text: /*
  ** 2001 September 15
  **
  ** The author disclaims copyright to this source code.  In place of
  ** a legal notice, here is a blessing:
  **
  **    May you do good and not evil.
  **    May you find forgiveness for yourself and forgive others.
  **    May you share freely, never taking more than you give.
  **
  *************************************************************************
  ** This is the implementation of the page cache subsystem or "pager".
  ** 
  ** The pager is used to access a database disk file.  It implements
  ** atomic commit and rollback through the use of a journal file that
  ** is separate from the database file.  The pager also implements file
  ** locking to prevent two processes from writing the same database
  ** file simultaneously, or one process from reading the database while
  ** another is writing.
  **
  ** @(#) $Id$
  */
  #include "os.h"         /* Must be first to enable large file support */
  #include "sqliteInt.h"
  #include "pager.h"
  #include <assert.h>
  #include <string.h>
  
  /*
  ** Macros for troubleshooting.  Normally turned off
  */
  #if 0
  static Pager *mainPager = 0;
  #define SET_PAGER(X)  if( mainPager==0 ) mainPager = (X)
  #define CLR_PAGER(X)  if( mainPager==(X) ) mainPager = 0
  #define TRACE1(X)     if( pPager==mainPager ) fprintf(stderr,X)
  #define TRACE2(X,Y)   if( pPager==mainPager ) fprintf(stderr,X,Y)
  #define TRACE3(X,Y,Z) if( pPager==mainPager ) fprintf(stderr,X,Y,Z)
  #else
  #define SET_PAGER(X)
  #define CLR_PAGER(X)
  #define TRACE1(X)
  #define TRACE2(X,Y)
  #define TRACE3(X,Y,Z)
  #endif
  
  
  /*
  ** The page cache as a whole is always in one of the following
  ** states:
  **
  **   SQLITE_UNLOCK       The page cache is not currently reading or 
  **                       writing the database file.  There is no
  **                       data held in memory.  This is the initial
  **                       state.
  **
  **   SQLITE_READLOCK     The page cache is reading the database.
  **                       Writing is not permitted.  There can be
  **                       multiple readers accessing the same database
  **                       file at the same time.
  **
  **   SQLITE_WRITELOCK    The page cache is writing the database.
  **                       Access is exclusive.  No other processes or
  **                       threads can be reading or writing while one
  **                       process is writing.
  **
  ** The page cache comes up in SQLITE_UNLOCK.  The first time a
  ** sqlite_page_get() occurs, the state transitions to SQLITE_READLOCK.
  ** After all pages have been released using sqlite_page_unref(),
  ** the state transitions back to SQLITE_UNLOCK.  The first time
  ** that sqlite_page_write() is called, the state transitions to
  ** SQLITE_WRITELOCK.  (Note that sqlite_page_write() can only be
  ** called on an outstanding page which means that the pager must
  ** be in SQLITE_READLOCK before it transitions to SQLITE_WRITELOCK.)
  ** The sqlite_page_rollback() and sqlite_page_commit() functions 
  ** transition the state from SQLITE_WRITELOCK back to SQLITE_READLOCK.
  */
  #define SQLITE_UNLOCK      0
  #define SQLITE_READLOCK    1
  #define SQLITE_WRITELOCK   2
  
  
  /*
  ** Each in-memory image of a page begins with the following header.
  ** This header is only visible to this pager module.  The client
  ** code that calls pager sees only the data that follows the header.
  **
  ** Client code should call sqlitepager_write() on a page prior to making
  ** any modifications to that page.  The first time sqlitepager_write()
  ** is called, the original page contents are written into the rollback
  ** journal and PgHdr.inJournal and PgHdr.needSync are set.  Later, once
  ** the journal page has made it onto the disk surface, PgHdr.needSync
  ** is cleared.  The modified page cannot be written back into the original
  ** database file until the journal pages has been synced to disk and the
  ** PgHdr.needSync has been cleared.
  **
  ** The PgHdr.dirty flag is set when sqlitepager_write() is called and
  ** is cleared again when the page content is written back to the original
  ** database file.
  */
  typedef struct PgHdr PgHdr;
  struct PgHdr {
    Pager *pPager;                 /* The pager to which this page belongs */
    Pgno pgno;                     /* The page number for this page */
    PgHdr *pNextHash, *pPrevHash;  /* Hash collision chain for PgHdr.pgno */
    int nRef;                      /* Number of users of this page */
    PgHdr *pNextFree, *pPrevFree;  /* Freelist of pages where nRef==0 */
    PgHdr *pNextAll, *pPrevAll;    /* A list of all pages */
    PgHdr *pNextCkpt, *pPrevCkpt;  /* List of pages in the checkpoint journal */
    u8 inJournal;                  /* TRUE if has been written to journal */
    u8 inCkpt;                     /* TRUE if written to the checkpoint journal */
    u8 dirty;                      /* TRUE if we need to write back changes */
    u8 needSync;                   /* Sync journal before writing this page */
    u8 alwaysRollback;             /* Disable dont_rollback() for this page */
    PgHdr *pDirty;                 /* Dirty pages sorted by PgHdr.pgno */
    /* SQLITE_PAGE_SIZE bytes of page data follow this header */
    /* Pager.nExtra bytes of local data follow the page data */
  };
  
  
  /*
  ** A macro used for invoking the codec if there is one
  */
  #ifdef SQLITE_HAS_CODEC
  # define CODEC(P,D,N,X) if( P->xCodec ){ P->xCodec(P->pCodecArg,D,N,X); }
  #else
  # define CODEC(P,D,N,X)
  #endif
  
  /*
  ** Convert a pointer to a PgHdr into a pointer to its data
  ** and back again.
  */
  #define PGHDR_TO_DATA(P)  ((void*)(&(P)[1]))
  #define DATA_TO_PGHDR(D)  (&((PgHdr*)(D))[-1])
  #define PGHDR_TO_EXTRA(P) ((void*)&((char*)(&(P)[1]))[SQLITE_PAGE_SIZE])
  
  /*
  ** How big to make the hash table used for locating in-memory pages
  ** by page number.
  */
  #define N_PG_HASH 2048
  
  /*
  ** Hash a page number
  */
  #define pager_hash(PN)  ((PN)&(N_PG_HASH-1))
  
  /*
  ** A open page cache is an instance of the following structure.
  */
  struct Pager {
    char *zFilename;            /* Name of the database file */
    char *zJournal;             /* Name of the journal file */
    char *zDirectory;           /* Directory hold database and journal files */
    OsFile fd, jfd;             /* File descriptors for database and journal */
    OsFile cpfd;                /* File descriptor for the checkpoint journal */
    int dbSize;                 /* Number of pages in the file */
    int origDbSize;             /* dbSize before the current change */
    int ckptSize;               /* Size of database (in pages) at ckpt_begin() */
    off_t ckptJSize;            /* Size of journal at ckpt_begin() */
    int nRec;                   /* Number of pages written to the journal */
    u32 cksumInit;              /* Quasi-random value added to every checksum */
    int ckptNRec;               /* Number of records in the checkpoint journal */
    int nExtra;                 /* Add this many bytes to each in-memory page */
    void (*xDestructor)(void*); /* Call this routine when freeing pages */
    int nPage;                  /* Total number of in-memory pages */
    int nRef;                   /* Number of in-memory pages with PgHdr.nRef>0 */
    int mxPage;                 /* Maximum number of pages to hold in cache */
    int nHit, nMiss, nOvfl;     /* Cache hits, missing, and LRU overflows */
    void (*xCodec)(void*,void*,Pgno,int); /* Routine for en/decoding data */
    void *pCodecArg;            /* First argument to xCodec() */
    u8 journalOpen;             /* True if journal file descriptors is valid */
    u8 journalStarted;          /* True if header of journal is synced */
    u8 useJournal;              /* Use a rollback journal on this file */
    u8 ckptOpen;                /* True if the checkpoint journal is open */
    u8 ckptInUse;               /* True we are in a checkpoint */
    u8 ckptAutoopen;            /* Open ckpt journal when main journal is opened*/
    u8 noSync;                  /* Do not sync the journal if true */
    u8 fullSync;                /* Do extra syncs of the journal for robustness */
    u8 state;                   /* SQLITE_UNLOCK, _READLOCK or _WRITELOCK */
    u8 errMask;                 /* One of several kinds of errors */
    u8 tempFile;                /* zFilename is a temporary file */
    u8 readOnly;                /* True for a read-only database */
    u8 needSync;                /* True if an fsync() is needed on the journal */
    u8 dirtyFile;               /* True if database file has changed in any way */
    u8 alwaysRollback;          /* Disable dont_rollback() for all pages */
    u8 *aInJournal;             /* One bit for each page in the database file */
    u8 *aInCkpt;                /* One bit for each page in the database */
    PgHdr *pFirst, *pLast;      /* List of free pages */
    PgHdr *pFirstSynced;        /* First free page with PgHdr.needSync==0 */
    PgHdr *pAll;                /* List of all pages */
    PgHdr *pCkpt;               /* List of pages in the checkpoint journal */
    PgHdr *aHash[N_PG_HASH];    /* Hash table to map page number of PgHdr */
  };
  
  /*
  ** These are bits that can be set in Pager.errMask.
  */
  #define PAGER_ERR_FULL     0x01  /* a write() failed */
  #define PAGER_ERR_MEM      0x02  /* malloc() failed */
  #define PAGER_ERR_LOCK     0x04  /* error in the locking protocol */
  #define PAGER_ERR_CORRUPT  0x08  /* database or journal corruption */
  #define PAGER_ERR_DISK     0x10  /* general disk I/O error - bad hard drive? */
  
  /*
  ** The journal file contains page records in the following
  ** format.
  **
  ** Actually, this structure is the complete page record for pager
  ** formats less than 3.  Beginning with format 3, this record is surrounded
  ** by two checksums.
  */
  typedef struct PageRecord PageRecord;
  struct PageRecord {
    Pgno pgno;                      /* The page number */
    char aData[SQLITE_PAGE_SIZE];   /* Original data for page pgno */
  };
  
