE_OK not possible for read-only connection */
    if( rc!=SQLITE_READONLY_CANTINIT ){
      rc = (rc==SQLITE_READONLY ? WAL_RETRY : rc);
      goto begin_unreliable_shm_out;
    }
  
    /* We reach this point only if the real shared-memory is still unreliable.
    ** Assume the in-memory WAL-index substitute is correct and load it
    ** into pWal->hdr.
    */
    memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));
  
    /* Make sure some writer hasn't come in and changed the WAL file out
    ** from under us, then disconnected, while we were not looking.
    */
    rc = sqlite3OsFileSize(pWal->pWalFd, &szWal);
    if( rc!=SQLITE_OK ){
      goto begin_unreliable_shm_out;
    }
    if( szWal<WAL_HDRSIZE ){
      /* If the wal file is too small to contain a wal-header and the
      ** wal-index header has mxFrame==0, then it must be safe to proceed
      ** reading the database file only. However, the page cache cannot
      ** be trusted, as a read/write connection may have connected, written
      ** the db, run a checkpoint, truncated the wal file and disconnected
      ** since this client's last read transaction.  */
      *pChanged = 1;
      rc = (pWal->hdr.mxFrame==0 ? SQLITE_OK : WAL_RETRY);
      goto begin_unreliable_shm_out;
    }
  
    /* Check the salt keys at the start of the wal file still match. */
    rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0);
    if( rc!=SQLITE_OK ){
      goto begin_unreliable_shm_out;
    }
    if( memcmp(&pWal->hdr.aSalt, &aBuf[16], 8) ){
      /* Some writer has wrapped the WAL file while we were not looking.
      ** Return WAL_RETRY which will cause the in-memory WAL-index to be
      ** rebuilt. */
      rc = WAL_RETRY;
      goto begin_unreliable_shm_out;
    }
  
    /* Allocate a buffer to read frames into */
    szFrame = pWal->hdr.szPage + WAL_FRAME_HDRSIZE;
    aFrame = (u8 *)sqlite3_malloc64(szFrame);
    if( aFrame==0 ){
      rc = SQLITE_NOMEM_BKPT;
      goto begin_unreliable_shm_out;
    }
    aData = &aFrame[WAL_FRAME_HDRSIZE];
  
    /* Check to see if a complete transaction has been appended to the
    ** wal file since the heap-memory wal-index was created. If so, the
    ** heap-memory wal-index is discarded and WAL_RETRY returned to
    ** the caller.  */
    aSaveCksum[0] = pWal->hdr.aFrameCksum[0];
    aSaveCksum[1] = pWal->hdr.aFrameCksum[1];
    for(iOffset=walFrameOffset(pWal->hdr.mxFrame+1, pWal->hdr.szPage); 
        iOffset+szFrame<=szWal; 
        iOffset+=szFrame
    ){
      u32 pgno;                   /* Database page number for frame */
      u32 nTruncate;              /* dbsize field from frame header */
  
      /* Read and decode the next log frame. */
      rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset);
      if( rc!=SQLITE_OK ) break;
      if( !walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame) ) break;
  
      /* If nTruncate is non-zero, then a complete transaction has been
      ** appended to this wal file. Set rc to WAL_RETRY and break out of
      ** the loop.  */
      if( nTruncate ){
        rc = WAL_RETRY;
        break;
      }
    }
    pWal->hdr.aFrameCksum[0] = aSaveCksum[0];
    pWal->hdr.aFrameCksum[1] = aSaveCksum[1];
  
   begin_unreliable_shm_out:
    sqlite3_free(aFrame);
    if( rc!=SQLITE_OK ){
      int i;
      for(i=0; i<pWal->nWiData; i++){
        sqlite3_free((void*)pWal->apWiData[i]);
        pWal->apWiData[i] = 0;
      }
      pWal->bShmUnreliable = 0;
      sqlite3WalEndReadTransaction(pWal);
      *pChanged = 1;
    }
    return rc;
  }
  
  /*
  ** Attempt to start a read transaction.  This might fail due to a race or
  ** other transient condition.  When that happens, it returns WAL_RETRY to
  ** indicate to the caller that it is safe to retry immediately.
  **
  ** On success return SQLITE_OK.  On a permanent failure (such an
  ** I/O error or an SQLITE_BUSY because another process is running
  ** recovery) return a positive error code.
  **
  ** The useWal parameter is true to force the use of the WAL and disable
  ** the case where the WAL is bypassed because it has been completely
  ** checkpointed.  If useWal==0 then this routine calls walIndexReadHdr() 
  ** to make a copy of the wal-index header into pWal->hdr.  If the 
  ** wal-index header has changed, *pChanged is set to 1 (as an indication 
  ** to the caller that the local page cache is obsolete and needs to be 
  ** flushed.)  When useWal==1, the wal-index header is assumed to already
  ** be loaded and the pChanged parameter is unused.
  **
  ** The caller must set the cnt parameter to the number of prior calls to
  ** this routine during the current read attempt that returned WAL_RETRY.
  ** This routine will start taking more aggressive measures to clear the
  ** race conditions after multiple WAL_RETRY returns, and after an excessive
  ** number of errors will ultimately return SQLITE_PROTOCOL.  The
  ** SQLITE_PROTOCOL return indicates that some other process has gone rogue
  ** and is not honoring the locking protocol.  There is a vanishingly small
  ** chance that SQLITE_PROTOCOL could be returned because of a run of really
  ** bad luck when there is lots of contention for the wal-index, but that
  ** possibility is so small that it can be safely neglected, we believe.
  **
  ** On success, this routine obtains a read lock on 
  ** WAL_READ_LOCK(pWal->readLock).  The pWal->readLock integer is
  ** in the range 0 <= pWal->readLock < WAL_NREADER.  If pWal->readLock==(-1)
  ** that means the Wal does not hold any read lock.  The reader must not
  ** access any database page that is modified by a WAL frame up to and
  ** including frame number aReadMark[pWal->readLock].  The reader will
  ** use WAL frames up to and including pWal->hdr.mxFrame if pWal->readLock>0
  ** Or if pWal->readLock==0, then the reader will ignore the WAL
  ** completely and get all content directly from the database file.
  ** If the useWal parameter is 1 then the WAL will never be ignored and
  ** this routine will always set pWal->readLock>0 on success.
  ** When the read transaction is completed, the caller must release the
  ** lock on WAL_READ_LOCK(pWal->readLock) and set pWal->readLock to -1.
  **
  ** This routine uses the nBackfill and aReadMark[] fields of the header
  ** to select a particular WAL_READ_LOCK() that strives to let the
  ** checkpoint process do as much work as possible.  This routine might
  ** update values of the aReadMark[] array in the header, but if it does
  ** so it takes care to hold an exclusive lock on the corresponding
  ** WAL_READ_LOCK() while changing values.
  */
  static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
    volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */
    u32 mxReadMark;                 /* Largest aReadMark[] value */
    int mxI;                        /* Index of largest aReadMark[] value */
    int i;                          /* Loop counter */
    int rc = SQLITE_OK;             /* Return code  */
    u32 mxFrame;                    /* Wal frame to lock to */
  
    assert( pWal->readLock<0 );     /* Not currently locked */
  
    /* useWal may only be set for read/write connections */
    assert( (pWal->readOnly & WAL_SHM_RDONLY)==0 || useWal==0 );
  
