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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Refactor wal.c to use the VFS. This check-in compiles and links and works ok as long as you leave WAL turned off, but WAL does not work.
Downloads: Tarball | ZIP archive | SQL archive
Timelines: family | ancestors | descendants | both | wal
Files: files | file ages | folders
SHA1: 62db5fa3b61be885b2d94e9b9ce3877b2c588350
User & Date: drh 2010-04-29 22:34:08
Context
2010-04-30
02:13
The first 6 WAL tests now work. It's a start. check-in: a92c1851 user: drh tags: wal
2010-04-29
22:34
Refactor wal.c to use the VFS. This check-in compiles and links and works ok as long as you leave WAL turned off, but WAL does not work. check-in: 62db5fa3 user: drh tags: wal
16:40
Untested implementation of the shared-memory dead-man-switch. check-in: 70661128 user: drh tags: wal
Changes
Hide Diffs Unified Diffs Ignore Whitespace Patch

Changes to src/btree.c.

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    ** The caller detects this and calls this function again. This is
    ** required as the version of page 1 currently in the page1 buffer
    ** may not be the latest version - there may be a newer one in the log
    ** file.
    */
    if( page1[19]==2 && pBt->doNotUseWAL==0 ){
      int isOpen = 0;
      rc = sqlite3PagerOpenLog(pBt->pPager, &isOpen);
      if( rc!=SQLITE_OK ){
        goto page1_init_failed;
      }else if( isOpen==0 ){
        releasePage(pPage1);
        return SQLITE_OK;
      }
      rc = SQLITE_NOTADB;







|







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    ** The caller detects this and calls this function again. This is
    ** required as the version of page 1 currently in the page1 buffer
    ** may not be the latest version - there may be a newer one in the log
    ** file.
    */
    if( page1[19]==2 && pBt->doNotUseWAL==0 ){
      int isOpen = 0;
      rc = sqlite3PagerOpenWal(pBt->pPager, &isOpen);
      if( rc!=SQLITE_OK ){
        goto page1_init_failed;
      }else if( isOpen==0 ){
        releasePage(pPage1);
        return SQLITE_OK;
      }
      rc = SQLITE_NOTADB;

Changes to src/main.c.

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void *sqlite3_wal_hook(
  sqlite3 *db,                    /* Attach the hook to this db handle */
  int(*xCallback)(void *, sqlite3*, const char*, int),
  void *pArg                      /* First argument passed to xCallback() */
){
  void *pRet;
  sqlite3_mutex_enter(db->mutex);
  pRet = db->pLogArg;
  db->xLogCallback = xCallback;
  db->pLogArg = pArg;
  sqlite3_mutex_leave(db->mutex);
  return pRet;
}

/*
** This function returns true if main-memory should be used instead of
** a temporary file for transient pager files and statement journals.







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void *sqlite3_wal_hook(
  sqlite3 *db,                    /* Attach the hook to this db handle */
  int(*xCallback)(void *, sqlite3*, const char*, int),
  void *pArg                      /* First argument passed to xCallback() */
){
  void *pRet;
  sqlite3_mutex_enter(db->mutex);
  pRet = db->pWalArg;
  db->xWalCallback = xCallback;
  db->pWalArg = pArg;
  sqlite3_mutex_leave(db->mutex);
  return pRet;
}

/*
** This function returns true if main-memory should be used instead of
** a temporary file for transient pager files and statement journals.

Changes to src/os_unix.c.

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*/
static int unixShmClose(sqlite3_shm *pSharedMem){
  unixShm *p;            /* The connection to be closed */
  unixShmFile *pFile;    /* The underlying shared-memory file */
  unixShm **pp;          /* For looping over sibling connections */
  int nRef;              /* Number of connections to pFile */


  p = (struct unixShm*)pSharedMem;
  pFile = p->pFile;

  /* Verify that the connection being closed holds no locks */
  assert( p->exclMask==0 );
  assert( p->sharedMask==0 );