  /*
  ** Journal files begin with the following magic string.  The data
  ** was obtained from /dev/random.  It is used only as a sanity check.
  **
  ** There are three journal formats (so far). The 1st journal format writes
  ** 32-bit integers in the byte-order of the host machine.  New
  ** formats writes integers as big-endian.  All new journals use the
  ** new format, but we have to be able to read an older journal in order
  ** to rollback journals created by older versions of the library.
  **
  ** The 3rd journal format (added for 2.8.0) adds additional sanity
  ** checking information to the journal.  If the power fails while the
  ** journal is being written, semi-random garbage data might appear in
  ** the journal file after power is restored.  If an attempt is then made
  ** to roll the journal back, the database could be corrupted.  The additional
  ** sanity checking data is an attempt to discover the garbage in the
  ** journal and ignore it.
  **
  ** The sanity checking information for the 3rd journal format consists
  ** of a 32-bit checksum on each page of data.  The checksum covers both
  ** the page number and the SQLITE_PAGE_SIZE bytes of data for the page.
  ** This cksum is initialized to a 32-bit random value that appears in the
  ** journal file right after the header.  The random initializer is important,
  ** because garbage data that appears at the end of a journal is likely
  ** data that was once in other files that have now been deleted.  If the
  ** garbage data came from an obsolete journal file, the checksums might
  ** be correct.  But by initializing the checksum to random value which
  ** is different for every journal, we minimize that risk.
  */
  static const unsigned char aJournalMagic1[] = {
    0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd4,
  };
  static const unsigned char aJournalMagic2[] = {
    0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd5,
  };
  static const unsigned char aJournalMagic3[] = {
    0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd6,
  };
  #define JOURNAL_FORMAT_1 1
  #define JOURNAL_FORMAT_2 2
  #define JOURNAL_FORMAT_3 3
  
  /*
  ** The following integer determines what format to use when creating
  ** new primary journal files.  By default we always use format 3.
  ** When testing, we can set this value to older journal formats in order to
  ** make sure that newer versions of the library are able to rollback older
  ** journal files.
  **
  ** Note that checkpoint journals always use format 2 and omit the header.
  */
  #ifdef SQLITE_TEST
  int journal_format = 3;
  #else
  # define journal_format 3
  #endif
  
  /*
  ** The size of the header and of each page in the journal varies according
  ** to which journal format is being used.  The following macros figure out
  ** the sizes based on format numbers.
  */
  #define JOURNAL_HDR_SZ(X) \
     (sizeof(aJournalMagic1) + sizeof(Pgno) + ((X)>=3)*2*sizeof(u32))
  #define JOURNAL_PG_SZ(X) \
     (SQLITE_PAGE_SIZE + sizeof(Pgno) + ((X)>=3)*sizeof(u32))
  
  /*
  ** Enable reference count tracking here:
  */
  #ifdef SQLITE_TEST
    int pager_refinfo_enable = 0;
    static void pager_refinfo(PgHdr *p){
      static int cnt = 0;
      if( !pager_refinfo_enable ) return;
      printf(
         "REFCNT: %4d addr=0x%08x nRef=%d\n",
         p->pgno, (int)PGHDR_TO_DATA(p), p->nRef
      );
      cnt++;   /* Something to set a breakpoint on */
    }
  # define REFINFO(X)  pager_refinfo(X)
  #else
  # define REFINFO(X)
  #endif
  
  /*
  ** Read a 32-bit integer from the given file descriptor.  Store the integer
  ** that is read in *pRes.  Return SQLITE_OK if everything worked, or an
  ** error code is something goes wrong.
  **
  ** If the journal format is 2 or 3, read a big-endian integer.  If the
  ** journal format is 1, read an integer in the native byte-order of the
  ** host machine.
  */
  static int read32bits(int format, OsFile *fd, u32 *pRes){
    u32 res;
    int rc;
    rc = sqliteOsRead(fd, &res, sizeof(res));
    if( rc==SQLITE_OK && format>JOURNAL_FORMAT_1 ){
      unsigned char ac[4];
      memcpy(ac, &res, 4);
      res = (ac[0]<<24) | (ac[1]<<16) | (ac[2]<<8) | ac[3];
    }
    *pRes = res;
    return rc;
  }
  
  /*
  ** Write a 32-bit integer into the given file descriptor.  Return SQLITE_OK
  ** on success or an error code is something goes wrong.
  **
  ** If the journal format is 2 or 3, write the integer as 4 big-endian
  ** bytes.  If the journal format is 1, write the integer in the native
  ** byte order.  In normal operation, only formats 2 and 3 are used.
  ** Journal format 1 is only used for testing.
  */
  static int write32bits(OsFile *fd, u32 val){
    unsigned char ac[4];
    if( journal_format<=1 ){
      return sqliteOsWrite(fd, &val, 4);
    }
    ac[0] = (val>>24) & 0xff;
    ac[1] = (val>>16) & 0xff;
    ac[2] = (val>>8) & 0xff;
    ac[3] = val & 0xff;
    return sqliteOsWrite(fd, ac, 4);
  }
  
  /*
  ** Write a 32-bit integer into a page header right before the
  ** page data.  This will overwrite the PgHdr.pDirty pointer.
  **
  ** The integer is big-endian for formats 2 and 3 and native byte order
  ** for journal format 1.
  */
  static void store32bits(u32 val, PgHdr *p, int offset){
    unsigned char *ac;
    ac = &((unsigned char*)PGHDR_TO_DATA(p))[offset];
    if( journal_format<=1 ){
      memcpy(ac, &val, 4);
    }else{
      ac[0] = (val>>24) & 0xff;
      ac[1] = (val>>16) & 0xff;
      ac[2] = (val>>8) & 0xff;
      ac[3] = val & 0xff;
    }
  }
  
  
  /*
  ** Convert the bits in the pPager->errMask into an approprate
  ** return code.
  */
  static int pager_errcode(Pager *pPager){
    int rc = SQLITE_OK;
    if( pPager->errMask & PAGER_ERR_LOCK )    rc = SQLITE_PROTOCOL;
    if( pPager->errMask & PAGER_ERR_DISK )    rc = SQLITE_IOERR;
    if( pPager->errMask & PAGER_ERR_FULL )    rc = SQLITE_FULL;
    if( pPager->errMask & PAGER_ERR_MEM )     rc = SQLITE_NOMEM;
    if( pPager->errMask & PAGER_ERR_CORRUPT ) rc = SQLITE_CORRUPT;
    return rc;
  }
  
  /*
  ** Add or remove a page from the list of all pages that are in the
  ** checkpoint journal.
  **
  ** The Pager keeps a separate list of pages that are currently in
  ** the checkpoint journal.  This helps the sqlitepager_ckpt_commit()
  ** routine run MUCH faster for the common case where there are many
  ** pages in memory but only a few are in the checkpoint journal.
  */
  static void page_add_to_ckpt_list(PgHdr *pPg){
    Pager *pPager = pPg->pPager;
    if( pPg->inCkpt ) return;
    assert( pPg->pPrevCkpt==0 && pPg->pNextCkpt==0 );
    pPg->pPrevCkpt = 0;
    if( pPager->pCkpt ){
      pPager->pCkpt->pPrevCkpt = pPg;
    }
    pPg->pNextCkpt = pPager->pCkpt;
    pPager->pCkpt = pPg;
    pPg->inCkpt = 1;
  }
  static void page_remove_from_ckpt_list(PgHdr *pPg){
    if( !pPg->inCkpt ) return;
    if( pPg->pPrevCkpt ){
      assert( pPg->pPrevCkpt->pNextCkpt==pPg );
      pPg->pPrevCkpt->pNextCkpt = pPg->pNextCkpt;
    }else{
      assert( pPg->pPager->pCkpt==pPg );
      pPg->pPager->pCkpt = pPg->pNextCkpt;
    }
    if( pPg->pNextCkpt ){
      assert( pPg->pNextCkpt->pPrevCkpt==pPg );
      pPg->pNextCkpt->pPrevCkpt = pPg->pPrevCkpt;
    }
    pPg->pNextCkpt = 0;
    pPg->pPrevCkpt = 0;
    pPg->inCkpt = 0;
  }
  
  /*
  ** Find a page in the hash table given its page number.  Return
  ** a pointer to the page or NULL if not found.
  */
  static PgHdr *pager_lookup(Pager *pPager, Pgno pgno){
    PgHdr *p = pPager->aHash[pager_hash(pgno)];
    while( p && p->pgno!=pgno ){
      p = p->pNextHash;
    }
    return p;
  }
  
  /*
  ** Unlock the database and clear the in-memory cache.  This routine
  ** sets the state of the pager back to what it was when it was first
  ** opened.  Any outstanding pages are invalidated and subsequent attempts
  ** to access those pages will likely result in a coredump.
  */
  static void pager_reset(Pager *pPager){
    PgHdr *pPg, *pNext;
    for(pPg=pPager->pAll; pPg; pPg=pNext){
      pNext = pPg->pNextAll;
      sqliteFree(pPg);
    }
    pPager->pFirst = 0;
    pPager->pFirstSynced = 0;
    pPager->pLast = 0;
    pPager->pAll = 0;
    memset(pPager->aHash, 0, sizeof(pPager->aHash));
    pPager->nPage = 0;
    if( pPager->state>=SQLITE_WRITELOCK ){
      sqlitepager_rollback(pPager);
    }
    sqliteOsUnlock(&pPager->fd);
    pPager->state = SQLITE_UNLOCK;
    pPager->dbSize = -1;
    pPager->nRef = 0;
    assert( pPager->journalOpen==0 );
  }
  
  /*
  ** When this routine is called, the pager has the journal file open and
  ** a write lock on the database.  This routine releases the database
  ** write lock and acquires a read lock in its place.  The journal file
  ** is deleted and closed.
  **
  ** TODO: Consider keeping the journal file open for temporary databases.
  ** This might give a performance improvement on windows where opening
  ** a file is an expensive operation.
  */
  static int pager_unwritelock(Pager *pPager){
    int rc;
    PgHdr *pPg;
    if( pPager->state<SQLITE_WRITELOCK ) return SQLITE_OK;
    sqlitepager_ckpt_commit(pPager);
    if( pPager->ckptOpen ){
      sqliteOsClose(&pPager->cpfd);
      pPager->ckptOpen = 0;
    }
    if( pPager->journalOpen ){
      sqliteOsClose(&pPager->jfd);
      pPager->journalOpen = 0;
      sqliteOsDelete(pPager->zJournal);
      sqliteFree( pPager->aInJournal );
      pPager->aInJournal = 0;
      for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
        pPg->inJournal = 0;
        pPg->dirty = 0;
        pPg->needSync = 0;
      }
    }else{
      assert( pPager->dirtyFile==0 || pPager->useJournal==0 );
    }
    rc = sqliteOsReadLock(&pPager->fd);
    if( rc==SQLITE_OK ){
      pPager->state = SQLITE_READLOCK;
    }else{
      /* This can only happen if a process does a BEGIN, then forks and the
      ** child process does the COMMIT.  Because of the semantics of unix
      ** file locking, the unlock will fail.
      */
      pPager->state = SQLITE_UNLOCK;
    }
    return rc;
  }
  