    /* Take steps to avoid spinning forever if there is a protocol error.
    **
    ** Circumstances that cause a RETRY should only last for the briefest
    ** instances of time.  No I/O or other system calls are done while the
    ** locks are held, so the locks should not be held for very long. But 
    ** if we are unlucky, another process that is holding a lock might get
    ** paged out or take a page-fault that is time-consuming to resolve, 
    ** during the few nanoseconds that it is holding the lock.  In that case,
    ** it might take longer than normal for the lock to free.
    **
    ** After 5 RETRYs, we begin calling sqlite3OsSleep().  The first few
    ** calls to sqlite3OsSleep() have a delay of 1 microsecond.  Really this
    ** is more of a scheduler yield than an actual delay.  But on the 10th
    ** an subsequent retries, the delays start becoming longer and longer, 
    ** so that on the 100th (and last) RETRY we delay for 323 milliseconds.
    ** The total delay time before giving up is less than 10 seconds.
    */
    if( cnt>5 ){
      int nDelay = 1;                      /* Pause time in microseconds */
      if( cnt>100 ){
        VVA_ONLY( pWal->lockError = 1; )
        return SQLITE_PROTOCOL;
      }
      if( cnt>=10 ) nDelay = (cnt-9)*(cnt-9)*39;
      sqlite3OsSleep(pWal->pVfs, nDelay);
    }
  
    if( !useWal ){
      assert( rc==SQLITE_OK );
      if( pWal->bShmUnreliable==0 ){
        rc = walIndexReadHdr(pWal, pChanged);
      }
      if( rc==SQLITE_BUSY ){
        /* If there is not a recovery running in another thread or process
        ** then convert BUSY errors to WAL_RETRY.  If recovery is known to
        ** be running, convert BUSY to BUSY_RECOVERY.  There is a race here
        ** which might cause WAL_RETRY to be returned even if BUSY_RECOVERY
        ** would be technically correct.  But the race is benign since with
        ** WAL_RETRY this routine will be called again and will probably be
        ** right on the second iteration.
        */
        if( pWal->apWiData[0]==0 ){
          /* This branch is taken when the xShmMap() method returns SQLITE_BUSY.
          ** We assume this is a transient condition, so return WAL_RETRY. The
          ** xShmMap() implementation used by the default unix and win32 VFS 
          ** modules may return SQLITE_BUSY due to a race condition in the 
          ** code that determines whether or not the shared-memory region 
          ** must be zeroed before the requested page is returned.
          */
          rc = WAL_RETRY;
        }else if( SQLITE_OK==(rc = walLockShared(pWal, WAL_RECOVER_LOCK)) ){
          walUnlockShared(pWal, WAL_RECOVER_LOCK);
          rc = WAL_RETRY;
        }else if( rc==SQLITE_BUSY ){
          rc = SQLITE_BUSY_RECOVERY;
        }
      }
      if( rc!=SQLITE_OK ){
        return rc;
      }
      else if( pWal->bShmUnreliable ){
        return walBeginShmUnreliable(pWal, pChanged);
      }
    }
  
    assert( pWal->nWiData>0 );
    assert( pWal->apWiData[0]!=0 );
    pInfo = walCkptInfo(pWal);
    if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame
  #ifdef SQLITE_ENABLE_SNAPSHOT
     && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)
  #endif
    ){
      /* The WAL has been completely backfilled (or it is empty).
      ** and can be safely ignored.
      */
      rc = walLockShared(pWal, WAL_READ_LOCK(0));
      walShmBarrier(pWal);
      if( rc==SQLITE_OK ){
        if( memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr)) ){
          /* It is not safe to allow the reader to continue here if frames
          ** may have been appended to the log before READ_LOCK(0) was obtained.
          ** When holding READ_LOCK(0), the reader ignores the entire log file,
          ** which implies that the database file contains a trustworthy
          ** snapshot. Since holding READ_LOCK(0) prevents a checkpoint from
          ** happening, this is usually correct.
          **
          ** However, if frames have been appended to the log (or if the log 
          ** is wrapped and written for that matter) before the READ_LOCK(0)
          ** is obtained, that is not necessarily true. A checkpointer may
          ** have started to backfill the appended frames but crashed before
          ** it finished. Leaving a corrupt image in the database file.
          */
          walUnlockShared(pWal, WAL_READ_LOCK(0));
          return WAL_RETRY;
        }
        pWal->readLock = 0;
        return SQLITE_OK;
      }else if( rc!=SQLITE_BUSY ){
        return rc;
      }
    }
  
    /* If we get this far, it means that the reader will want to use
    ** the WAL to get at content from recent commits.  The job now is
    ** to select one of the aReadMark[] entries that is closest to
    ** but not exceeding pWal->hdr.mxFrame and lock that entry.
    */
    mxReadMark = 0;
    mxI = 0;
    mxFrame = pWal->hdr.mxFrame;
  #ifdef SQLITE_ENABLE_SNAPSHOT
    if( pWal->pSnapshot && pWal->pSnapshot->mxFrame<mxFrame ){
      mxFrame = pWal->pSnapshot->mxFrame;
    }
  #endif
    for(i=1; i<WAL_NREADER; i++){
      u32 thisMark = AtomicLoad(pInfo->aReadMark+i);
      if( mxReadMark<=thisMark && thisMark<=mxFrame ){
        assert( thisMark!=READMARK_NOT_USED );
        mxReadMark = thisMark;
        mxI = i;
      }
    }
    if( (pWal->readOnly & WAL_SHM_RDONLY)==0
     && (mxReadMark<mxFrame || mxI==0)
    ){
      for(i=1; i<WAL_NREADER; i++){
        rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
        if( rc==SQLITE_OK ){
          mxReadMark = AtomicStore(pInfo->aReadMark+i,mxFrame);
          mxI = i;
          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          break;
        }else if( rc!=SQLITE_BUSY ){
          return rc;
        }
      }
    }
    if( mxI==0 ){
      assert( rc==SQLITE_BUSY || (pWal->readOnly & WAL_SHM_RDONLY)!=0 );
      return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTINIT;
    }
  
    rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
    if( rc ){
      return rc==SQLITE_BUSY ? WAL_RETRY : rc;
    }
    /* Now that the read-lock has been obtained, check that neither the
    ** value in the aReadMark[] array or the contents of the wal-index
    ** header have changed.
    **
    ** It is necessary to check that the wal-index header did not change
    ** between the time it was read and when the shared-lock was obtained
    ** on WAL_READ_LOCK(mxI) was obtained to account for the possibility
    ** that the log file may have been wrapped by a writer, or that frames
    ** that occur later in the log than pWal->hdr.mxFrame may have been
    ** copied into the database by a checkpointer. If either of these things
    ** happened, then reading the database with the current value of
    ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
    ** instead.
    **
    ** Before checking that the live wal-index header has not changed
    ** since it was read, set Wal.minFrame to the first frame in the wal
    ** file that has not yet been checkpointed. This client will not need
    ** to read any frames earlier than minFrame from the wal file - they
    ** can be safely read directly from the database file.
    **
    ** Because a ShmBarrier() call is made between taking the copy of 
    ** nBackfill and checking that the wal-header in shared-memory still
    ** matches the one cached in pWal->hdr, it is guaranteed that the 
    ** checkpointer that set nBackfill was not working with a wal-index
    ** header newer than that cached in pWal->hdr. If it were, that could
    ** cause a problem. The checkpointer could omit to checkpoint
    ** a version of page X that lies before pWal->minFrame (call that version
    ** A) on the basis that there is a newer version (version B) of the same
    ** page later in the wal file. But if version B happens to like past
    ** frame pWal->hdr.mxFrame - then the client would incorrectly assume
    ** that it can read version A from the database file. However, since
    ** we can guarantee that the checkpointer that set nBackfill could not
    ** see any pages past pWal->hdr.mxFrame, this problem does not come up.
    */
    pWal->minFrame = AtomicLoad(&pInfo->nBackfill)+1;
    walShmBarrier(pWal);
    if( AtomicLoad(pInfo->aReadMark+mxI)!=mxReadMark
     || memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr))
    ){
      walUnlockShared(pWal, WAL_READ_LOCK(mxI));
      return WAL_RETRY;
    }else{
      assert( mxReadMark<=pWal->hdr.mxFrame );
      pWal->readLock = (i16)mxI;
    }
    return rc;
  }
  