................................................................................
** shared memory segment.  xShmRelease() must be called to release the
** lock.
*/
static int unixShmSize(
  sqlite3_shm *pSharedMem,  /* Pointer returned by unixShmOpen() */
  int reqSize,              /* Requested size.  -1 for query only */
  int *pNewSize,            /* Write new size here */
  char **ppBuf              /* Write new buffer origin here */
){
  unixShm *p = (unixShm*)pSharedMem;
  unixShmFile *pFile = p->pFile;
  int rc = SQLITE_OK;

  sqlite3_mutex_enter(pFile->mutexBuf);
  sqlite3_mutex_enter(pFile->mutex);
................................................................................
  /* Return directly if this is just a lock state query, or if
  ** the connection is already in the desired locking state.
  */
  if( desiredLock==SQLITE_SHM_QUERY
   || desiredLock==p->lockState
   || (desiredLock==SQLITE_SHM_READ && p->lockState==SQLITE_SHM_READ_FULL)
  ){
    *pGotLock = p->lockState;
    return SQLITE_OK;
  }

  sqlite3_mutex_enter(pFile->mutex);
  switch( desiredLock ){
    case SQLITE_SHM_UNLOCK: {
      assert( p->lockState!=SQLITE_SHM_RECOVER );
................................................................................
      if( rc==SQLITE_OK ){
        p->lockState = SQLITE_SHM_RECOVER;
      }
      break;
    }
  }
  sqlite3_mutex_leave(pFile->mutex);
  *pGotLock = p->lockState;
  return rc;
}

/*
** Delete a shared-memory segment from the system.
*/
static int unixShmDelete(sqlite3_vfs *pVfs, const char *zName){







>







 







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*/
static int unixShmClose(sqlite3_shm *pSharedMem){
  unixShm *p;            /* The connection to be closed */
  unixShmFile *pFile;    /* The underlying shared-memory file */
  unixShm **pp;          /* For looping over sibling connections */
  int nRef;              /* Number of connections to pFile */

  if( pSharedMem==0 ) return SQLITE_OK;
  p = (struct unixShm*)pSharedMem;
  pFile = p->pFile;

  /* Verify that the connection being closed holds no locks */
  assert( p->exclMask==0 );
  assert( p->sharedMask==0 );

................................................................................
** shared memory segment.  xShmRelease() must be called to release the
** lock.
*/
static int unixShmSize(
  sqlite3_shm *pSharedMem,  /* Pointer returned by unixShmOpen() */
  int reqSize,              /* Requested size.  -1 for query only */
  int *pNewSize,            /* Write new size here */
  void **ppBuf              /* Write new buffer origin here */
){
  unixShm *p = (unixShm*)pSharedMem;
  unixShmFile *pFile = p->pFile;
  int rc = SQLITE_OK;

  sqlite3_mutex_enter(pFile->mutexBuf);
  sqlite3_mutex_enter(pFile->mutex);
................................................................................
  /* Return directly if this is just a lock state query, or if
  ** the connection is already in the desired locking state.
  */
  if( desiredLock==SQLITE_SHM_QUERY
   || desiredLock==p->lockState
   || (desiredLock==SQLITE_SHM_READ && p->lockState==SQLITE_SHM_READ_FULL)
  ){
    if( pGotLock ) *pGotLock = p->lockState;
    return SQLITE_OK;
  }

  sqlite3_mutex_enter(pFile->mutex);
  switch( desiredLock ){
    case SQLITE_SHM_UNLOCK: {
      assert( p->lockState!=SQLITE_SHM_RECOVER );
................................................................................
      if( rc==SQLITE_OK ){
        p->lockState = SQLITE_SHM_RECOVER;
      }
      break;
    }
  }
  sqlite3_mutex_leave(pFile->mutex);
  if( pGotLock ) *pGotLock = p->lockState;
  return rc;
}

/*
** Delete a shared-memory segment from the system.
*/
static int unixShmDelete(sqlite3_vfs *pVfs, const char *zName){

Changes to src/pager.c.

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  void (*xCodecSizeChng)(void*,int,int); /* Notify of page size changes */
  void (*xCodecFree)(void*);             /* Destructor for the codec */
  void *pCodec;               /* First argument to xCodec... methods */
#endif
  char *pTmpSpace;            /* Pager.pageSize bytes of space for tmp use */
  PCache *pPCache;            /* Pointer to page cache object */
  sqlite3_backup *pBackup;    /* Pointer to list of ongoing backup processes */
  Log *pLog;                  /* Log used by "journal_mode=wal" */
};

/*
** The following global variables hold counters used for
** testing purposes only.  These variables do not exist in
** a non-testing build.  These variables are not thread-safe.
*/
................................................................................
  return rc;
}

/*
** Return true if this pager uses a write-ahead log instead of the usual
** rollback journal. Otherwise false.
*/
static int pagerUseLog(Pager *pPager){
  return (pPager->pLog!=0);
}