  /*
  ** Compute and return a checksum for the page of data.
  **
  ** This is not a real checksum.  It is really just the sum of the 
  ** random initial value and the page number.  We considered do a checksum
  ** of the database, but that was found to be too slow.
  */
  static u32 pager_cksum(Pager *pPager, Pgno pgno, const char *aData){
    u32 cksum = pPager->cksumInit + pgno;
    return cksum;
  }
  
  /*
  ** Read a single page from the journal file opened on file descriptor
  ** jfd.  Playback this one page.
  **
  ** There are three different journal formats.  The format parameter determines
  ** which format is used by the journal that is played back.
  */
  static int pager_playback_one_page(Pager *pPager, OsFile *jfd, int format){
    int rc;
    PgHdr *pPg;              /* An existing page in the cache */
    PageRecord pgRec;
    u32 cksum;
  
    rc = read32bits(format, jfd, &pgRec.pgno);
    if( rc!=SQLITE_OK ) return rc;
    rc = sqliteOsRead(jfd, &pgRec.aData, sizeof(pgRec.aData));
    if( rc!=SQLITE_OK ) return rc;
  
    /* Sanity checking on the page.  This is more important that I originally
    ** thought.  If a power failure occurs while the journal is being written,
    ** it could cause invalid data to be written into the journal.  We need to
    ** detect this invalid data (with high probability) and ignore it.
    */
    if( pgRec.pgno==0 ){
      return SQLITE_DONE;
    }
    if( pgRec.pgno>(unsigned)pPager->dbSize ){
      return SQLITE_OK;
    }
    if( format>=JOURNAL_FORMAT_3 ){
      rc = read32bits(format, jfd, &cksum);
      if( rc ) return rc;
      if( pager_cksum(pPager, pgRec.pgno, pgRec.aData)!=cksum ){
        return SQLITE_DONE;
      }
    }
  
    /* Playback the page.  Update the in-memory copy of the page
    ** at the same time, if there is one.
    */
    pPg = pager_lookup(pPager, pgRec.pgno);
    TRACE2("PLAYBACK %d\n", pgRec.pgno);
    sqliteOsSeek(&pPager->fd, (pgRec.pgno-1)*(off_t)SQLITE_PAGE_SIZE);
    rc = sqliteOsWrite(&pPager->fd, pgRec.aData, SQLITE_PAGE_SIZE);
    if( pPg ){
      /* No page should ever be rolled back that is in use, except for page
      ** 1 which is held in use in order to keep the lock on the database
      ** active.
      */
      assert( pPg->nRef==0 || pPg->pgno==1 );
      memcpy(PGHDR_TO_DATA(pPg), pgRec.aData, SQLITE_PAGE_SIZE);
      memset(PGHDR_TO_EXTRA(pPg), 0, pPager->nExtra);
      pPg->dirty = 0;
      pPg->needSync = 0;
      CODEC(pPager, PGHDR_TO_DATA(pPg), pPg->pgno, 3);
    }
    return rc;
  }
  
  /*
  ** Playback the journal and thus restore the database file to
  ** the state it was in before we started making changes.  
  **
  ** The journal file format is as follows: 
  **
  **    *  8 byte prefix.  One of the aJournalMagic123 vectors defined
  **       above.  The format of the journal file is determined by which
  **       of the three prefix vectors is seen.
  **    *  4 byte big-endian integer which is the number of valid page records
  **       in the journal.  If this value is 0xffffffff, then compute the
  **       number of page records from the journal size.  This field appears
  **       in format 3 only.
  **    *  4 byte big-endian integer which is the initial value for the 
  **       sanity checksum.  This field appears in format 3 only.
  **    *  4 byte integer which is the number of pages to truncate the
  **       database to during a rollback.
  **    *  Zero or more pages instances, each as follows:
  **        +  4 byte page number.
  **        +  SQLITE_PAGE_SIZE bytes of data.
  **        +  4 byte checksum (format 3 only)
  **
  ** When we speak of the journal header, we mean the first 4 bullets above.
  ** Each entry in the journal is an instance of the 5th bullet.  Note that
  ** bullets 2 and 3 only appear in format-3 journals.
  **
  ** Call the value from the second bullet "nRec".  nRec is the number of
  ** valid page entries in the journal.  In most cases, you can compute the
  ** value of nRec from the size of the journal file.  But if a power
  ** failure occurred while the journal was being written, it could be the
  ** case that the size of the journal file had already been increased but
  ** the extra entries had not yet made it safely to disk.  In such a case,
  ** the value of nRec computed from the file size would be too large.  For
  ** that reason, we always use the nRec value in the header.
  **
  ** If the nRec value is 0xffffffff it means that nRec should be computed
  ** from the file size.  This value is used when the user selects the
  ** no-sync option for the journal.  A power failure could lead to corruption
  ** in this case.  But for things like temporary table (which will be
  ** deleted when the power is restored) we don't care.  
  **
  ** Journal formats 1 and 2 do not have an nRec value in the header so we
  ** have to compute nRec from the file size.  This has risks (as described
  ** above) which is why all persistent tables have been changed to use
  ** format 3.
  **
  ** If the file opened as the journal file is not a well-formed
  ** journal file then the database will likely already be
  ** corrupted, so the PAGER_ERR_CORRUPT bit is set in pPager->errMask
  ** and SQLITE_CORRUPT is returned.  If it all works, then this routine
  ** returns SQLITE_OK.
  */
  static int pager_playback(Pager *pPager, int useJournalSize){
    off_t szJ;               /* Size of the journal file in bytes */
    int nRec;                /* Number of Records in the journal */
    int i;                   /* Loop counter */
    Pgno mxPg = 0;           /* Size of the original file in pages */
    int format;              /* Format of the journal file. */
    unsigned char aMagic[sizeof(aJournalMagic1)];
    int rc;
  
    /* Figure out how many records are in the journal.  Abort early if
    ** the journal is empty.
    */
    assert( pPager->journalOpen );
    sqliteOsSeek(&pPager->jfd, 0);
    rc = sqliteOsFileSize(&pPager->jfd, &szJ);
    if( rc!=SQLITE_OK ){
      goto end_playback;
    }
  
    /* If the journal file is too small to contain a complete header,
    ** it must mean that the process that created the journal was just
    ** beginning to write the journal file when it died.  In that case,
    ** the database file should have still been completely unchanged.
    ** Nothing needs to be rolled back.  We can safely ignore this journal.
    */
    if( szJ < sizeof(aMagic)+sizeof(Pgno) ){
      goto end_playback;
    }
  
    /* Read the beginning of the journal and truncate the
    ** database file back to its original size.
    */
    rc = sqliteOsRead(&pPager->jfd, aMagic, sizeof(aMagic));
    if( rc!=SQLITE_OK ){
      rc = SQLITE_PROTOCOL;
      goto end_playback;
    }
    if( memcmp(aMagic, aJournalMagic3, sizeof(aMagic))==0 ){
      format = JOURNAL_FORMAT_3;
    }else if( memcmp(aMagic, aJournalMagic2, sizeof(aMagic))==0 ){
      format = JOURNAL_FORMAT_2;
    }else if( memcmp(aMagic, aJournalMagic1, sizeof(aMagic))==0 ){
      format = JOURNAL_FORMAT_1;
    }else{
      rc = SQLITE_PROTOCOL;
      goto end_playback;
    }
    if( format>=JOURNAL_FORMAT_3 ){
      if( szJ < sizeof(aMagic) + 3*sizeof(u32) ){
        /* Ignore the journal if it is too small to contain a complete
        ** header.  We already did this test once above, but at the prior
        ** test, we did not know the journal format and so we had to assume
        ** the smallest possible header.  Now we know the header is bigger
        ** than the minimum so we test again.
        */
        goto end_playback;
      }
      rc = read32bits(format, &pPager->jfd, (u32*)&nRec);
      if( rc ) goto end_playback;
      rc = read32bits(format, &pPager->jfd, &pPager->cksumInit);
      if( rc ) goto end_playback;
      if( nRec==0xffffffff || useJournalSize ){
        nRec = (szJ - JOURNAL_HDR_SZ(3))/JOURNAL_PG_SZ(3);
      }
    }else{
      nRec = (szJ - JOURNAL_HDR_SZ(2))/JOURNAL_PG_SZ(2);
      assert( nRec*JOURNAL_PG_SZ(2)+JOURNAL_HDR_SZ(2)==szJ );
    }
    rc = read32bits(format, &pPager->jfd, &mxPg);
    if( rc!=SQLITE_OK ){
      goto end_playback;
    }
    assert( pPager->origDbSize==0 || pPager->origDbSize==mxPg );
    rc = sqliteOsTruncate(&pPager->fd, SQLITE_PAGE_SIZE*(off_t)mxPg);
    if( rc!=SQLITE_OK ){
      goto end_playback;
    }
    pPager->dbSize = mxPg;
    
    /* Copy original pages out of the journal and back into the database file.
    */
    for(i=0; i<nRec; i++){
      rc = pager_playback_one_page(pPager, &pPager->jfd, format);
      if( rc!=SQLITE_OK ){
        if( rc==SQLITE_DONE ){
          rc = SQLITE_OK;
        }
        break;
      }
    }
  
    /* Pages that have been written to the journal but never synced
    ** where not restored by the loop above.  We have to restore those
    ** pages by reading them back from the original database.
    */
    if( rc==SQLITE_OK ){
      PgHdr *pPg;
      for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
        char zBuf[SQLITE_PAGE_SIZE];
        if( !pPg->dirty ) continue;
        if( (int)pPg->pgno <= pPager->origDbSize ){
          sqliteOsSeek(&pPager->fd, SQLITE_PAGE_SIZE*(off_t)(pPg->pgno-1));
          rc = sqliteOsRead(&pPager->fd, zBuf, SQLITE_PAGE_SIZE);
          TRACE2("REFETCH %d\n", pPg->pgno);
          CODEC(pPager, zBuf, pPg->pgno, 2);
          if( rc ) break;
        }else{
          memset(zBuf, 0, SQLITE_PAGE_SIZE);
        }
        if( pPg->nRef==0 || memcmp(zBuf, PGHDR_TO_DATA(pPg), SQLITE_PAGE_SIZE) ){
          memcpy(PGHDR_TO_DATA(pPg), zBuf, SQLITE_PAGE_SIZE);
          memset(PGHDR_TO_EXTRA(pPg), 0, pPager->nExtra);
        }
        pPg->needSync = 0;
        pPg->dirty = 0;
      }
    }
  
  end_playback:
    if( rc!=SQLITE_OK ){
      pager_unwritelock(pPager);
      pPager->errMask |= PAGER_ERR_CORRUPT;
      rc = SQLITE_CORRUPT;
    }else{
      rc = pager_unwritelock(pPager);
    }
    return rc;
  }
  