  #ifdef SQLITE_ENABLE_SNAPSHOT
  /*
  ** Attempt to reduce the value of the WalCkptInfo.nBackfillAttempted 
  ** variable so that older snapshots can be accessed. To do this, loop
  ** through all wal frames from nBackfillAttempted to (nBackfill+1), 
  ** comparing their content to the corresponding page with the database
  ** file, if any. Set nBackfillAttempted to the frame number of the
  ** first frame for which the wal file content matches the db file.
  **
  ** This is only really safe if the file-system is such that any page 
  ** writes made by earlier checkpointers were atomic operations, which 
  ** is not always true. It is also possible that nBackfillAttempted
  ** may be left set to a value larger than expected, if a wal frame
  ** contains content that duplicate of an earlier version of the same
  ** page.
  **
  ** SQLITE_OK is returned if successful, or an SQLite error code if an
  ** error occurs. It is not an error if nBackfillAttempted cannot be
  ** decreased at all.
  */
  int sqlite3WalSnapshotRecover(Wal *pWal){
    int rc;
  
    assert( pWal->readLock>=0 );
    rc = walLockExclusive(pWal, WAL_CKPT_LOCK, 1);
    if( rc==SQLITE_OK ){
      volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
      int szPage = (int)pWal->szPage;
      i64 szDb;                   /* Size of db file in bytes */
  
      rc = sqlite3OsFileSize(pWal->pDbFd, &szDb);
      if( rc==SQLITE_OK ){
        void *pBuf1 = sqlite3_malloc(szPage);
        void *pBuf2 = sqlite3_malloc(szPage);
        if( pBuf1==0 || pBuf2==0 ){
          rc = SQLITE_NOMEM;
        }else{
          u32 i = pInfo->nBackfillAttempted;
          for(i=pInfo->nBackfillAttempted; i>pInfo->nBackfill; i--){
            WalHashLoc sLoc;          /* Hash table location */
            u32 pgno;                 /* Page number in db file */
            i64 iDbOff;               /* Offset of db file entry */
            i64 iWalOff;              /* Offset of wal file entry */
  
            rc = walHashGet(pWal, walFramePage(i), &sLoc);
            if( rc!=SQLITE_OK ) break;
            pgno = sLoc.aPgno[i-sLoc.iZero];
            iDbOff = (i64)(pgno-1) * szPage;
  
            if( iDbOff+szPage<=szDb ){
              iWalOff = walFrameOffset(i, szPage) + WAL_FRAME_HDRSIZE;
              rc = sqlite3OsRead(pWal->pWalFd, pBuf1, szPage, iWalOff);
  
              if( rc==SQLITE_OK ){
                rc = sqlite3OsRead(pWal->pDbFd, pBuf2, szPage, iDbOff);
              }
  
              if( rc!=SQLITE_OK || 0==memcmp(pBuf1, pBuf2, szPage) ){
                break;
              }
            }
  
            pInfo->nBackfillAttempted = i-1;
          }
        }
  
        sqlite3_free(pBuf1);
        sqlite3_free(pBuf2);
      }
      walUnlockExclusive(pWal, WAL_CKPT_LOCK, 1);
    }
  
    return rc;
  }
  #endif /* SQLITE_ENABLE_SNAPSHOT */
  
  /*
  ** Begin a read transaction on the database.
  **
  ** This routine used to be called sqlite3OpenSnapshot() and with good reason:
  ** it takes a snapshot of the state of the WAL and wal-index for the current
  ** instant in time.  The current thread will continue to use this snapshot.
  ** Other threads might append new content to the WAL and wal-index but
  ** that extra content is ignored by the current thread.
  **
  ** If the database contents have changes since the previous read
  ** transaction, then *pChanged is set to 1 before returning.  The
  ** Pager layer will use this to know that its cache is stale and
  ** needs to be flushed.
  */
  int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
    int rc;                         /* Return code */
    int cnt = 0;                    /* Number of TryBeginRead attempts */
  
  #ifdef SQLITE_ENABLE_SNAPSHOT
    int bChanged = 0;
    WalIndexHdr *pSnapshot = pWal->pSnapshot;
    if( pSnapshot && memcmp(pSnapshot, &pWal->hdr, sizeof(WalIndexHdr))!=0 ){
      bChanged = 1;
    }
  #endif
  
    do{
      rc = walTryBeginRead(pWal, pChanged, 0, ++cnt);
    }while( rc==WAL_RETRY );
    testcase( (rc&0xff)==SQLITE_BUSY );
    testcase( (rc&0xff)==SQLITE_IOERR );
    testcase( rc==SQLITE_PROTOCOL );
    testcase( rc==SQLITE_OK );
  
  #ifdef SQLITE_ENABLE_SNAPSHOT
    if( rc==SQLITE_OK ){
      if( pSnapshot && memcmp(pSnapshot, &pWal->hdr, sizeof(WalIndexHdr))!=0 ){
        /* At this point the client has a lock on an aReadMark[] slot holding
        ** a value equal to or smaller than pSnapshot->mxFrame, but pWal->hdr
        ** is populated with the wal-index header corresponding to the head
        ** of the wal file. Verify that pSnapshot is still valid before
        ** continuing.  Reasons why pSnapshot might no longer be valid:
        **
        **    (1)  The WAL file has been reset since the snapshot was taken.
        **         In this case, the salt will have changed.
        **
        **    (2)  A checkpoint as been attempted that wrote frames past
        **         pSnapshot->mxFrame into the database file.  Note that the
        **         checkpoint need not have completed for this to cause problems.
        */
        volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
  
        assert( pWal->readLock>0 || pWal->hdr.mxFrame==0 );
        assert( pInfo->aReadMark[pWal->readLock]<=pSnapshot->mxFrame );
  
        /* It is possible that there is a checkpointer thread running 
        ** concurrent with this code. If this is the case, it may be that the
        ** checkpointer has already determined that it will checkpoint 
        ** snapshot X, where X is later in the wal file than pSnapshot, but 
        ** has not yet set the pInfo->nBackfillAttempted variable to indicate 
        ** its intent. To avoid the race condition this leads to, ensure that
        ** there is no checkpointer process by taking a shared CKPT lock 
        ** before checking pInfo->nBackfillAttempted.  
        **
        ** TODO: Does the aReadMark[] lock prevent a checkpointer from doing
        **       this already?
        */
        rc = walLockShared(pWal, WAL_CKPT_LOCK);
  
        if( rc==SQLITE_OK ){
          /* Check that the wal file has not been wrapped. Assuming that it has
          ** not, also check that no checkpointer has attempted to checkpoint any
          ** frames beyond pSnapshot->mxFrame. If either of these conditions are
          ** true, return SQLITE_ERROR_SNAPSHOT. Otherwise, overwrite pWal->hdr
          ** with *pSnapshot and set *pChanged as appropriate for opening the
          ** snapshot.  */
          if( !memcmp(pSnapshot->aSalt, pWal->hdr.aSalt, sizeof(pWal->hdr.aSalt))
           && pSnapshot->mxFrame>=pInfo->nBackfillAttempted
          ){
            assert( pWal->readLock>0 );
            memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
            *pChanged = bChanged;
          }else{
            rc = SQLITE_ERROR_SNAPSHOT;
          }
  
          /* Release the shared CKPT lock obtained above. */
          walUnlockShared(pWal, WAL_CKPT_LOCK);
          pWal->minFrame = 1;
        }
  
  
        if( rc!=SQLITE_OK ){
          sqlite3WalEndReadTransaction(pWal);
        }
      }
    }
  #endif
    return rc;
  }
  
  /*
  ** Finish with a read transaction.  All this does is release the
  ** read-lock.
  */
  void sqlite3WalEndReadTransaction(Wal *pWal){
    sqlite3WalEndWriteTransaction(pWal);
    if( pWal->readLock>=0 ){
      walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
      pWal->readLock = -1;
    }
  }
  
  /*
  ** Search the wal file for page pgno. If found, set *piRead to the frame that
  ** contains the page. Otherwise, if pgno is not in the wal file, set *piRead
  ** to zero.
  **
  ** Return SQLITE_OK if successful, or an error code if an error occurs. If an
  ** error does occur, the final value of *piRead is undefined.
  */
  int sqlite3WalFindFrame(
    Wal *pWal,                      /* WAL handle */
    Pgno pgno,                      /* Database page number to read data for */
    u32 *piRead                     /* OUT: Frame number (or zero) */
  ){
    u32 iRead = 0;                  /* If !=0, WAL frame to return data from */
    u32 iLast = pWal->hdr.mxFrame;  /* Last page in WAL for this reader */
    int iHash;                      /* Used to loop through N hash tables */
    int iMinHash;
  