/*
** Unlock the database file. This function is a no-op if the pager
** is in exclusive mode.
**
** If the pager is currently in error state, discard the contents of 
................................................................................
    ** values stored in Pager.dbSize etc. might become invalid if
    ** this happens.  One can argue that this doesn't need to be cleared
    ** until the change-counter check fails in PagerSharedLock().
    ** Clearing the page size cache here is being conservative.
    */
    pPager->dbSizeValid = 0;

    if( pagerUseLog(pPager) ){
      sqlite3WalCloseSnapshot(pPager->pLog);
    }else{
      rc = osUnlock(pPager->fd, NO_LOCK);
    }
    if( rc ){
      pPager->errCode = rc;
    }
    IOTRACE(("UNLOCK %p\n", pPager))
................................................................................
  if( pPager->state<PAGER_RESERVED ){
    return SQLITE_OK;
  }
  releaseAllSavepoints(pPager);

  assert( isOpen(pPager->jfd) || pPager->pInJournal==0 );
  if( isOpen(pPager->jfd) ){
    assert( !pagerUseLog(pPager) );

    /* Finalize the journal file. */
    if( sqlite3IsMemJournal(pPager->jfd) ){
      assert( pPager->journalMode==PAGER_JOURNALMODE_MEMORY );
      sqlite3OsClose(pPager->jfd);
    }else if( pPager->journalMode==PAGER_JOURNALMODE_TRUNCATE ){
      if( pPager->journalOff==0 ){
................................................................................
#endif
  }
  sqlite3BitvecDestroy(pPager->pInJournal);
  pPager->pInJournal = 0;
  pPager->nRec = 0;
  sqlite3PcacheCleanAll(pPager->pPCache);

  if( pagerUseLog(pPager) ){
    rc2 = sqlite3WalWriteLock(pPager->pLog, 0);
    pPager->state = PAGER_SHARED;
  }else if( !pPager->exclusiveMode ){
    rc2 = osUnlock(pPager->fd, SHARED_LOCK);
    pPager->state = PAGER_SHARED;
    pPager->changeCountDone = 0;
  }else if( pPager->state==PAGER_SYNCED ){
    pPager->state = PAGER_EXCLUSIVE;
................................................................................
  assert( (isMainJrnl&~1)==0 );      /* isMainJrnl is 0 or 1 */
  assert( (isSavepnt&~1)==0 );       /* isSavepnt is 0 or 1 */
  assert( isMainJrnl || pDone );     /* pDone always used on sub-journals */
  assert( isSavepnt || pDone==0 );   /* pDone never used on non-savepoint */

  aData = pPager->pTmpSpace;
  assert( aData );         /* Temp storage must have already been allocated */
  assert( pagerUseLog(pPager)==0 || (!isMainJrnl && isSavepnt) );

  /* Read the page number and page data from the journal or sub-journal
  ** file. Return an error code to the caller if an IO error occurs.
  */
  jfd = isMainJrnl ? pPager->jfd : pPager->sjfd;
  rc = read32bits(jfd, *pOffset, &pgno);
  if( rc!=SQLITE_OK ) return rc;
................................................................................
  ** in the main journal either because the page is not in cache or else
  ** the page is marked as needSync==0.
  **
  ** 2008-04-14:  When attempting to vacuum a corrupt database file, it
  ** is possible to fail a statement on a database that does not yet exist.
  ** Do not attempt to write if database file has never been opened.
  */
  if( pagerUseLog(pPager) ){
    pPg = 0;
  }else{
    pPg = pager_lookup(pPager, pgno);
  }
  assert( pPg || !MEMDB );
  PAGERTRACE(("PLAYBACK %d page %d hash(%08x) %s\n",
           PAGERID(pPager), pgno, pager_datahash(pPager->pageSize, (u8*)aData),
................................................................................
  }
  if( (pPager->state>=PAGER_EXCLUSIVE)
   && isOpen(pPager->fd)
   && isSynced
  ){
    i64 ofst = (pgno-1)*(i64)pPager->pageSize;
    testcase( !isSavepnt && pPg!=0 && (pPg->flags&PGHDR_NEED_SYNC)!=0 );
    assert( !pagerUseLog(pPager) );
    rc = sqlite3OsWrite(pPager->fd, (u8*)aData, pPager->pageSize, ofst);
    if( pgno>pPager->dbFileSize ){
      pPager->dbFileSize = pgno;
    }
    if( pPager->pBackup ){
      CODEC1(pPager, aData, pgno, 3, rc=SQLITE_NOMEM);
      sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)aData);
................................................................................
      ** the PGHDR_NEED_SYNC flag is cleared, if the page is written to
      ** again within this transaction, it will be marked as dirty but
      ** the PGHDR_NEED_SYNC flag will not be set. It could then potentially
      ** be written out into the database file before its journal file
      ** segment is synced. If a crash occurs during or following this,
      ** database corruption may ensue.
      */
      assert( !pagerUseLog(pPager) );
      sqlite3PcacheMakeClean(pPg);
    }
#ifdef SQLITE_CHECK_PAGES
    pPg->pageHash = pager_pagehash(pPg);
#endif
    /* If this was page 1, then restore the value of Pager.dbFileVers.
    ** Do this before any decoding. */
................................................................................
** Otherwise, SQLITE_OK is returned.
*/
static int readDbPage(PgHdr *pPg){
  Pager *pPager = pPg->pPager; /* Pager object associated with page pPg */
  Pgno pgno = pPg->pgno;       /* Page number to read */
  int rc = SQLITE_OK;          /* Return code */
  i64 iOffset;                 /* Byte offset of file to read from */
  int isInLog = 0;             /* True if page is in log file */