  /*
  ** Playback the checkpoint journal.
  **
  ** This is similar to playing back the transaction journal but with
  ** a few extra twists.
  **
  **    (1)  The number of pages in the database file at the start of
  **         the checkpoint is stored in pPager->ckptSize, not in the
  **         journal file itself.
  **
  **    (2)  In addition to playing back the checkpoint journal, also
  **         playback all pages of the transaction journal beginning
  **         at offset pPager->ckptJSize.
  */
  static int pager_ckpt_playback(Pager *pPager){
    off_t szJ;               /* Size of the full journal */
    int nRec;                /* Number of Records */
    int i;                   /* Loop counter */
    int rc;
  
    /* Truncate the database back to its original size.
    */
    rc = sqliteOsTruncate(&pPager->fd, SQLITE_PAGE_SIZE*(off_t)pPager->ckptSize);
    pPager->dbSize = pPager->ckptSize;
  
    /* Figure out how many records are in the checkpoint journal.
    */
    assert( pPager->ckptInUse && pPager->journalOpen );
    sqliteOsSeek(&pPager->cpfd, 0);
    nRec = pPager->ckptNRec;
    
    /* Copy original pages out of the checkpoint journal and back into the
    ** database file.  Note that the checkpoint journal always uses format
    ** 2 instead of format 3 since it does not need to be concerned with
    ** power failures corrupting the journal and can thus omit the checksums.
    */
    for(i=nRec-1; i>=0; i--){
      rc = pager_playback_one_page(pPager, &pPager->cpfd, 2);
      assert( rc!=SQLITE_DONE );
      if( rc!=SQLITE_OK ) goto end_ckpt_playback;
    }
  
    /* Figure out how many pages need to be copied out of the transaction
    ** journal.
    */
    rc = sqliteOsSeek(&pPager->jfd, pPager->ckptJSize);
    if( rc!=SQLITE_OK ){
      goto end_ckpt_playback;
    }
    rc = sqliteOsFileSize(&pPager->jfd, &szJ);
    if( rc!=SQLITE_OK ){
      goto end_ckpt_playback;
    }
    nRec = (szJ - pPager->ckptJSize)/JOURNAL_PG_SZ(journal_format);
    for(i=nRec-1; i>=0; i--){
      rc = pager_playback_one_page(pPager, &pPager->jfd, journal_format);
      if( rc!=SQLITE_OK ){
        assert( rc!=SQLITE_DONE );
        goto end_ckpt_playback;
      }
    }
    
  end_ckpt_playback:
    if( rc!=SQLITE_OK ){
      pPager->errMask |= PAGER_ERR_CORRUPT;
      rc = SQLITE_CORRUPT;
    }
    return rc;
  }
  
  /*
  ** Change the maximum number of in-memory pages that are allowed.
  **
  ** The maximum number is the absolute value of the mxPage parameter.
  ** If mxPage is negative, the noSync flag is also set.  noSync bypasses
  ** calls to sqliteOsSync().  The pager runs much faster with noSync on,
  ** but if the operating system crashes or there is an abrupt power 
  ** failure, the database file might be left in an inconsistent and
  ** unrepairable state.  
  */
  void sqlitepager_set_cachesize(Pager *pPager, int mxPage){
    if( mxPage>=0 ){
      pPager->noSync = pPager->tempFile;
      if( pPager->noSync==0 ) pPager->needSync = 0;
    }else{
      pPager->noSync = 1;
      mxPage = -mxPage;
    }
    if( mxPage>10 ){
      pPager->mxPage = mxPage;
    }
  }
  
  /*
  ** Adjust the robustness of the database to damage due to OS crashes
  ** or power failures by changing the number of syncs()s when writing
  ** the rollback journal.  There are three levels:
  **
  **    OFF       sqliteOsSync() is never called.  This is the default
  **              for temporary and transient files.
  **
  **    NORMAL    The journal is synced once before writes begin on the
  **              database.  This is normally adequate protection, but
  **              it is theoretically possible, though very unlikely,
  **              that an inopertune power failure could leave the journal
  **              in a state which would cause damage to the database
  **              when it is rolled back.
  **
  **    FULL      The journal is synced twice before writes begin on the
  **              database (with some additional information - the nRec field
  **              of the journal header - being written in between the two
  **              syncs).  If we assume that writing a
  **              single disk sector is atomic, then this mode provides
  **              assurance that the journal will not be corrupted to the
  **              point of causing damage to the database during rollback.
  **
  ** Numeric values associated with these states are OFF==1, NORMAL=2,
  ** and FULL=3.
  */
  void sqlitepager_set_safety_level(Pager *pPager, int level){
    pPager->noSync =  level==1 || pPager->tempFile;
    pPager->fullSync = level==3 && !pPager->tempFile;
    if( pPager->noSync==0 ) pPager->needSync = 0;
  }
  
  /*
  ** Open a temporary file.  Write the name of the file into zName
  ** (zName must be at least SQLITE_TEMPNAME_SIZE bytes long.)  Write
  ** the file descriptor into *fd.  Return SQLITE_OK on success or some
  ** other error code if we fail.
  **
  ** The OS will automatically delete the temporary file when it is
  ** closed.
  */
  static int sqlitepager_opentemp(char *zFile, OsFile *fd){
    int cnt = 8;
    int rc;
    do{
      cnt--;
      sqliteOsTempFileName(zFile);
      rc = sqliteOsOpenExclusive(zFile, fd, 1);
    }while( cnt>0 && rc!=SQLITE_OK );
    return rc;
  }
  
  /*
  ** Create a new page cache and put a pointer to the page cache in *ppPager.
  ** The file to be cached need not exist.  The file is not locked until
  ** the first call to sqlitepager_get() and is only held open until the
  ** last page is released using sqlitepager_unref().
  **
  ** If zFilename is NULL then a randomly-named temporary file is created
  ** and used as the file to be cached.  The file will be deleted
  ** automatically when it is closed.
  */
  int sqlitepager_open(
    Pager **ppPager,         /* Return the Pager structure here */
    const char *zFilename,   /* Name of the database file to open */
    int mxPage,              /* Max number of in-memory cache pages */
    int nExtra,              /* Extra bytes append to each in-memory page */
    int useJournal           /* TRUE to use a rollback journal on this file */
  ){
    Pager *pPager;
    char *zFullPathname;
    int nameLen;
    OsFile fd;
    int rc, i;
    int tempFile;
    int readOnly = 0;
    char zTemp[SQLITE_TEMPNAME_SIZE];
  
    *ppPager = 0;
    if( sqlite_malloc_failed ){
      return SQLITE_NOMEM;
    }
    if( zFilename && zFilename[0] ){
      zFullPathname = sqliteOsFullPathname(zFilename);
      rc = sqliteOsOpenReadWrite(zFullPathname, &fd, &readOnly);
      tempFile = 0;
    }else{
      rc = sqlitepager_opentemp(zTemp, &fd);
      zFilename = zTemp;
      zFullPathname = sqliteOsFullPathname(zFilename);
      tempFile = 1;
    }
    if( sqlite_malloc_failed ){
      return SQLITE_NOMEM;
    }
    if( rc!=SQLITE_OK ){
      sqliteFree(zFullPathname);
      return SQLITE_CANTOPEN;
    }
    nameLen = strlen(zFullPathname);
    pPager = sqliteMalloc( sizeof(*pPager) + nameLen*3 + 30 );
    if( pPager==0 ){
      sqliteOsClose(&fd);
      sqliteFree(zFullPathname);
      return SQLITE_NOMEM;
    }
    SET_PAGER(pPager);
    pPager->zFilename = (char*)&pPager[1];
    pPager->zDirectory = &pPager->zFilename[nameLen+1];
    pPager->zJournal = &pPager->zDirectory[nameLen+1];
    strcpy(pPager->zFilename, zFullPathname);
    strcpy(pPager->zDirectory, zFullPathname);
    for(i=nameLen; i>0 && pPager->zDirectory[i-1]!='/'; i--){}
    if( i>0 ) pPager->zDirectory[i-1] = 0;
    strcpy(pPager->zJournal, zFullPathname);
    sqliteFree(zFullPathname);
    strcpy(&pPager->zJournal[nameLen], "-journal");
    pPager->fd = fd;
    pPager->journalOpen = 0;
    pPager->useJournal = useJournal;
    pPager->ckptOpen = 0;
    pPager->ckptInUse = 0;
    pPager->nRef = 0;
    pPager->dbSize = -1;
    pPager->ckptSize = 0;
    pPager->ckptJSize = 0;
    pPager->nPage = 0;
    pPager->mxPage = mxPage>5 ? mxPage : 10;
    pPager->state = SQLITE_UNLOCK;
    pPager->errMask = 0;
    pPager->tempFile = tempFile;
    pPager->readOnly = readOnly;
    pPager->needSync = 0;
    pPager->noSync = pPager->tempFile || !useJournal;
    pPager->pFirst = 0;
    pPager->pFirstSynced = 0;
    pPager->pLast = 0;
    pPager->nExtra = nExtra;
    memset(pPager->aHash, 0, sizeof(pPager->aHash));
    *ppPager = pPager;
    return SQLITE_OK;
  }
  
  /*
  ** Set the destructor for this pager.  If not NULL, the destructor is called
  ** when the reference count on each page reaches zero.  The destructor can
  ** be used to clean up information in the extra segment appended to each page.
  **
  ** The destructor is not called as a result sqlitepager_close().  
  ** Destructors are only called by sqlitepager_unref().
  */
  void sqlitepager_set_destructor(Pager *pPager, void (*xDesc)(void*)){
    pPager->xDestructor = xDesc;
  }
  
  /*
  ** Return the total number of pages in the disk file associated with
  ** pPager.
  */
  int sqlitepager_pagecount(Pager *pPager){
    off_t n;
    assert( pPager!=0 );
    if( pPager->dbSize>=0 ){
      return pPager->dbSize;
    }
    if( sqliteOsFileSize(&pPager->fd, &n)!=SQLITE_OK ){
      pPager->errMask |= PAGER_ERR_DISK;
      return 0;
    }
    n /= SQLITE_PAGE_SIZE;
    if( pPager->state!=SQLITE_UNLOCK ){
      pPager->dbSize = n;
    }
    return n;
  }
  
  /*
  ** Forward declaration
  */
  static int syncJournal(Pager*);
  
  /*
  ** Truncate the file to the number of pages specified.
  */
  int sqlitepager_truncate(Pager *pPager, Pgno nPage){
    int rc;
    if( pPager->dbSize<0 ){
      sqlitepager_pagecount(pPager);
    }
    if( pPager->errMask!=0 ){
      rc = pager_errcode(pPager);
      return rc;
    }
    if( nPage>=(unsigned)pPager->dbSize ){
      return SQLITE_OK;
    }
    syncJournal(pPager);
    rc = sqliteOsTruncate(&pPager->fd, SQLITE_PAGE_SIZE*(off_t)nPage);
    if( rc==SQLITE_OK ){
      pPager->dbSize = nPage;
    }
    return rc;
  }
  