    /* This routine is only be called from within a read transaction. */
    assert( pWal->readLock>=0 || pWal->lockError );
  
    /* If the "last page" field of the wal-index header snapshot is 0, then
    ** no data will be read from the wal under any circumstances. Return early
    ** in this case as an optimization.  Likewise, if pWal->readLock==0, 
    ** then the WAL is ignored by the reader so return early, as if the 
    ** WAL were empty.
    */
    if( iLast==0 || (pWal->readLock==0 && pWal->bShmUnreliable==0) ){
      *piRead = 0;
      return SQLITE_OK;
    }
  
    /* Search the hash table or tables for an entry matching page number
    ** pgno. Each iteration of the following for() loop searches one
    ** hash table (each hash table indexes up to HASHTABLE_NPAGE frames).
    **
    ** This code might run concurrently to the code in walIndexAppend()
    ** that adds entries to the wal-index (and possibly to this hash 
    ** table). This means the value just read from the hash 
    ** slot (aHash[iKey]) may have been added before or after the 
    ** current read transaction was opened. Values added after the
    ** read transaction was opened may have been written incorrectly -
    ** i.e. these slots may contain garbage data. However, we assume
    ** that any slots written before the current read transaction was
    ** opened remain unmodified.
    **
    ** For the reasons above, the if(...) condition featured in the inner
    ** loop of the following block is more stringent that would be required 
    ** if we had exclusive access to the hash-table:
    **
    **   (aPgno[iFrame]==pgno): 
    **     This condition filters out normal hash-table collisions.
    **
    **   (iFrame<=iLast): 
    **     This condition filters out entries that were added to the hash
    **     table after the current read-transaction had started.
    */
    iMinHash = walFramePage(pWal->minFrame);
    for(iHash=walFramePage(iLast); iHash>=iMinHash; iHash--){
      WalHashLoc sLoc;              /* Hash table location */
      int iKey;                     /* Hash slot index */
      int nCollide;                 /* Number of hash collisions remaining */
      int rc;                       /* Error code */
  
      rc = walHashGet(pWal, iHash, &sLoc);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      nCollide = HASHTABLE_NSLOT;
      for(iKey=walHash(pgno); sLoc.aHash[iKey]; iKey=walNextHash(iKey)){
        u32 iH = sLoc.aHash[iKey];
        u32 iFrame = iH + sLoc.iZero;
        if( iFrame<=iLast && iFrame>=pWal->minFrame && sLoc.aPgno[iH]==pgno ){
          assert( iFrame>iRead || CORRUPT_DB );
          iRead = iFrame;
        }
        if( (nCollide--)==0 ){
          return SQLITE_CORRUPT_BKPT;
        }
      }
      if( iRead ) break;
    }
  
  #ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
    /* If expensive assert() statements are available, do a linear search
    ** of the wal-index file content. Make sure the results agree with the
    ** result obtained using the hash indexes above.  */
    {
      u32 iRead2 = 0;
      u32 iTest;
      assert( pWal->bShmUnreliable || pWal->minFrame>0 );
      for(iTest=iLast; iTest>=pWal->minFrame && iTest>0; iTest--){
        if( walFramePgno(pWal, iTest)==pgno ){
          iRead2 = iTest;
          break;
        }
      }
      assert( iRead==iRead2 );
    }
  #endif
  
    *piRead = iRead;
    return SQLITE_OK;
  }
  
  /*
  ** Read the contents of frame iRead from the wal file into buffer pOut
  ** (which is nOut bytes in size). Return SQLITE_OK if successful, or an
  ** error code otherwise.
  */
  int sqlite3WalReadFrame(
    Wal *pWal,                      /* WAL handle */
    u32 iRead,                      /* Frame to read */
    int nOut,                       /* Size of buffer pOut in bytes */
    u8 *pOut                        /* Buffer to write page data to */
  ){
    int sz;
    i64 iOffset;
    sz = pWal->hdr.szPage;
    sz = (sz&0xfe00) + ((sz&0x0001)<<16);
    testcase( sz<=32768 );
    testcase( sz>=65536 );
    iOffset = walFrameOffset(iRead, sz) + WAL_FRAME_HDRSIZE;
    /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL */
    return sqlite3OsRead(pWal->pWalFd, pOut, (nOut>sz ? sz : nOut), iOffset);
  }
  
  /* 
  ** Return the size of the database in pages (or zero, if unknown).
  */
  Pgno sqlite3WalDbsize(Wal *pWal){
    if( pWal && ALWAYS(pWal->readLock>=0) ){
      return pWal->hdr.nPage;
    }
    return 0;
  }
  
  
  /* 
  ** This function starts a write transaction on the WAL.
  **
  ** A read transaction must have already been started by a prior call
  ** to sqlite3WalBeginReadTransaction().
  **
  ** If another thread or process has written into the database since
  ** the read transaction was started, then it is not possible for this
  ** thread to write as doing so would cause a fork.  So this routine
  ** returns SQLITE_BUSY in that case and no write transaction is started.
  **
  ** There can only be a single writer active at a time.
  */
  int sqlite3WalBeginWriteTransaction(Wal *pWal){
    int rc;
  
    /* Cannot start a write transaction without first holding a read
    ** transaction. */
    assert( pWal->readLock>=0 );
    assert( pWal->writeLock==0 && pWal->iReCksum==0 );
  
    if( pWal->readOnly ){
      return SQLITE_READONLY;
    }
  
    /* Only one writer allowed at a time.  Get the write lock.  Return
    ** SQLITE_BUSY if unable.
    */
    rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
    if( rc ){
      return rc;
    }
    pWal->writeLock = 1;
  
    /* If another connection has written to the database file since the
    ** time the read transaction on this connection was started, then
    ** the write is disallowed.
    */
    if( memcmp(&pWal->hdr, (void *)walIndexHdr(pWal), sizeof(WalIndexHdr))!=0 ){
      walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
      pWal->writeLock = 0;
      rc = SQLITE_BUSY_SNAPSHOT;
    }
  
    return rc;
  }
  
  /*
  ** End a write transaction.  The commit has already been done.  This
  ** routine merely releases the lock.
  */
  int sqlite3WalEndWriteTransaction(Wal *pWal){
    if( pWal->writeLock ){
      walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
      pWal->writeLock = 0;
      pWal->iReCksum = 0;
      pWal->truncateOnCommit = 0;
    }
    return SQLITE_OK;
  }
  
  /*
  ** If any data has been written (but not committed) to the log file, this
  ** function moves the write-pointer back to the start of the transaction.
  **
  ** Additionally, the callback function is invoked for each frame written
  ** to the WAL since the start of the transaction. If the callback returns
  ** other than SQLITE_OK, it is not invoked again and the error code is
  ** returned to the caller.
  **
  ** Otherwise, if the callback function does not return an error, this
  ** function returns SQLITE_OK.
  */
  int sqlite3WalUndo(Wal *pWal, int (*xUndo)(void *, Pgno), void *pUndoCtx){
    int rc = SQLITE_OK;
    if( ALWAYS(pWal->writeLock) ){
      Pgno iMax = pWal->hdr.mxFrame;
      Pgno iFrame;
    
      /* Restore the clients cache of the wal-index header to the state it
      ** was in before the client began writing to the database. 
      */
      memcpy(&pWal->hdr, (void *)walIndexHdr(pWal), sizeof(WalIndexHdr));
  
      for(iFrame=pWal->hdr.mxFrame+1; 
          ALWAYS(rc==SQLITE_OK) && iFrame<=iMax; 
          iFrame++
      ){
        /* This call cannot fail. Unless the page for which the page number
        ** is passed as the second argument is (a) in the cache and 
        ** (b) has an outstanding reference, then xUndo is either a no-op
        ** (if (a) is false) or simply expels the page from the cache (if (b)
        ** is false).
        **
        ** If the upper layer is doing a rollback, it is guaranteed that there
        ** are no outstanding references to any page other than page 1. And
        ** page 1 is never written to the log until the transaction is
        ** committed. As a result, the call to xUndo may not fail.
        */
        assert( walFramePgno(pWal, iFrame)!=1 );
        rc = xUndo(pUndoCtx, walFramePgno(pWal, iFrame));
      }
      if( iMax!=pWal->hdr.mxFrame ) walCleanupHash(pWal);
    }
    return rc;
  }
  