  assert( pPager->state>=PAGER_SHARED && !MEMDB );
  assert( isOpen(pPager->fd) );

  if( NEVER(!isOpen(pPager->fd)) ){
    assert( pPager->tempFile );
    memset(pPg->pData, 0, pPager->pageSize);
    return SQLITE_OK;
  }

  if( pagerUseLog(pPager) ){
    /* Try to pull the page from the write-ahead log. */
    rc = sqlite3WalRead(pPager->pLog, pgno, &isInLog, pPg->pData);
  }
  if( rc==SQLITE_OK && !isInLog ){
    iOffset = (pgno-1)*(i64)pPager->pageSize;
    rc = sqlite3OsRead(pPager->fd, pPg->pData, pPager->pageSize, iOffset);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
    }
  }

................................................................................

  return rc;
}

/*
** This function is called to rollback a transaction on a WAL database.
*/
static int pagerRollbackLog(Pager *pPager){
  int rc;                         /* Return Code */
  PgHdr *pList;                   /* List of dirty pages to revert */

  /* Normally, if a transaction is rolled back, any backup processes are
  ** updated as data is copied out of the rollback journal and into the
  ** database. This is not generally possible with a WAL database, as
  ** rollback involves simply truncating the log file. Therefore, if one
  ** or more frames have already been written to the log (and therefore 
  ** also copied into the backup databases) as part of this transaction,
  ** the backups must be restarted.
  */
  if( sqlite3WalDirty(pPager->pLog) ){
    sqlite3BackupRestart(pPager->pBackup);
  }

  /* For all pages in the cache that are currently dirty or have already
  ** been written (but not committed) to the log file, do one of the 
  ** following:
  **
  **   + Discard the cached page (if refcount==0), or
  **   + Reload page content from the database (if refcount>0).
  */
  pPager->dbSize = pPager->dbOrigSize;
  rc = sqlite3WalUndo(pPager->pLog, pagerUndoCallback, (void *)pPager);
  pList = sqlite3PcacheDirtyList(pPager->pPCache);
  while( pList && rc==SQLITE_OK ){
    PgHdr *pNext = pList->pDirty;
    rc = pagerUndoCallback((void *)pPager, pList->pgno);
    pList = pNext;
  }

................................................................................
  }

  /* Set the database size back to the value it was before the savepoint 
  ** being reverted was opened.
  */
  pPager->dbSize = pSavepoint ? pSavepoint->nOrig : pPager->dbOrigSize;

  if( !pSavepoint && pagerUseLog(pPager) ){
    return pagerRollbackLog(pPager);
  }

  /* Use pPager->journalOff as the effective size of the main rollback
  ** journal.  The actual file might be larger than this in
  ** PAGER_JOURNALMODE_TRUNCATE or PAGER_JOURNALMODE_PERSIST.  But anything
  ** past pPager->journalOff is off-limits to us.
  */
  szJ = pPager->journalOff;
  assert( pagerUseLog(pPager)==0 || szJ==0 );