  /*
  ** Shutdown the page cache.  Free all memory and close all files.
  **
  ** If a transaction was in progress when this routine is called, that
  ** transaction is rolled back.  All outstanding pages are invalidated
  ** and their memory is freed.  Any attempt to use a page associated
  ** with this page cache after this function returns will likely
  ** result in a coredump.
  */
  int sqlitepager_close(Pager *pPager){
    PgHdr *pPg, *pNext;
    switch( pPager->state ){
      case SQLITE_WRITELOCK: {
        sqlitepager_rollback(pPager);
        sqliteOsUnlock(&pPager->fd);
        assert( pPager->journalOpen==0 );
        break;
      }
      case SQLITE_READLOCK: {
        sqliteOsUnlock(&pPager->fd);
        break;
      }
      default: {
        /* Do nothing */
        break;
      }
    }
    for(pPg=pPager->pAll; pPg; pPg=pNext){
      pNext = pPg->pNextAll;
      sqliteFree(pPg);
    }
    sqliteOsClose(&pPager->fd);
    assert( pPager->journalOpen==0 );
    /* Temp files are automatically deleted by the OS
    ** if( pPager->tempFile ){
    **   sqliteOsDelete(pPager->zFilename);
    ** }
    */
    CLR_PAGER(pPager);
    if( pPager->zFilename!=(char*)&pPager[1] ){
      assert( 0 );  /* Cannot happen */
      sqliteFree(pPager->zFilename);
      sqliteFree(pPager->zJournal);
      sqliteFree(pPager->zDirectory);
    }
    sqliteFree(pPager);
    return SQLITE_OK;
  }
  
  /*
  ** Return the page number for the given page data.
  */
  Pgno sqlitepager_pagenumber(void *pData){
    PgHdr *p = DATA_TO_PGHDR(pData);
    return p->pgno;
  }
  
  /*
  ** Increment the reference count for a page.  If the page is
  ** currently on the freelist (the reference count is zero) then
  ** remove it from the freelist.
  */
  #define page_ref(P)   ((P)->nRef==0?_page_ref(P):(void)(P)->nRef++)
  static void _page_ref(PgHdr *pPg){
    if( pPg->nRef==0 ){
      /* The page is currently on the freelist.  Remove it. */
      if( pPg==pPg->pPager->pFirstSynced ){
        PgHdr *p = pPg->pNextFree;
        while( p && p->needSync ){ p = p->pNextFree; }
        pPg->pPager->pFirstSynced = p;
      }
      if( pPg->pPrevFree ){
        pPg->pPrevFree->pNextFree = pPg->pNextFree;
      }else{
        pPg->pPager->pFirst = pPg->pNextFree;
      }
      if( pPg->pNextFree ){
        pPg->pNextFree->pPrevFree = pPg->pPrevFree;
      }else{
        pPg->pPager->pLast = pPg->pPrevFree;
      }
      pPg->pPager->nRef++;
    }
    pPg->nRef++;
    REFINFO(pPg);
  }
  
  /*
  ** Increment the reference count for a page.  The input pointer is
  ** a reference to the page data.
  */
  int sqlitepager_ref(void *pData){
    PgHdr *pPg = DATA_TO_PGHDR(pData);
    page_ref(pPg);
    return SQLITE_OK;
  }
  
  /*
  ** Sync the journal.  In other words, make sure all the pages that have
  ** been written to the journal have actually reached the surface of the
  ** disk.  It is not safe to modify the original database file until after
  ** the journal has been synced.  If the original database is modified before
  ** the journal is synced and a power failure occurs, the unsynced journal
  ** data would be lost and we would be unable to completely rollback the
  ** database changes.  Database corruption would occur.
  ** 
  ** This routine also updates the nRec field in the header of the journal.
  ** (See comments on the pager_playback() routine for additional information.)
  ** If the sync mode is FULL, two syncs will occur.  First the whole journal
  ** is synced, then the nRec field is updated, then a second sync occurs.
  **
  ** For temporary databases, we do not care if we are able to rollback
  ** after a power failure, so sync occurs.
  **
  ** This routine clears the needSync field of every page current held in
  ** memory.
  */
  static int syncJournal(Pager *pPager){
    PgHdr *pPg;
    int rc = SQLITE_OK;
  
    /* Sync the journal before modifying the main database
    ** (assuming there is a journal and it needs to be synced.)
    */
    if( pPager->needSync ){
      if( !pPager->tempFile ){
        assert( pPager->journalOpen );
        /* assert( !pPager->noSync ); // noSync might be set if synchronous
        ** was turned off after the transaction was started.  Ticket #615 */
  #ifndef NDEBUG
        {
          /* Make sure the pPager->nRec counter we are keeping agrees
          ** with the nRec computed from the size of the journal file.
          */
          off_t hdrSz, pgSz, jSz;
          hdrSz = JOURNAL_HDR_SZ(journal_format);
          pgSz = JOURNAL_PG_SZ(journal_format);
          rc = sqliteOsFileSize(&pPager->jfd, &jSz);
          if( rc!=0 ) return rc;
          assert( pPager->nRec*pgSz+hdrSz==jSz );
        }
  #endif
        if( journal_format>=3 ){
          /* Write the nRec value into the journal file header */
          off_t szJ;
          if( pPager->fullSync ){
            TRACE1("SYNC\n");
            rc = sqliteOsSync(&pPager->jfd);
            if( rc!=0 ) return rc;
          }
          sqliteOsSeek(&pPager->jfd, sizeof(aJournalMagic1));
          rc = write32bits(&pPager->jfd, pPager->nRec);
          if( rc ) return rc;
          szJ = JOURNAL_HDR_SZ(journal_format) +
                   pPager->nRec*JOURNAL_PG_SZ(journal_format);
          sqliteOsSeek(&pPager->jfd, szJ);
        }
        TRACE1("SYNC\n");
        rc = sqliteOsSync(&pPager->jfd);
        if( rc!=0 ) return rc;
        pPager->journalStarted = 1;
      }
      pPager->needSync = 0;
  
      /* Erase the needSync flag from every page.
      */
      for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
        pPg->needSync = 0;
      }
      pPager->pFirstSynced = pPager->pFirst;
    }
  
  #ifndef NDEBUG
    /* If the Pager.needSync flag is clear then the PgHdr.needSync
    ** flag must also be clear for all pages.  Verify that this
    ** invariant is true.
    */
    else{
      for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
        assert( pPg->needSync==0 );
      }
      assert( pPager->pFirstSynced==pPager->pFirst );
    }
  #endif
  
    return rc;
  }
  
  /*
  ** Given a list of pages (connected by the PgHdr.pDirty pointer) write
  ** every one of those pages out to the database file and mark them all
  ** as clean.
  */
  static int pager_write_pagelist(PgHdr *pList){
    Pager *pPager;
    int rc;
  
    if( pList==0 ) return SQLITE_OK;
    pPager = pList->pPager;
    while( pList ){
      assert( pList->dirty );
      sqliteOsSeek(&pPager->fd, (pList->pgno-1)*(off_t)SQLITE_PAGE_SIZE);
      CODEC(pPager, PGHDR_TO_DATA(pList), pList->pgno, 6);
      TRACE2("STORE %d\n", pList->pgno);
      rc = sqliteOsWrite(&pPager->fd, PGHDR_TO_DATA(pList), SQLITE_PAGE_SIZE);
      CODEC(pPager, PGHDR_TO_DATA(pList), pList->pgno, 0);
      if( rc ) return rc;
      pList->dirty = 0;
      pList = pList->pDirty;
    }
    return SQLITE_OK;
  }
  
  /*
  ** Collect every dirty page into a dirty list and
  ** return a pointer to the head of that list.  All pages are
  ** collected even if they are still in use.
  */
  static PgHdr *pager_get_all_dirty_pages(Pager *pPager){
    PgHdr *p, *pList;
    pList = 0;
    for(p=pPager->pAll; p; p=p->pNextAll){
      if( p->dirty ){
        p->pDirty = pList;
        pList = p;
      }
    }
    return pList;
  }
  
  /*
  ** Acquire a page.
  **
  ** A read lock on the disk file is obtained when the first page is acquired. 
  ** This read lock is dropped when the last page is released.
  **
  ** A _get works for any page number greater than 0.  If the database
  ** file is smaller than the requested page, then no actual disk
  ** read occurs and the memory image of the page is initialized to
  ** all zeros.  The extra data appended to a page is always initialized
  ** to zeros the first time a page is loaded into memory.
  **
  ** The acquisition might fail for several reasons.  In all cases,
  ** an appropriate error code is returned and *ppPage is set to NULL.
  **
  ** See also sqlitepager_lookup().  Both this routine and _lookup() attempt
  ** to find a page in the in-memory cache first.  If the page is not already
  ** in memory, this routine goes to disk to read it in whereas _lookup()
  ** just returns 0.  This routine acquires a read-lock the first time it
  ** has to go to disk, and could also playback an old journal if necessary.
  ** Since _lookup() never goes to disk, it never has to deal with locks
  ** or journal files.
  */
  int sqlitepager_get(Pager *pPager, Pgno pgno, void **ppPage){
    PgHdr *pPg;
    int rc;
  
    /* Make sure we have not hit any critical errors.
    */ 
    assert( pPager!=0 );
    assert( pgno!=0 );
    *ppPage = 0;
    if( pPager->errMask & ~(PAGER_ERR_FULL) ){
      return pager_errcode(pPager);
    }
  
    /* If this is the first page accessed, then get a read lock
    ** on the database file.
    */
    if( pPager->nRef==0 ){
      rc = sqliteOsReadLock(&pPager->fd);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      pPager->state = SQLITE_READLOCK;
  
      /* If a journal file exists, try to play it back.
      */
      if( pPager->useJournal && sqliteOsFileExists(pPager->zJournal) ){
         int rc;
  
         /* Get a write lock on the database
         */
         rc = sqliteOsWriteLock(&pPager->fd);
         if( rc!=SQLITE_OK ){
           if( sqliteOsUnlock(&pPager->fd)!=SQLITE_OK ){
             /* This should never happen! */
             rc = SQLITE_INTERNAL;
           }
           return rc;
         }
         pPager->state = SQLITE_WRITELOCK;
  