  /* 
  ** Argument aWalData must point to an array of WAL_SAVEPOINT_NDATA u32 
  ** values. This function populates the array with values required to 
  ** "rollback" the write position of the WAL handle back to the current 
  ** point in the event of a savepoint rollback (via WalSavepointUndo()).
  */
  void sqlite3WalSavepoint(Wal *pWal, u32 *aWalData){
    assert( pWal->writeLock );
    aWalData[0] = pWal->hdr.mxFrame;
    aWalData[1] = pWal->hdr.aFrameCksum[0];
    aWalData[2] = pWal->hdr.aFrameCksum[1];
    aWalData[3] = pWal->nCkpt;
  }
  
  /* 
  ** Move the write position of the WAL back to the point identified by
  ** the values in the aWalData[] array. aWalData must point to an array
  ** of WAL_SAVEPOINT_NDATA u32 values that has been previously populated
  ** by a call to WalSavepoint().
  */
  int sqlite3WalSavepointUndo(Wal *pWal, u32 *aWalData){
    int rc = SQLITE_OK;
  
    assert( pWal->writeLock );
    assert( aWalData[3]!=pWal->nCkpt || aWalData[0]<=pWal->hdr.mxFrame );
  
    if( aWalData[3]!=pWal->nCkpt ){
      /* This savepoint was opened immediately after the write-transaction
      ** was started. Right after that, the writer decided to wrap around
      ** to the start of the log. Update the savepoint values to match.
      */
      aWalData[0] = 0;
      aWalData[3] = pWal->nCkpt;
    }
  
    if( aWalData[0]<pWal->hdr.mxFrame ){
      pWal->hdr.mxFrame = aWalData[0];
      pWal->hdr.aFrameCksum[0] = aWalData[1];
      pWal->hdr.aFrameCksum[1] = aWalData[2];
      walCleanupHash(pWal);
    }
  
    return rc;
  }
  
  /*
  ** This function is called just before writing a set of frames to the log
  ** file (see sqlite3WalFrames()). It checks to see if, instead of appending
  ** to the current log file, it is possible to overwrite the start of the
  ** existing log file with the new frames (i.e. "reset" the log). If so,
  ** it sets pWal->hdr.mxFrame to 0. Otherwise, pWal->hdr.mxFrame is left
  ** unchanged.
  **
  ** SQLITE_OK is returned if no error is encountered (regardless of whether
  ** or not pWal->hdr.mxFrame is modified). An SQLite error code is returned
  ** if an error occurs.
  */
  static int walRestartLog(Wal *pWal){
    int rc = SQLITE_OK;
    int cnt;
  
    if( pWal->readLock==0 ){
      volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
      assert( pInfo->nBackfill==pWal->hdr.mxFrame );
      if( pInfo->nBackfill>0 ){
        u32 salt1;
        sqlite3_randomness(4, &salt1);
        rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
        if( rc==SQLITE_OK ){
          /* If all readers are using WAL_READ_LOCK(0) (in other words if no
          ** readers are currently using the WAL), then the transactions
          ** frames will overwrite the start of the existing log. Update the
          ** wal-index header to reflect this.
          **
          ** In theory it would be Ok to update the cache of the header only
          ** at this point. But updating the actual wal-index header is also
          ** safe and means there is no special case for sqlite3WalUndo()
          ** to handle if this transaction is rolled back.  */
          walRestartHdr(pWal, salt1);
          walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
        }else if( rc!=SQLITE_BUSY ){
          return rc;
        }
      }
      walUnlockShared(pWal, WAL_READ_LOCK(0));
      pWal->readLock = -1;
      cnt = 0;
      do{
        int notUsed;
        rc = walTryBeginRead(pWal, &notUsed, 1, ++cnt);
      }while( rc==WAL_RETRY );
      assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
      testcase( (rc&0xff)==SQLITE_IOERR );
      testcase( rc==SQLITE_PROTOCOL );
      testcase( rc==SQLITE_OK );
    }
    return rc;
  }
  
  /*
  ** Information about the current state of the WAL file and where
  ** the next fsync should occur - passed from sqlite3WalFrames() into
  ** walWriteToLog().
  */
  typedef struct WalWriter {
    Wal *pWal;                   /* The complete WAL information */
    sqlite3_file *pFd;           /* The WAL file to which we write */
    sqlite3_int64 iSyncPoint;    /* Fsync at this offset */
    int syncFlags;               /* Flags for the fsync */
    int szPage;                  /* Size of one page */
  } WalWriter;
  
  /*
  ** Write iAmt bytes of content into the WAL file beginning at iOffset.
  ** Do a sync when crossing the p->iSyncPoint boundary.
  **
  ** In other words, if iSyncPoint is in between iOffset and iOffset+iAmt,
  ** first write the part before iSyncPoint, then sync, then write the
  ** rest.
  */
  static int walWriteToLog(
    WalWriter *p,              /* WAL to write to */
    void *pContent,            /* Content to be written */
    int iAmt,                  /* Number of bytes to write */
    sqlite3_int64 iOffset      /* Start writing at this offset */
  ){
    int rc;
    if( iOffset<p->iSyncPoint && iOffset+iAmt>=p->iSyncPoint ){
      int iFirstAmt = (int)(p->iSyncPoint - iOffset);
      rc = sqlite3OsWrite(p->pFd, pContent, iFirstAmt, iOffset);
      if( rc ) return rc;
      iOffset += iFirstAmt;
      iAmt -= iFirstAmt;
      pContent = (void*)(iFirstAmt + (char*)pContent);
      assert( WAL_SYNC_FLAGS(p->syncFlags)!=0 );
      rc = sqlite3OsSync(p->pFd, WAL_SYNC_FLAGS(p->syncFlags));
      if( iAmt==0 || rc ) return rc;
    }
    rc = sqlite3OsWrite(p->pFd, pContent, iAmt, iOffset);
    return rc;
  }
  
  /*
  ** Write out a single frame of the WAL
  */
  static int walWriteOneFrame(
    WalWriter *p,               /* Where to write the frame */
    PgHdr *pPage,               /* The page of the frame to be written */
    int nTruncate,              /* The commit flag.  Usually 0.  >0 for commit */
    sqlite3_int64 iOffset       /* Byte offset at which to write */
  ){
    int rc;                         /* Result code from subfunctions */
    void *pData;                    /* Data actually written */
    u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-header in */
  #if defined(SQLITE_HAS_CODEC)
    if( (pData = sqlite3PagerCodec(pPage))==0 ) return SQLITE_NOMEM_BKPT;
  #else
    pData = pPage->pData;
  #endif
    walEncodeFrame(p->pWal, pPage->pgno, nTruncate, pData, aFrame);
    rc = walWriteToLog(p, aFrame, sizeof(aFrame), iOffset);
    if( rc ) return rc;
    /* Write the page data */
    rc = walWriteToLog(p, pData, p->szPage, iOffset+sizeof(aFrame));
    return rc;
  }
  
  /*
  ** This function is called as part of committing a transaction within which
  ** one or more frames have been overwritten. It updates the checksums for
  ** all frames written to the wal file by the current transaction starting
  ** with the earliest to have been overwritten.
  **
  ** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
  */
  static int walRewriteChecksums(Wal *pWal, u32 iLast){
    const int szPage = pWal->szPage;/* Database page size */
    int rc = SQLITE_OK;             /* Return code */
    u8 *aBuf;                       /* Buffer to load data from wal file into */
    u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-headers in */
    u32 iRead;                      /* Next frame to read from wal file */
    i64 iCksumOff;
  
    aBuf = sqlite3_malloc(szPage + WAL_FRAME_HDRSIZE);
    if( aBuf==0 ) return SQLITE_NOMEM_BKPT;
  