  /* Begin by rolling back records from the main journal starting at
  ** PagerSavepoint.iOffset and continuing to the next journal header.
  ** There might be records in the main journal that have a page number
  ** greater than the current database size (pPager->dbSize) but those
  ** will be skipped automatically.  Pages are added to pDone as they
  ** are played back.
  */
  if( pSavepoint && !pagerUseLog(pPager) ){
    iHdrOff = pSavepoint->iHdrOffset ? pSavepoint->iHdrOffset : szJ;
    pPager->journalOff = pSavepoint->iOffset;
    while( rc==SQLITE_OK && pPager->journalOff<iHdrOff ){
      rc = pager_playback_one_page(pPager, &pPager->journalOff, pDone, 1, 1);
    }
    assert( rc!=SQLITE_DONE );
  }else{
................................................................................
  ** previously rolled back out of the main journal (and are hence in pDone)
  ** will be skipped.  Out-of-range pages are also skipped.
  */
  if( pSavepoint ){
    u32 ii;            /* Loop counter */
    i64 offset = pSavepoint->iSubRec*(4+pPager->pageSize);

    if( pagerUseLog(pPager) ){
      rc = sqlite3WalSavepointUndo(pPager->pLog, pSavepoint->iFrame);
    }
    for(ii=pSavepoint->iSubRec; rc==SQLITE_OK && ii<pPager->nSubRec; ii++){
      assert( offset==ii*(4+pPager->pageSize) );
      rc = pager_playback_one_page(pPager, &offset, pDone, 0, 1);
    }
    assert( rc!=SQLITE_DONE );
  }
................................................................................
  /* Determine the number of pages in the file. Store this in nPage. */
  if( pPager->dbSizeValid ){
    nPage = pPager->dbSize;
  }else{
    int rc;                 /* Error returned by OsFileSize() */
    i64 n = 0;              /* File size in bytes returned by OsFileSize() */

    if( pagerUseLog(pPager) ){
      sqlite3WalDbsize(pPager->pLog, &nPage);
    }

    if( nPage==0 ){
      assert( isOpen(pPager->fd) || pPager->tempFile );
      if( isOpen(pPager->fd) ){
        if( SQLITE_OK!=(rc = sqlite3OsFileSize(pPager->fd, &n)) ){
          pager_error(pPager, rc);
................................................................................
int sqlite3PagerClose(Pager *pPager){
  u8 *pTmp = (u8 *)pPager->pTmpSpace;

  disable_simulated_io_errors();
  sqlite3BeginBenignMalloc();
  pPager->errCode = 0;
  pPager->exclusiveMode = 0;
  sqlite3WalClose(pPager->pLog, pPager->fd, 
    (pPager->noSync ? 0 : pPager->sync_flags), pTmp
  );
  pPager->pLog = 0;
  pager_reset(pPager);
  if( MEMDB ){
    pager_unlock(pPager);
  }else{
    /* Set Pager.journalHdr to -1 for the benefit of the pager_playback() 
    ** call which may be made from within pagerUnlockAndRollback(). If it
    ** is not -1, then the unsynced portion of an open journal file may
................................................................................
  **
  ** While the pager is in the RESERVED state, the original database file
  ** is unchanged and we can rollback without having to playback the
  ** journal into the original database file.  Once we transition to
  ** EXCLUSIVE, it means the database file has been changed and any rollback
  ** will require a journal playback.
  */
  assert( !pagerUseLog(pList->pPager) );
  assert( pPager->state>=PAGER_RESERVED );
  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);

  /* If the file is a temp-file has not yet been opened, open it now. It
  ** is not possible for rc to be other than SQLITE_OK if this branch
  ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
  */
................................................................................
    void *pData = pPg->pData;
    i64 offset = pPager->nSubRec*(4+pPager->pageSize);
    char *pData2;

    CODEC2(pPager, pData, pPg->pgno, 7, return SQLITE_NOMEM, pData2);
    PAGERTRACE(("STMT-JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno));
  
    assert( pagerUseLog(pPager) 
         || pageInJournal(pPg) 
         || pPg->pgno>pPager->dbOrigSize 
    );
    rc = write32bits(pPager->sjfd, offset, pPg->pgno);
    if( rc==SQLITE_OK ){
      rc = sqlite3OsWrite(pPager->sjfd, pData2, pPager->pageSize, offset+4);
    }
................................................................................