         /* Open the journal for reading only.  Return SQLITE_BUSY if
         ** we are unable to open the journal file. 
         **
         ** The journal file does not need to be locked itself.  The
         ** journal file is never open unless the main database file holds
         ** a write lock, so there is never any chance of two or more
         ** processes opening the journal at the same time.
         */
         rc = sqliteOsOpenReadOnly(pPager->zJournal, &pPager->jfd);
         if( rc!=SQLITE_OK ){
           rc = sqliteOsUnlock(&pPager->fd);
           assert( rc==SQLITE_OK );
           return SQLITE_BUSY;
         }
         pPager->journalOpen = 1;
         pPager->journalStarted = 0;
  
         /* Playback and delete the journal.  Drop the database write
         ** lock and reacquire the read lock.
         */
         rc = pager_playback(pPager, 0);
         if( rc!=SQLITE_OK ){
           return rc;
         }
      }
      pPg = 0;
    }else{
      /* Search for page in cache */
      pPg = pager_lookup(pPager, pgno);
    }
    if( pPg==0 ){
      /* The requested page is not in the page cache. */
      int h;
      pPager->nMiss++;
      if( pPager->nPage<pPager->mxPage || pPager->pFirst==0 ){
        /* Create a new page */
        pPg = sqliteMallocRaw( sizeof(*pPg) + SQLITE_PAGE_SIZE 
                                + sizeof(u32) + pPager->nExtra );
        if( pPg==0 ){
          pager_unwritelock(pPager);
          pPager->errMask |= PAGER_ERR_MEM;
          return SQLITE_NOMEM;
        }
        memset(pPg, 0, sizeof(*pPg));
        pPg->pPager = pPager;
        pPg->pNextAll = pPager->pAll;
        if( pPager->pAll ){
          pPager->pAll->pPrevAll = pPg;
        }
        pPg->pPrevAll = 0;
        pPager->pAll = pPg;
        pPager->nPage++;
      }else{
        /* Find a page to recycle.  Try to locate a page that does not
        ** require us to do an fsync() on the journal.
        */
        pPg = pPager->pFirstSynced;
  
        /* If we could not find a page that does not require an fsync()
        ** on the journal file then fsync the journal file.  This is a
        ** very slow operation, so we work hard to avoid it.  But sometimes
        ** it can't be helped.
        */
        if( pPg==0 ){
          int rc = syncJournal(pPager);
          if( rc!=0 ){
            sqlitepager_rollback(pPager);
            return SQLITE_IOERR;
          }
          pPg = pPager->pFirst;
        }
        assert( pPg->nRef==0 );
  
        /* Write the page to the database file if it is dirty.
        */
        if( pPg->dirty ){
          assert( pPg->needSync==0 );
          pPg->pDirty = 0;
          rc = pager_write_pagelist( pPg );
          if( rc!=SQLITE_OK ){
            sqlitepager_rollback(pPager);
            return SQLITE_IOERR;
          }
        }
        assert( pPg->dirty==0 );
  
        /* If the page we are recycling is marked as alwaysRollback, then
        ** set the global alwaysRollback flag, thus disabling the
        ** sqlite_dont_rollback() optimization for the rest of this transaction.
        ** It is necessary to do this because the page marked alwaysRollback
        ** might be reloaded at a later time but at that point we won't remember
        ** that is was marked alwaysRollback.  This means that all pages must
        ** be marked as alwaysRollback from here on out.
        */
        if( pPg->alwaysRollback ){
          pPager->alwaysRollback = 1;
        }
  
        /* Unlink the old page from the free list and the hash table
        */
        if( pPg==pPager->pFirstSynced ){
          PgHdr *p = pPg->pNextFree;
          while( p && p->needSync ){ p = p->pNextFree; }
          pPager->pFirstSynced = p;
        }
        if( pPg->pPrevFree ){
          pPg->pPrevFree->pNextFree = pPg->pNextFree;
        }else{
          assert( pPager->pFirst==pPg );
          pPager->pFirst = pPg->pNextFree;
        }
        if( pPg->pNextFree ){
          pPg->pNextFree->pPrevFree = pPg->pPrevFree;
        }else{
          assert( pPager->pLast==pPg );
          pPager->pLast = pPg->pPrevFree;
        }
        pPg->pNextFree = pPg->pPrevFree = 0;
        if( pPg->pNextHash ){
          pPg->pNextHash->pPrevHash = pPg->pPrevHash;
        }
        if( pPg->pPrevHash ){
          pPg->pPrevHash->pNextHash = pPg->pNextHash;
        }else{
          h = pager_hash(pPg->pgno);
          assert( pPager->aHash[h]==pPg );
          pPager->aHash[h] = pPg->pNextHash;
        }
        pPg->pNextHash = pPg->pPrevHash = 0;
        pPager->nOvfl++;
      }
      pPg->pgno = pgno;
      if( pPager->aInJournal && (int)pgno<=pPager->origDbSize ){
        sqliteCheckMemory(pPager->aInJournal, pgno/8);
        assert( pPager->journalOpen );
        pPg->inJournal = (pPager->aInJournal[pgno/8] & (1<<(pgno&7)))!=0;
        pPg->needSync = 0;
      }else{
        pPg->inJournal = 0;
        pPg->needSync = 0;
      }
      if( pPager->aInCkpt && (int)pgno<=pPager->ckptSize
               && (pPager->aInCkpt[pgno/8] & (1<<(pgno&7)))!=0 ){
        page_add_to_ckpt_list(pPg);
      }else{
        page_remove_from_ckpt_list(pPg);
      }
      pPg->dirty = 0;
      pPg->nRef = 1;
      REFINFO(pPg);
      pPager->nRef++;
      h = pager_hash(pgno);
      pPg->pNextHash = pPager->aHash[h];
      pPager->aHash[h] = pPg;
      if( pPg->pNextHash ){
        assert( pPg->pNextHash->pPrevHash==0 );
        pPg->pNextHash->pPrevHash = pPg;
      }
      if( pPager->nExtra>0 ){
        memset(PGHDR_TO_EXTRA(pPg), 0, pPager->nExtra);
      }
      if( pPager->dbSize<0 ) sqlitepager_pagecount(pPager);
      if( pPager->errMask!=0 ){
        sqlitepager_unref(PGHDR_TO_DATA(pPg));
        rc = pager_errcode(pPager);
        return rc;
      }
      if( pPager->dbSize<(int)pgno ){
        memset(PGHDR_TO_DATA(pPg), 0, SQLITE_PAGE_SIZE);
      }else{
        int rc;
        sqliteOsSeek(&pPager->fd, (pgno-1)*(off_t)SQLITE_PAGE_SIZE);
        rc = sqliteOsRead(&pPager->fd, PGHDR_TO_DATA(pPg), SQLITE_PAGE_SIZE);
        TRACE2("FETCH %d\n", pPg->pgno);
        CODEC(pPager, PGHDR_TO_DATA(pPg), pPg->pgno, 3);
        if( rc!=SQLITE_OK ){
          off_t fileSize;
          if( sqliteOsFileSize(&pPager->fd,&fileSize)!=SQLITE_OK
                 || fileSize>=pgno*SQLITE_PAGE_SIZE ){
            sqlitepager_unref(PGHDR_TO_DATA(pPg));
            return rc;
          }else{
            memset(PGHDR_TO_DATA(pPg), 0, SQLITE_PAGE_SIZE);
          }
        }
      }
    }else{
      /* The requested page is in the page cache. */
      pPager->nHit++;
      page_ref(pPg);
    }
    *ppPage = PGHDR_TO_DATA(pPg);
    return SQLITE_OK;
  }
  
  /*
  ** Acquire a page if it is already in the in-memory cache.  Do
  ** not read the page from disk.  Return a pointer to the page,
  ** or 0 if the page is not in cache.
  **
  ** See also sqlitepager_get().  The difference between this routine
  ** and sqlitepager_get() is that _get() will go to the disk and read
  ** in the page if the page is not already in cache.  This routine
  ** returns NULL if the page is not in cache or if a disk I/O error 
  ** has ever happened.
  */
  void *sqlitepager_lookup(Pager *pPager, Pgno pgno){
    PgHdr *pPg;
  
    assert( pPager!=0 );
    assert( pgno!=0 );
    if( pPager->errMask & ~(PAGER_ERR_FULL) ){
      return 0;
    }
    /* if( pPager->nRef==0 ){
    **  return 0;
    ** }
    */
    pPg = pager_lookup(pPager, pgno);
    if( pPg==0 ) return 0;
    page_ref(pPg);
    return PGHDR_TO_DATA(pPg);
  }
  
  /*
  ** Release a page.
  **
  ** If the number of references to the page drop to zero, then the
  ** page is added to the LRU list.  When all references to all pages
  ** are released, a rollback occurs and the lock on the database is
  ** removed.
  */
  int sqlitepager_unref(void *pData){
    PgHdr *pPg;
  
    /* Decrement the reference count for this page
    */
    pPg = DATA_TO_PGHDR(pData);
    assert( pPg->nRef>0 );
    pPg->nRef--;
    REFINFO(pPg);
  
    /* When the number of references to a page reach 0, call the
    ** destructor and add the page to the freelist.
    */
    if( pPg->nRef==0 ){
      Pager *pPager;
      pPager = pPg->pPager;
      pPg->pNextFree = 0;
      pPg->pPrevFree = pPager->pLast;
      pPager->pLast = pPg;
      if( pPg->pPrevFree ){
        pPg->pPrevFree->pNextFree = pPg;
      }else{
        pPager->pFirst = pPg;
      }
      if( pPg->needSync==0 && pPager->pFirstSynced==0 ){
        pPager->pFirstSynced = pPg;
      }
      if( pPager->xDestructor ){
        pPager->xDestructor(pData);
      }
    
      /* When all pages reach the freelist, drop the read lock from
      ** the database file.
      */
      pPager->nRef--;
      assert( pPager->nRef>=0 );
      if( pPager->nRef==0 ){
        pager_reset(pPager);
      }
    }
    return SQLITE_OK;
  }
  