    /* Find the checksum values to use as input for the recalculating the
    ** first checksum. If the first frame is frame 1 (implying that the current
    ** transaction restarted the wal file), these values must be read from the
    ** wal-file header. Otherwise, read them from the frame header of the
    ** previous frame.  */
    assert( pWal->iReCksum>0 );
    if( pWal->iReCksum==1 ){
      iCksumOff = 24;
    }else{
      iCksumOff = walFrameOffset(pWal->iReCksum-1, szPage) + 16;
    }
    rc = sqlite3OsRead(pWal->pWalFd, aBuf, sizeof(u32)*2, iCksumOff);
    pWal->hdr.aFrameCksum[0] = sqlite3Get4byte(aBuf);
    pWal->hdr.aFrameCksum[1] = sqlite3Get4byte(&aBuf[sizeof(u32)]);
  
    iRead = pWal->iReCksum;
    pWal->iReCksum = 0;
    for(; rc==SQLITE_OK && iRead<=iLast; iRead++){
      i64 iOff = walFrameOffset(iRead, szPage);
      rc = sqlite3OsRead(pWal->pWalFd, aBuf, szPage+WAL_FRAME_HDRSIZE, iOff);
      if( rc==SQLITE_OK ){
        u32 iPgno, nDbSize;
        iPgno = sqlite3Get4byte(aBuf);
        nDbSize = sqlite3Get4byte(&aBuf[4]);
  
        walEncodeFrame(pWal, iPgno, nDbSize, &aBuf[WAL_FRAME_HDRSIZE], aFrame);
        rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOff);
      }
    }
  
    sqlite3_free(aBuf);
    return rc;
  }
  
  /* 
  ** Write a set of frames to the log. The caller must hold the write-lock
  ** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
  */
  int sqlite3WalFrames(
    Wal *pWal,                      /* Wal handle to write to */
    int szPage,                     /* Database page-size in bytes */
    PgHdr *pList,                   /* List of dirty pages to write */
    Pgno nTruncate,                 /* Database size after this commit */
    int isCommit,                   /* True if this is a commit */
    int sync_flags                  /* Flags to pass to OsSync() (or 0) */
  ){
    int rc;                         /* Used to catch return codes */
    u32 iFrame;                     /* Next frame address */
    PgHdr *p;                       /* Iterator to run through pList with. */
    PgHdr *pLast = 0;               /* Last frame in list */
    int nExtra = 0;                 /* Number of extra copies of last page */
    int szFrame;                    /* The size of a single frame */
    i64 iOffset;                    /* Next byte to write in WAL file */
    WalWriter w;                    /* The writer */
    u32 iFirst = 0;                 /* First frame that may be overwritten */
    WalIndexHdr *pLive;             /* Pointer to shared header */
  
    assert( pList );
    assert( pWal->writeLock );
  
    /* If this frame set completes a transaction, then nTruncate>0.  If
    ** nTruncate==0 then this frame set does not complete the transaction. */
    assert( (isCommit!=0)==(nTruncate!=0) );
  
  #if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
    { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
      WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
                pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
    }
  #endif
  
    pLive = (WalIndexHdr*)walIndexHdr(pWal);
    if( memcmp(&pWal->hdr, (void *)pLive, sizeof(WalIndexHdr))!=0 ){
      iFirst = pLive->mxFrame+1;
    }
  
    /* See if it is possible to write these frames into the start of the
    ** log file, instead of appending to it at pWal->hdr.mxFrame.
    */
    if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
      return rc;
    }
  
    /* If this is the first frame written into the log, write the WAL
    ** header to the start of the WAL file. See comments at the top of
    ** this source file for a description of the WAL header format.
    */
    iFrame = pWal->hdr.mxFrame;
    if( iFrame==0 ){
      u8 aWalHdr[WAL_HDRSIZE];      /* Buffer to assemble wal-header in */
      u32 aCksum[2];                /* Checksum for wal-header */
  
      sqlite3Put4byte(&aWalHdr[0], (WAL_MAGIC | SQLITE_BIGENDIAN));
      sqlite3Put4byte(&aWalHdr[4], WAL_MAX_VERSION);
      sqlite3Put4byte(&aWalHdr[8], szPage);
      sqlite3Put4byte(&aWalHdr[12], pWal->nCkpt);
      if( pWal->nCkpt==0 ) sqlite3_randomness(8, pWal->hdr.aSalt);
      memcpy(&aWalHdr[16], pWal->hdr.aSalt, 8);
      walChecksumBytes(1, aWalHdr, WAL_HDRSIZE-2*4, 0, aCksum);
      sqlite3Put4byte(&aWalHdr[24], aCksum[0]);
      sqlite3Put4byte(&aWalHdr[28], aCksum[1]);
      
      pWal->szPage = szPage;
      pWal->hdr.bigEndCksum = SQLITE_BIGENDIAN;
      pWal->hdr.aFrameCksum[0] = aCksum[0];
      pWal->hdr.aFrameCksum[1] = aCksum[1];
      pWal->truncateOnCommit = 1;
  
      rc = sqlite3OsWrite(pWal->pWalFd, aWalHdr, sizeof(aWalHdr), 0);
      WALTRACE(("WAL%p: wal-header write %s\n", pWal, rc ? "failed" : "ok"));
      if( rc!=SQLITE_OK ){
        return rc;
      }
  
      /* Sync the header (unless SQLITE_IOCAP_SEQUENTIAL is true or unless
      ** all syncing is turned off by PRAGMA synchronous=OFF).  Otherwise
      ** an out-of-order write following a WAL restart could result in
      ** database corruption.  See the ticket:
      **
      **     https://sqlite.org/src/info/ff5be73dee
      */
      if( pWal->syncHeader ){
        rc = sqlite3OsSync(pWal->pWalFd, CKPT_SYNC_FLAGS(sync_flags));
        if( rc ) return rc;
      }
    }
    assert( (int)pWal->szPage==szPage );
  
    /* Setup information needed to write frames into the WAL */
    w.pWal = pWal;
    w.pFd = pWal->pWalFd;
    w.iSyncPoint = 0;
    w.syncFlags = sync_flags;
    w.szPage = szPage;
    iOffset = walFrameOffset(iFrame+1, szPage);
    szFrame = szPage + WAL_FRAME_HDRSIZE;
  
    /* Write all frames into the log file exactly once */
    for(p=pList; p; p=p->pDirty){
      int nDbSize;   /* 0 normally.  Positive == commit flag */
  
      /* Check if this page has already been written into the wal file by
      ** the current transaction. If so, overwrite the existing frame and
      ** set Wal.writeLock to WAL_WRITELOCK_RECKSUM - indicating that 
      ** checksums must be recomputed when the transaction is committed.  */
      if( iFirst && (p->pDirty || isCommit==0) ){
        u32 iWrite = 0;
        VVA_ONLY(rc =) sqlite3WalFindFrame(pWal, p->pgno, &iWrite);
        assert( rc==SQLITE_OK || iWrite==0 );
        if( iWrite>=iFirst ){
          i64 iOff = walFrameOffset(iWrite, szPage) + WAL_FRAME_HDRSIZE;
          void *pData;
          if( pWal->iReCksum==0 || iWrite<pWal->iReCksum ){
            pWal->iReCksum = iWrite;
          }
  #if defined(SQLITE_HAS_CODEC)
          if( (pData = sqlite3PagerCodec(p))==0 ) return SQLITE_NOMEM;
  #else
          pData = p->pData;
  #endif
          rc = sqlite3OsWrite(pWal->pWalFd, pData, szPage, iOff);
          if( rc ) return rc;
          p->flags &= ~PGHDR_WAL_APPEND;
          continue;
        }
      }
  
      iFrame++;
      assert( iOffset==walFrameOffset(iFrame, szPage) );
      nDbSize = (isCommit && p->pDirty==0) ? nTruncate : 0;
      rc = walWriteOneFrame(&w, p, nDbSize, iOffset);
      if( rc ) return rc;
      pLast = p;
      iOffset += szFrame;
      p->flags |= PGHDR_WAL_APPEND;
    }
  
    /* Recalculate checksums within the wal file if required. */
    if( isCommit && pWal->iReCksum ){
      rc = walRewriteChecksums(pWal, iFrame);
      if( rc ) return rc;
    }
  