/*
** This function is a wrapper around sqlite3WalFrames(). As well as logging
** the contents of the list of pages headed by pList (connected by pDirty),
** this function notifies any active backup processes that the pages have
** changed. 
*/ 
static int pagerLogFrames(
  Pager *pPager,                  /* Pager object */
  PgHdr *pList,                   /* List of frames to log */
  Pgno nTruncate,                 /* Database size after this commit */
  int isCommit,                   /* True if this is a commit */
  int sync_flags                  /* Flags to pass to OsSync() (or 0) */
){
  int rc;                         /* Return code */

  assert( pPager->pLog );
  rc = sqlite3WalFrames(pPager->pLog, 
      pPager->pageSize, pList, nTruncate, isCommit, sync_flags
  );
  if( rc==SQLITE_OK && pPager->pBackup ){
    PgHdr *p;
    for(p=pList; p; p=p->pDirty){
      sqlite3BackupUpdate(pPager->pBackup, p->pgno, (u8 *)p->pData);
    }
................................................................................
  Pager *pPager = (Pager *)p;
  int rc = SQLITE_OK;

  assert( pPg->pPager==pPager );
  assert( pPg->flags&PGHDR_DIRTY );

  pPg->pDirty = 0;
  if( pagerUseLog(pPager) ){
    /* Write a single frame for this page to the log. */
    if( subjRequiresPage(pPg) ){ 
      rc = subjournalPage(pPg); 
    }
    if( rc==SQLITE_OK ){
      rc = pagerLogFrames(pPager, pPg, 0, 0, 0);
    }
  }else{
    /* The doNotSync flag is set by the sqlite3PagerWrite() function while it
    ** is journalling a set of two or more database pages that are stored
    ** on the same disk sector. Syncing the journal is not allowed while
    ** this is happening as it is important that all members of such a
    ** set of pages are synced to disk together. So, if the page this function
................................................................................
** a WAL, this ensures there is no race condition between the xAccess() 
** below and an xDelete() being executed by some other connection.
*/
static int pagerHasWAL(Pager *pPager, int *pExists){
  int rc;                         /* Return code */

  if( !pPager->tempFile ){
    char *zLog = sqlite3_mprintf("%s-wal", pPager->zFilename);
    if( !zLog ){
      rc = SQLITE_NOMEM;
    }else{
      rc = sqlite3OsAccess(pPager->pVfs, zLog, SQLITE_ACCESS_EXISTS, pExists);
      sqlite3_free(zLog);
    }
  }else{
    rc = SQLITE_OK;
    *pExists = 0;
  }
  return rc;
}

static int pagerOpenSnapshot(Pager *pPager){
  int rc;                         /* Return code */
  int changed = 0;                /* True if cache must be reset */

  assert( pagerUseLog(pPager) );

  rc = sqlite3WalOpenSnapshot(pPager->pLog, &changed);
  if( rc==SQLITE_OK ){
    int dummy;
    if( changed ){
      pager_reset(pPager);
      assert( pPager->errCode || pPager->dbSizeValid==0 );
    }
    rc = sqlite3PagerPagecount(pPager, &dummy);
................................................................................
    if( isOpen(pPager->jfd) || pPager->zJournal ){
      isErrorReset = 1;
    }
    pPager->errCode = SQLITE_OK;
    pager_reset(pPager);
  }

  if( pagerUseLog(pPager) ){
    rc = pagerOpenSnapshot(pPager);
  }else if( pPager->state==PAGER_UNLOCK || isErrorReset ){
    sqlite3_vfs * const pVfs = pPager->pVfs;
    int isHotJournal = 0;
    int isWal = 0;
    assert( !MEMDB );
    assert( sqlite3PcacheRefCount(pPager->pPCache)==0 );
................................................................................