  /*
  ** Create a journal file for pPager.  There should already be a write
  ** lock on the database file when this routine is called.
  **
  ** Return SQLITE_OK if everything.  Return an error code and release the
  ** write lock if anything goes wrong.
  */
  static int pager_open_journal(Pager *pPager){
    int rc;
    assert( pPager->state==SQLITE_WRITELOCK );
    assert( pPager->journalOpen==0 );
    assert( pPager->useJournal );
    sqlitepager_pagecount(pPager);
    pPager->aInJournal = sqliteMalloc( pPager->dbSize/8 + 1 );
    if( pPager->aInJournal==0 ){
      sqliteOsReadLock(&pPager->fd);
      pPager->state = SQLITE_READLOCK;
      return SQLITE_NOMEM;
    }
    rc = sqliteOsOpenExclusive(pPager->zJournal, &pPager->jfd,pPager->tempFile);
    if( rc!=SQLITE_OK ){
      sqliteFree(pPager->aInJournal);
      pPager->aInJournal = 0;
      sqliteOsReadLock(&pPager->fd);
      pPager->state = SQLITE_READLOCK;
      return SQLITE_CANTOPEN;
    }
    sqliteOsOpenDirectory(pPager->zDirectory, &pPager->jfd);
    pPager->journalOpen = 1;
    pPager->journalStarted = 0;
    pPager->needSync = 0;
    pPager->alwaysRollback = 0;
    pPager->nRec = 0;
    if( pPager->errMask!=0 ){
      rc = pager_errcode(pPager);
      return rc;
    }
    pPager->origDbSize = pPager->dbSize;
    if( journal_format==JOURNAL_FORMAT_3 ){
      rc = sqliteOsWrite(&pPager->jfd, aJournalMagic3, sizeof(aJournalMagic3));
      if( rc==SQLITE_OK ){
        rc = write32bits(&pPager->jfd, pPager->noSync ? 0xffffffff : 0);
      }
      if( rc==SQLITE_OK ){
        sqliteRandomness(sizeof(pPager->cksumInit), &pPager->cksumInit);
        rc = write32bits(&pPager->jfd, pPager->cksumInit);
      }
    }else if( journal_format==JOURNAL_FORMAT_2 ){
      rc = sqliteOsWrite(&pPager->jfd, aJournalMagic2, sizeof(aJournalMagic2));
    }else{
      assert( journal_format==JOURNAL_FORMAT_1 );
      rc = sqliteOsWrite(&pPager->jfd, aJournalMagic1, sizeof(aJournalMagic1));
    }
    if( rc==SQLITE_OK ){
      rc = write32bits(&pPager->jfd, pPager->dbSize);
    }
    if( pPager->ckptAutoopen && rc==SQLITE_OK ){
      rc = sqlitepager_ckpt_begin(pPager);
    }
    if( rc!=SQLITE_OK ){
      rc = pager_unwritelock(pPager);
      if( rc==SQLITE_OK ){
        rc = SQLITE_FULL;
      }
    }
    return rc;  
  }
  
  /*
  ** Acquire a write-lock on the database.  The lock is removed when
  ** the any of the following happen:
  **
  **   *  sqlitepager_commit() is called.
  **   *  sqlitepager_rollback() is called.
  **   *  sqlitepager_close() is called.
  **   *  sqlitepager_unref() is called to on every outstanding page.
  **
  ** The parameter to this routine is a pointer to any open page of the
  ** database file.  Nothing changes about the page - it is used merely
  ** to acquire a pointer to the Pager structure and as proof that there
  ** is already a read-lock on the database.
  **
  ** A journal file is opened if this is not a temporary file.  For
  ** temporary files, the opening of the journal file is deferred until
  ** there is an actual need to write to the journal.
  **
  ** If the database is already write-locked, this routine is a no-op.
  */
  int sqlitepager_begin(void *pData){
    PgHdr *pPg = DATA_TO_PGHDR(pData);
    Pager *pPager = pPg->pPager;
    int rc = SQLITE_OK;
    assert( pPg->nRef>0 );
    assert( pPager->state!=SQLITE_UNLOCK );
    if( pPager->state==SQLITE_READLOCK ){
      assert( pPager->aInJournal==0 );
      rc = sqliteOsWriteLock(&pPager->fd);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      pPager->state = SQLITE_WRITELOCK;
      pPager->dirtyFile = 0;
      TRACE1("TRANSACTION\n");
      if( pPager->useJournal && !pPager->tempFile ){
        rc = pager_open_journal(pPager);
      }
    }
    return rc;
  }
  
  /*
  ** Mark a data page as writeable.  The page is written into the journal 
  ** if it is not there already.  This routine must be called before making
  ** changes to a page.
  **
  ** The first time this routine is called, the pager creates a new
  ** journal and acquires a write lock on the database.  If the write
  ** lock could not be acquired, this routine returns SQLITE_BUSY.  The
  ** calling routine must check for that return value and be careful not to
  ** change any page data until this routine returns SQLITE_OK.
  **
  ** If the journal file could not be written because the disk is full,
  ** then this routine returns SQLITE_FULL and does an immediate rollback.
  ** All subsequent write attempts also return SQLITE_FULL until there
  ** is a call to sqlitepager_commit() or sqlitepager_rollback() to
  ** reset.
  */
  int sqlitepager_write(void *pData){
    PgHdr *pPg = DATA_TO_PGHDR(pData);
    Pager *pPager = pPg->pPager;
    int rc = SQLITE_OK;
  
    /* Check for errors
    */
    if( pPager->errMask ){ 
      return pager_errcode(pPager);
    }
    if( pPager->readOnly ){
      return SQLITE_PERM;
    }
  
    /* Mark the page as dirty.  If the page has already been written
    ** to the journal then we can return right away.
    */
    pPg->dirty = 1;
    if( pPg->inJournal && (pPg->inCkpt || pPager->ckptInUse==0) ){
      pPager->dirtyFile = 1;
      return SQLITE_OK;
    }
  
    /* If we get this far, it means that the page needs to be
    ** written to the transaction journal or the ckeckpoint journal
    ** or both.
    **
    ** First check to see that the transaction journal exists and
    ** create it if it does not.
    */
    assert( pPager->state!=SQLITE_UNLOCK );
    rc = sqlitepager_begin(pData);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    assert( pPager->state==SQLITE_WRITELOCK );
    if( !pPager->journalOpen && pPager->useJournal ){
      rc = pager_open_journal(pPager);
      if( rc!=SQLITE_OK ) return rc;
    }
    assert( pPager->journalOpen || !pPager->useJournal );
    pPager->dirtyFile = 1;
  
    /* The transaction journal now exists and we have a write lock on the
    ** main database file.  Write the current page to the transaction 
    ** journal if it is not there already.
    */
    if( !pPg->inJournal && pPager->useJournal ){
      if( (int)pPg->pgno <= pPager->origDbSize ){
        int szPg;
        u32 saved;
        if( journal_format>=JOURNAL_FORMAT_3 ){
          u32 cksum = pager_cksum(pPager, pPg->pgno, pData);
          saved = *(u32*)PGHDR_TO_EXTRA(pPg);
          store32bits(cksum, pPg, SQLITE_PAGE_SIZE);
          szPg = SQLITE_PAGE_SIZE+8;
        }else{
          szPg = SQLITE_PAGE_SIZE+4;
        }
        store32bits(pPg->pgno, pPg, -4);
        CODEC(pPager, pData, pPg->pgno, 7);
        rc = sqliteOsWrite(&pPager->jfd, &((char*)pData)[-4], szPg);
        TRACE3("JOURNAL %d %d\n", pPg->pgno, pPg->needSync);
        CODEC(pPager, pData, pPg->pgno, 0);
        if( journal_format>=JOURNAL_FORMAT_3 ){
          *(u32*)PGHDR_TO_EXTRA(pPg) = saved;
        }
        if( rc!=SQLITE_OK ){
          sqlitepager_rollback(pPager);
          pPager->errMask |= PAGER_ERR_FULL;
          return rc;
        }
        pPager->nRec++;
        assert( pPager->aInJournal!=0 );
        pPager->aInJournal[pPg->pgno/8] |= 1<<(pPg->pgno&7);
        pPg->needSync = !pPager->noSync;
        pPg->inJournal = 1;
        if( pPager->ckptInUse ){
          pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
          page_add_to_ckpt_list(pPg);
        }
      }else{
        pPg->needSync = !pPager->journalStarted && !pPager->noSync;
        TRACE3("APPEND %d %d\n", pPg->pgno, pPg->needSync);
      }
      if( pPg->needSync ){
        pPager->needSync = 1;
      }
    }
  
    /* If the checkpoint journal is open and the page is not in it,
    ** then write the current page to the checkpoint journal.  Note that
    ** the checkpoint journal always uses the simpler format 2 that lacks
    ** checksums.  The header is also omitted from the checkpoint journal.
    */
    if( pPager->ckptInUse && !pPg->inCkpt && (int)pPg->pgno<=pPager->ckptSize ){
      assert( pPg->inJournal || (int)pPg->pgno>pPager->origDbSize );
      store32bits(pPg->pgno, pPg, -4);
      CODEC(pPager, pData, pPg->pgno, 7);
      rc = sqliteOsWrite(&pPager->cpfd, &((char*)pData)[-4], SQLITE_PAGE_SIZE+4);
      TRACE2("CKPT-JOURNAL %d\n", pPg->pgno);
      CODEC(pPager, pData, pPg->pgno, 0);
      if( rc!=SQLITE_OK ){
        sqlitepager_rollback(pPager);
        pPager->errMask |= PAGER_ERR_FULL;
        return rc;
      }
      pPager->ckptNRec++;
      assert( pPager->aInCkpt!=0 );
      pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
      page_add_to_ckpt_list(pPg);
    }
  
    /* Update the database size and return.
    */
    if( pPager->dbSize<(int)pPg->pgno ){
      pPager->dbSize = pPg->pgno;
    }
    return rc;
  }
  
  /*
  ** Return TRUE if the page given in the argument was previously passed
  ** to sqlitepager_write().  In other words, return TRUE if it is ok
  ** to change the content of the page.
  */
  int sqlitepager_iswriteable(void *pData){
    PgHdr *pPg = DATA_TO_PGHDR(pData);
    return pPg->dirty;
  }
  
  /*
  ** Replace the content of a single page with the information in the third
  ** argument.
  */
  int sqlitepager_overwrite(Pager *pPager, Pgno pgno, void *pData){
    void *pPage;
    int rc;
  
    rc = sqlitepager_get(pPager, pgno, &pPage);
    if( rc==SQLITE_OK ){
      rc = sqlitepager_write(pPage);
      if( rc==SQLITE_OK ){
        memcpy(pPage, pData, SQLITE_PAGE_SIZE);
      }
      sqlitepager_unref(pPage);
    }
    return rc;
  }
  