    /* If this is the end of a transaction, then we might need to pad
    ** the transaction and/or sync the WAL file.
    **
    ** Padding and syncing only occur if this set of frames complete a
    ** transaction and if PRAGMA synchronous=FULL.  If synchronous==NORMAL
    ** or synchronous==OFF, then no padding or syncing are needed.
    **
    ** If SQLITE_IOCAP_POWERSAFE_OVERWRITE is defined, then padding is not
    ** needed and only the sync is done.  If padding is needed, then the
    ** final frame is repeated (with its commit mark) until the next sector
    ** boundary is crossed.  Only the part of the WAL prior to the last
    ** sector boundary is synced; the part of the last frame that extends
    ** past the sector boundary is written after the sync.
    */
    if( isCommit && WAL_SYNC_FLAGS(sync_flags)!=0 ){
      int bSync = 1;
      if( pWal->padToSectorBoundary ){
        int sectorSize = sqlite3SectorSize(pWal->pWalFd);
        w.iSyncPoint = ((iOffset+sectorSize-1)/sectorSize)*sectorSize;
        bSync = (w.iSyncPoint==iOffset);
        testcase( bSync );
        while( iOffset<w.iSyncPoint ){
          rc = walWriteOneFrame(&w, pLast, nTruncate, iOffset);
          if( rc ) return rc;
          iOffset += szFrame;
          nExtra++;
          assert( pLast!=0 );
        }
      }
      if( bSync ){
        assert( rc==SQLITE_OK );
        rc = sqlite3OsSync(w.pFd, WAL_SYNC_FLAGS(sync_flags));
      }
    }
  
    /* If this frame set completes the first transaction in the WAL and
    ** if PRAGMA journal_size_limit is set, then truncate the WAL to the
    ** journal size limit, if possible.
    */
    if( isCommit && pWal->truncateOnCommit && pWal->mxWalSize>=0 ){
      i64 sz = pWal->mxWalSize;
      if( walFrameOffset(iFrame+nExtra+1, szPage)>pWal->mxWalSize ){
        sz = walFrameOffset(iFrame+nExtra+1, szPage);
      }
      walLimitSize(pWal, sz);
      pWal->truncateOnCommit = 0;
    }
  
    /* Append data to the wal-index. It is not necessary to lock the 
    ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index
    ** guarantees that there are no other writers, and no data that may
    ** be in use by existing readers is being overwritten.
    */
    iFrame = pWal->hdr.mxFrame;
    for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){
      if( (p->flags & PGHDR_WAL_APPEND)==0 ) continue;
      iFrame++;
      rc = walIndexAppend(pWal, iFrame, p->pgno);
    }
    assert( pLast!=0 || nExtra==0 );
    while( rc==SQLITE_OK && nExtra>0 ){
      iFrame++;
      nExtra--;
      rc = walIndexAppend(pWal, iFrame, pLast->pgno);
    }
  
    if( rc==SQLITE_OK ){
      /* Update the private copy of the header. */
      pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
      testcase( szPage<=32768 );
      testcase( szPage>=65536 );
      pWal->hdr.mxFrame = iFrame;
      if( isCommit ){
        pWal->hdr.iChange++;
        pWal->hdr.nPage = nTruncate;
      }
      /* If this is a commit, update the wal-index header too. */
      if( isCommit ){
        walIndexWriteHdr(pWal);
        pWal->iCallback = iFrame;
      }
    }
  
    WALTRACE(("WAL%p: frame write %s\n", pWal, rc ? "failed" : "ok"));
    return rc;
  }
  
  /* 
  ** This routine is called to implement sqlite3_wal_checkpoint() and
  ** related interfaces.
  **
  ** Obtain a CHECKPOINT lock and then backfill as much information as
  ** we can from WAL into the database.
  **
  ** If parameter xBusy is not NULL, it is a pointer to a busy-handler
  ** callback. In this case this function runs a blocking checkpoint.
  */
  int sqlite3WalCheckpoint(
    Wal *pWal,                      /* Wal connection */
    sqlite3 *db,                    /* Check this handle's interrupt flag */
    int eMode,                      /* PASSIVE, FULL, RESTART, or TRUNCATE */
    int (*xBusy)(void*),            /* Function to call when busy */
    void *pBusyArg,                 /* Context argument for xBusyHandler */
    int sync_flags,                 /* Flags to sync db file with (or 0) */
    int nBuf,                       /* Size of temporary buffer */
    u8 *zBuf,                       /* Temporary buffer to use */
    int *pnLog,                     /* OUT: Number of frames in WAL */
    int *pnCkpt                     /* OUT: Number of backfilled frames in WAL */
  ){
    int rc;                         /* Return code */
    int isChanged = 0;              /* True if a new wal-index header is loaded */
    int eMode2 = eMode;             /* Mode to pass to walCheckpoint() */
    int (*xBusy2)(void*) = xBusy;   /* Busy handler for eMode2 */
  
    assert( pWal->ckptLock==0 );
    assert( pWal->writeLock==0 );
  
    /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
    ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
    assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );
  
    if( pWal->readOnly ) return SQLITE_READONLY;
    WALTRACE(("WAL%p: checkpoint begins\n", pWal));
  
    /* IMPLEMENTATION-OF: R-62028-47212 All calls obtain an exclusive 
    ** "checkpoint" lock on the database file. */
    rc = walLockExclusive(pWal, WAL_CKPT_LOCK, 1);
    if( rc ){
      /* EVIDENCE-OF: R-10421-19736 If any other process is running a
      ** checkpoint operation at the same time, the lock cannot be obtained and
      ** SQLITE_BUSY is returned.
      ** EVIDENCE-OF: R-53820-33897 Even if there is a busy-handler configured,
      ** it will not be invoked in this case.
      */
      testcase( rc==SQLITE_BUSY );
      testcase( xBusy!=0 );
      return rc;
    }
    pWal->ckptLock = 1;
  
    /* IMPLEMENTATION-OF: R-59782-36818 The SQLITE_CHECKPOINT_FULL, RESTART and
    ** TRUNCATE modes also obtain the exclusive "writer" lock on the database
    ** file.
    **
    ** EVIDENCE-OF: R-60642-04082 If the writer lock cannot be obtained
    ** immediately, and a busy-handler is configured, it is invoked and the
    ** writer lock retried until either the busy-handler returns 0 or the
    ** lock is successfully obtained.
    */
    if( eMode!=SQLITE_CHECKPOINT_PASSIVE ){
      rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_WRITE_LOCK, 1);
      if( rc==SQLITE_OK ){
        pWal->writeLock = 1;
      }else if( rc==SQLITE_BUSY ){
        eMode2 = SQLITE_CHECKPOINT_PASSIVE;
        xBusy2 = 0;
        rc = SQLITE_OK;
      }
    }
  
    /* Read the wal-index header. */
    if( rc==SQLITE_OK ){
      rc = walIndexReadHdr(pWal, &isChanged);
      if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
        sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
      }
    }
  
    /* Copy data from the log to the database file. */
    if( rc==SQLITE_OK ){
  
      if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){
        rc = SQLITE_CORRUPT_BKPT;
      }else{
        rc = walCheckpoint(pWal, db, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
      }
  
      /* If no error occurred, set the output variables. */
      if( rc==SQLITE_OK || rc==SQLITE_BUSY ){
        if( pnLog ) *pnLog = (int)pWal->hdr.mxFrame;
        if( pnCkpt ) *pnCkpt = (int)(walCkptInfo(pWal)->nBackfill);
      }
    }
  
    if( isChanged ){
      /* If a new wal-index header was loaded before the checkpoint was 
      ** performed, then the pager-cache associated with pWal is now
      ** out of date. So zero the cached wal-index header to ensure that
      ** next time the pager opens a snapshot on this database it knows that
      ** the cache needs to be reset.
      */
      memset(&pWal->hdr, 0, sizeof(WalIndexHdr));
    }
  