  /*
  ** A call to this routine tells the pager that it is not necessary to
  ** write the information on page "pgno" back to the disk, even though
  ** that page might be marked as dirty.
  **
  ** The overlying software layer calls this routine when all of the data
  ** on the given page is unused.  The pager marks the page as clean so
  ** that it does not get written to disk.
  **
  ** Tests show that this optimization, together with the
  ** sqlitepager_dont_rollback() below, more than double the speed
  ** of large INSERT operations and quadruple the speed of large DELETEs.
  **
  ** When this routine is called, set the alwaysRollback flag to true.
  ** Subsequent calls to sqlitepager_dont_rollback() for the same page
  ** will thereafter be ignored.  This is necessary to avoid a problem
  ** where a page with data is added to the freelist during one part of
  ** a transaction then removed from the freelist during a later part
  ** of the same transaction and reused for some other purpose.  When it
  ** is first added to the freelist, this routine is called.  When reused,
  ** the dont_rollback() routine is called.  But because the page contains
  ** critical data, we still need to be sure it gets rolled back in spite
  ** of the dont_rollback() call.
  */
  void sqlitepager_dont_write(Pager *pPager, Pgno pgno){
    PgHdr *pPg;
  
    pPg = pager_lookup(pPager, pgno);
    pPg->alwaysRollback = 1;
    if( pPg && pPg->dirty ){
      if( pPager->dbSize==(int)pPg->pgno && pPager->origDbSize<pPager->dbSize ){
        /* If this pages is the last page in the file and the file has grown
        ** during the current transaction, then do NOT mark the page as clean.
        ** When the database file grows, we must make sure that the last page
        ** gets written at least once so that the disk file will be the correct
        ** size. If you do not write this page and the size of the file
        ** on the disk ends up being too small, that can lead to database
        ** corruption during the next transaction.
        */
      }else{
        TRACE2("DONT_WRITE %d\n", pgno);
        pPg->dirty = 0;
      }
    }
  }
  
  /*
  ** A call to this routine tells the pager that if a rollback occurs,
  ** it is not necessary to restore the data on the given page.  This
  ** means that the pager does not have to record the given page in the
  ** rollback journal.
  */
  void sqlitepager_dont_rollback(void *pData){
    PgHdr *pPg = DATA_TO_PGHDR(pData);
    Pager *pPager = pPg->pPager;
  
    if( pPager->state!=SQLITE_WRITELOCK || pPager->journalOpen==0 ) return;
    if( pPg->alwaysRollback || pPager->alwaysRollback ) return;
    if( !pPg->inJournal && (int)pPg->pgno <= pPager->origDbSize ){
      assert( pPager->aInJournal!=0 );
      pPager->aInJournal[pPg->pgno/8] |= 1<<(pPg->pgno&7);
      pPg->inJournal = 1;
      if( pPager->ckptInUse ){
        pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
        page_add_to_ckpt_list(pPg);
      }
      TRACE2("DONT_ROLLBACK %d\n", pPg->pgno);
    }
    if( pPager->ckptInUse && !pPg->inCkpt && (int)pPg->pgno<=pPager->ckptSize ){
      assert( pPg->inJournal || (int)pPg->pgno>pPager->origDbSize );
      assert( pPager->aInCkpt!=0 );
      pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
      page_add_to_ckpt_list(pPg);
    }
  }
  
  /*
  ** Commit all changes to the database and release the write lock.
  **
  ** If the commit fails for any reason, a rollback attempt is made
  ** and an error code is returned.  If the commit worked, SQLITE_OK
  ** is returned.
  */
  int sqlitepager_commit(Pager *pPager){
    int rc;
    PgHdr *pPg;
  
    if( pPager->errMask==PAGER_ERR_FULL ){
      rc = sqlitepager_rollback(pPager);
      if( rc==SQLITE_OK ){
        rc = SQLITE_FULL;
      }
      return rc;
    }
    if( pPager->errMask!=0 ){
      rc = pager_errcode(pPager);
      return rc;
    }
    if( pPager->state!=SQLITE_WRITELOCK ){
      return SQLITE_ERROR;
    }
    TRACE1("COMMIT\n");
    if( pPager->dirtyFile==0 ){
      /* Exit early (without doing the time-consuming sqliteOsSync() calls)
      ** if there have been no changes to the database file. */
      assert( pPager->needSync==0 );
      rc = pager_unwritelock(pPager);
      pPager->dbSize = -1;
      return rc;
    }
    assert( pPager->journalOpen );
    rc = syncJournal(pPager);
    if( rc!=SQLITE_OK ){
      goto commit_abort;
    }
    pPg = pager_get_all_dirty_pages(pPager);
    if( pPg ){
      rc = pager_write_pagelist(pPg);
      if( rc || (!pPager->noSync && sqliteOsSync(&pPager->fd)!=SQLITE_OK) ){
        goto commit_abort;
      }
    }
    rc = pager_unwritelock(pPager);
    pPager->dbSize = -1;
    return rc;
  
    /* Jump here if anything goes wrong during the commit process.
    */
  commit_abort:
    rc = sqlitepager_rollback(pPager);
    if( rc==SQLITE_OK ){
      rc = SQLITE_FULL;
    }
    return rc;
  }
  
  /*
  ** Rollback all changes.  The database falls back to read-only mode.
  ** All in-memory cache pages revert to their original data contents.
  ** The journal is deleted.
  **
  ** This routine cannot fail unless some other process is not following
  ** the correct locking protocol (SQLITE_PROTOCOL) or unless some other
  ** process is writing trash into the journal file (SQLITE_CORRUPT) or
  ** unless a prior malloc() failed (SQLITE_NOMEM).  Appropriate error
  ** codes are returned for all these occasions.  Otherwise,
  ** SQLITE_OK is returned.
  */
  int sqlitepager_rollback(Pager *pPager){
    int rc;
    TRACE1("ROLLBACK\n");
    if( !pPager->dirtyFile || !pPager->journalOpen ){
      rc = pager_unwritelock(pPager);
      pPager->dbSize = -1;
      return rc;
    }
  
    if( pPager->errMask!=0 && pPager->errMask!=PAGER_ERR_FULL ){
      if( pPager->state>=SQLITE_WRITELOCK ){
        pager_playback(pPager, 1);
      }
      return pager_errcode(pPager);
    }
    if( pPager->state!=SQLITE_WRITELOCK ){
      return SQLITE_OK;
    }
    rc = pager_playback(pPager, 1);
    if( rc!=SQLITE_OK ){
      rc = SQLITE_CORRUPT;
      pPager->errMask |= PAGER_ERR_CORRUPT;
    }
    pPager->dbSize = -1;
    return rc;
  }
  
  /*
  ** Return TRUE if the database file is opened read-only.  Return FALSE
  ** if the database is (in theory) writable.
  */
  int sqlitepager_isreadonly(Pager *pPager){
    return pPager->readOnly;
  }
  
  /*
  ** This routine is used for testing and analysis only.
  */
  int *sqlitepager_stats(Pager *pPager){
    static int a[9];
    a[0] = pPager->nRef;
    a[1] = pPager->nPage;
    a[2] = pPager->mxPage;
    a[3] = pPager->dbSize;
    a[4] = pPager->state;
    a[5] = pPager->errMask;
    a[6] = pPager->nHit;
    a[7] = pPager->nMiss;
    a[8] = pPager->nOvfl;
    return a;
  }
  
  /*
  ** Set the checkpoint.
  **
  ** This routine should be called with the transaction journal already
  ** open.  A new checkpoint journal is created that can be used to rollback
  ** changes of a single SQL command within a larger transaction.
  */
  int sqlitepager_ckpt_begin(Pager *pPager){
    int rc;
    char zTemp[SQLITE_TEMPNAME_SIZE];
    if( !pPager->journalOpen ){
      pPager->ckptAutoopen = 1;
      return SQLITE_OK;
    }
    assert( pPager->journalOpen );
    assert( !pPager->ckptInUse );
    pPager->aInCkpt = sqliteMalloc( pPager->dbSize/8 + 1 );
    if( pPager->aInCkpt==0 ){
      sqliteOsReadLock(&pPager->fd);
      return SQLITE_NOMEM;
    }
  #ifndef NDEBUG
    rc = sqliteOsFileSize(&pPager->jfd, &pPager->ckptJSize);
    if( rc ) goto ckpt_begin_failed;
    assert( pPager->ckptJSize == 
      pPager->nRec*JOURNAL_PG_SZ(journal_format)+JOURNAL_HDR_SZ(journal_format) );
  #endif
    pPager->ckptJSize = pPager->nRec*JOURNAL_PG_SZ(journal_format)
                           + JOURNAL_HDR_SZ(journal_format);
    pPager->ckptSize = pPager->dbSize;
    if( !pPager->ckptOpen ){
      rc = sqlitepager_opentemp(zTemp, &pPager->cpfd);
      if( rc ) goto ckpt_begin_failed;
      pPager->ckptOpen = 1;
      pPager->ckptNRec = 0;
    }
    pPager->ckptInUse = 1;
    return SQLITE_OK;
   
  ckpt_begin_failed:
    if( pPager->aInCkpt ){
      sqliteFree(pPager->aInCkpt);
      pPager->aInCkpt = 0;
    }
    return rc;
  }
  
  /*
  ** Commit a checkpoint.
  */
  int sqlitepager_ckpt_commit(Pager *pPager){
    if( pPager->ckptInUse ){
      PgHdr *pPg, *pNext;
      sqliteOsSeek(&pPager->cpfd, 0);
      /* sqliteOsTruncate(&pPager->cpfd, 0); */
      pPager->ckptNRec = 0;
      pPager->ckptInUse = 0;
      sqliteFree( pPager->aInCkpt );
      pPager->aInCkpt = 0;
      for(pPg=pPager->pCkpt; pPg; pPg=pNext){
        pNext = pPg->pNextCkpt;
        assert( pPg->inCkpt );
        pPg->inCkpt = 0;
        pPg->pPrevCkpt = pPg->pNextCkpt = 0;
      }
      pPager->pCkpt = 0;
    }
    pPager->ckptAutoopen = 0;
    return SQLITE_OK;
  }
  
  /*
  ** Rollback a checkpoint.
  */
  int sqlitepager_ckpt_rollback(Pager *pPager){
    int rc;
    if( pPager->ckptInUse ){
      rc = pager_ckpt_playback(pPager);
      sqlitepager_ckpt_commit(pPager);
    }else{
      rc = SQLITE_OK;
    }
    pPager->ckptAutoopen = 0;
    return rc;
  }
  
  /*
  ** Return the full pathname of the database file.
  */
  const char *sqlitepager_filename(Pager *pPager){
    return pPager->zFilename;
  }
  
  /*
  ** Set the codec for this pager
  */
  void sqlitepager_set_codec(
    Pager *pPager,
    void (*xCodec)(void*,void*,Pgno,int),
    void *pCodecArg
  ){
    pPager->xCodec = xCodec;
    pPager->pCodecArg = pCodecArg;
  }
  
  #ifdef SQLITE_TEST
  /*
  ** Print a listing of all referenced pages and their ref count.
  */
  void sqlitepager_refdump(Pager *pPager){
    PgHdr *pPg;
    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
      if( pPg->nRef<=0 ) continue;
      printf("PAGE %3d addr=0x%08x nRef=%d\n", 
         pPg->pgno, (int)PGHDR_TO_DATA(pPg), pPg->nRef);
    }
  }
  #endif
  