    /* Release the locks. */
    sqlite3WalEndWriteTransaction(pWal);
    walUnlockExclusive(pWal, WAL_CKPT_LOCK, 1);
    pWal->ckptLock = 0;
    WALTRACE(("WAL%p: checkpoint %s\n", pWal, rc ? "failed" : "ok"));
    return (rc==SQLITE_OK && eMode!=eMode2 ? SQLITE_BUSY : rc);
  }
  
  /* Return the value to pass to a sqlite3_wal_hook callback, the
  ** number of frames in the WAL at the point of the last commit since
  ** sqlite3WalCallback() was called.  If no commits have occurred since
  ** the last call, then return 0.
  */
  int sqlite3WalCallback(Wal *pWal){
    u32 ret = 0;
    if( pWal ){
      ret = pWal->iCallback;
      pWal->iCallback = 0;
    }
    return (int)ret;
  }
  
  /*
  ** This function is called to change the WAL subsystem into or out
  ** of locking_mode=EXCLUSIVE.
  **
  ** If op is zero, then attempt to change from locking_mode=EXCLUSIVE
  ** into locking_mode=NORMAL.  This means that we must acquire a lock
  ** on the pWal->readLock byte.  If the WAL is already in locking_mode=NORMAL
  ** or if the acquisition of the lock fails, then return 0.  If the
  ** transition out of exclusive-mode is successful, return 1.  This
  ** operation must occur while the pager is still holding the exclusive
  ** lock on the main database file.
  **
  ** If op is one, then change from locking_mode=NORMAL into 
  ** locking_mode=EXCLUSIVE.  This means that the pWal->readLock must
  ** be released.  Return 1 if the transition is made and 0 if the
  ** WAL is already in exclusive-locking mode - meaning that this
  ** routine is a no-op.  The pager must already hold the exclusive lock
  ** on the main database file before invoking this operation.
  **
  ** If op is negative, then do a dry-run of the op==1 case but do
  ** not actually change anything. The pager uses this to see if it
  ** should acquire the database exclusive lock prior to invoking
  ** the op==1 case.
  */
  int sqlite3WalExclusiveMode(Wal *pWal, int op){
    int rc;
    assert( pWal->writeLock==0 );
    assert( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE || op==-1 );
  
    /* pWal->readLock is usually set, but might be -1 if there was a 
    ** prior error while attempting to acquire are read-lock. This cannot 
    ** happen if the connection is actually in exclusive mode (as no xShmLock
    ** locks are taken in this case). Nor should the pager attempt to
    ** upgrade to exclusive-mode following such an error.
    */
    assert( pWal->readLock>=0 || pWal->lockError );
    assert( pWal->readLock>=0 || (op<=0 && pWal->exclusiveMode==0) );
  
    if( op==0 ){
      if( pWal->exclusiveMode!=WAL_NORMAL_MODE ){
        pWal->exclusiveMode = WAL_NORMAL_MODE;
        if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){
          pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
        }
        rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
      }else{
        /* Already in locking_mode=NORMAL */
        rc = 0;
      }
    }else if( op>0 ){
      assert( pWal->exclusiveMode==WAL_NORMAL_MODE );
      assert( pWal->readLock>=0 );
      walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
      pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
      rc = 1;
    }else{
      rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
    }
    return rc;
  }
  
  /* 
  ** Return true if the argument is non-NULL and the WAL module is using
  ** heap-memory for the wal-index. Otherwise, if the argument is NULL or the
  ** WAL module is using shared-memory, return false. 
  */
  int sqlite3WalHeapMemory(Wal *pWal){
    return (pWal && pWal->exclusiveMode==WAL_HEAPMEMORY_MODE );
  }
  
  #ifdef SQLITE_ENABLE_SNAPSHOT
  /* Create a snapshot object.  The content of a snapshot is opaque to
  ** every other subsystem, so the WAL module can put whatever it needs
  ** in the object.
  */
  int sqlite3WalSnapshotGet(Wal *pWal, sqlite3_snapshot **ppSnapshot){
    int rc = SQLITE_OK;
    WalIndexHdr *pRet;
    static const u32 aZero[4] = { 0, 0, 0, 0 };
  
    assert( pWal->readLock>=0 && pWal->writeLock==0 );
  
    if( memcmp(&pWal->hdr.aFrameCksum[0],aZero,16)==0 ){
      *ppSnapshot = 0;
      return SQLITE_ERROR;
    }
    pRet = (WalIndexHdr*)sqlite3_malloc(sizeof(WalIndexHdr));
    if( pRet==0 ){
      rc = SQLITE_NOMEM_BKPT;
    }else{
      memcpy(pRet, &pWal->hdr, sizeof(WalIndexHdr));
      *ppSnapshot = (sqlite3_snapshot*)pRet;
    }
  
    return rc;
  }
  
  /* Try to open on pSnapshot when the next read-transaction starts
  */
  void sqlite3WalSnapshotOpen(Wal *pWal, sqlite3_snapshot *pSnapshot){
    pWal->pSnapshot = (WalIndexHdr*)pSnapshot;
  }
  
  /* 
  ** Return a +ve value if snapshot p1 is newer than p2. A -ve value if
  ** p1 is older than p2 and zero if p1 and p2 are the same snapshot.
  */
  int sqlite3_snapshot_cmp(sqlite3_snapshot *p1, sqlite3_snapshot *p2){
    WalIndexHdr *pHdr1 = (WalIndexHdr*)p1;
    WalIndexHdr *pHdr2 = (WalIndexHdr*)p2;
  
    /* aSalt[0] is a copy of the value stored in the wal file header. It
    ** is incremented each time the wal file is restarted.  */
    if( pHdr1->aSalt[0]<pHdr2->aSalt[0] ) return -1;
    if( pHdr1->aSalt[0]>pHdr2->aSalt[0] ) return +1;
    if( pHdr1->mxFrame<pHdr2->mxFrame ) return -1;
    if( pHdr1->mxFrame>pHdr2->mxFrame ) return +1;
    return 0;
  }
  
  /*
  ** The caller currently has a read transaction open on the database.
  ** This function takes a SHARED lock on the CHECKPOINTER slot and then
  ** checks if the snapshot passed as the second argument is still 
  ** available. If so, SQLITE_OK is returned.
  **
  ** If the snapshot is not available, SQLITE_ERROR is returned. Or, if
  ** the CHECKPOINTER lock cannot be obtained, SQLITE_BUSY. If any error
  ** occurs (any value other than SQLITE_OK is returned), the CHECKPOINTER
  ** lock is released before returning.
  */
  int sqlite3WalSnapshotCheck(Wal *pWal, sqlite3_snapshot *pSnapshot){
    int rc;
    rc = walLockShared(pWal, WAL_CKPT_LOCK);
    if( rc==SQLITE_OK ){
      WalIndexHdr *pNew = (WalIndexHdr*)pSnapshot;
      if( memcmp(pNew->aSalt, pWal->hdr.aSalt, sizeof(pWal->hdr.aSalt))
       || pNew->mxFrame<walCkptInfo(pWal)->nBackfillAttempted
      ){
        rc = SQLITE_ERROR_SNAPSHOT;
        walUnlockShared(pWal, WAL_CKPT_LOCK);
      }
    }
    return rc;
  }
  
  /*
  ** Release a lock obtained by an earlier successful call to
  ** sqlite3WalSnapshotCheck().
  */
  void sqlite3WalSnapshotUnlock(Wal *pWal){
    assert( pWal );
    walUnlockShared(pWal, WAL_CKPT_LOCK);
  }
  
  
  #endif /* SQLITE_ENABLE_SNAPSHOT */
  
  #ifdef SQLITE_ENABLE_ZIPVFS
  /*
  ** If the argument is not NULL, it points to a Wal object that holds a
  ** read-lock. This function returns the database page-size if it is known,
  ** or zero if it is not (or if pWal is NULL).
  */
  int sqlite3WalFramesize(Wal *pWal){
    assert( pWal==0 || pWal->readLock>=0 );
    return (pWal ? pWal->szPage : 0);
  }
  #endif
  
  /* Return the sqlite3_file object for the WAL file
  */
  sqlite3_file *sqlite3WalFile(Wal *pWal){
    return pWal->pWalFd;
  }
  
  #endif /* #ifndef SQLITE_OMIT_WAL */
  