    rc = pagerHasWAL(pPager, &isWal);
    if( rc!=SQLITE_OK ){
      goto failed;
    }
    if( isWal ){
      pager_reset(pPager);
      rc = sqlite3PagerOpenLog(pPager, 0);
      if( rc==SQLITE_OK ){
        rc = pagerOpenSnapshot(pPager);
      }
    }else if( pPager->journalMode==PAGER_JOURNALMODE_WAL ){
      pPager->journalMode = PAGER_JOURNALMODE_DELETE;
    }
  }
................................................................................
**
** SQLITE_OK is returned if everything goes according to plan. 
** An SQLITE_IOERR_XXX error code is returned if a call to 
** sqlite3OsOpen() fails.
*/
static int openSubJournal(Pager *pPager){
  int rc = SQLITE_OK;
  if( (pagerUseLog(pPager) || isOpen(pPager->jfd)) && !isOpen(pPager->sjfd) ){
    if( pPager->journalMode==PAGER_JOURNALMODE_MEMORY || pPager->subjInMemory ){
      sqlite3MemJournalOpen(pPager->sjfd);
    }else{
      rc = pagerOpentemp(pPager, pPager->sjfd, SQLITE_OPEN_SUBJOURNAL);
    }
  }
  return rc;
................................................................................
  int rc = SQLITE_OK;
  assert( pPager->state!=PAGER_UNLOCK );
  pPager->subjInMemory = (u8)subjInMemory;
  if( pPager->state==PAGER_SHARED ){
    assert( pPager->pInJournal==0 );
    assert( !MEMDB && !pPager->tempFile );

    if( pagerUseLog(pPager) ){
      /* Grab the write lock on the log file. If successful, upgrade to
      ** PAGER_EXCLUSIVE state. Otherwise, return an error code to the caller.
      ** The busy-handler is not invoked if another connection already
      ** holds the write-lock. If possible, the upper layer will call it.
      */
      rc = sqlite3WalWriteLock(pPager->pLog, 1);
      if( rc==SQLITE_OK ){
        pPager->dbOrigSize = pPager->dbSize;
        pPager->state = PAGER_RESERVED;
        pPager->journalOff = 0;
      }
    }else{
      /* Obtain a RESERVED lock on the database file. If the exFlag parameter
................................................................................
  }else if( isOpen(pPager->jfd) && pPager->journalOff==0 ){
    /* This happens when the pager was in exclusive-access mode the last
    ** time a (read or write) transaction was successfully concluded
    ** by this connection. Instead of deleting the journal file it was 
    ** kept open and either was truncated to 0 bytes or its header was
    ** overwritten with zeros.
    */
    assert( pagerUseLog(pPager)==0 );
    assert( pPager->nRec==0 );
    assert( pPager->dbOrigSize==0 );
    assert( pPager->pInJournal==0 );
    rc = pager_open_journal(pPager);
  }

  PAGERTRACE(("TRANSACTION %d\n", PAGERID(pPager)));
................................................................................
  CHECK_PAGE(pPg);

  /* Mark the page as dirty.  If the page has already been written
  ** to the journal then we can return right away.
  */
  sqlite3PcacheMakeDirty(pPg);
  if( pageInJournal(pPg) && !subjRequiresPage(pPg) ){
    assert( !pagerUseLog(pPager) );
    pPager->dbModified = 1;
  }else{

    /* If we get this far, it means that the page needs to be
    ** written to the transaction journal or the ckeckpoint journal
    ** or both.
    **
................................................................................
    */
    rc = sqlite3PagerBegin(pPager, 0, pPager->subjInMemory);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    if( !isOpen(pPager->jfd) 
     && pPager->journalMode!=PAGER_JOURNALMODE_OFF 
     && !pagerUseLog(pPager)
    ){
      assert( pPager->useJournal );
      rc = pager_open_journal(pPager);
      if( rc!=SQLITE_OK ) return rc;
    }
    pPager->dbModified = 1;
  
    /* The transaction journal now exists and we have a RESERVED or an
    ** EXCLUSIVE lock on the main database file.  Write the current page to
    ** the transaction journal if it is not there already.
    */
    if( !pageInJournal(pPg) && isOpen(pPager->jfd) ){
      assert( !pagerUseLog(pPager) );
      if( pPg->pgno<=pPager->dbOrigSize ){
        u32 cksum;
        char *pData2;

        /* We should never write to the journal file the page that
        ** contains the database locks.  The following assert verifies
        ** that we do not. */
................................................................................
  if( MEMDB && pPager->dbModified ){
    /* If this is an in-memory db, or no pages have been written to, or this
    ** function has already been called, it is mostly a no-op.  However, any
    ** backup in progress needs to be restarted.
    */
    sqlite3BackupRestart(pPager->pBackup);
  }else if( pPager->state!=PAGER_SYNCED && pPager->dbModified ){
    if( pagerUseLog(pPager) ){
      PgHdr *pList = sqlite3PcacheDirtyList(pPager->pPCache);
      if( pList ){
        rc = pagerLogFrames(pPager, pList, pPager->dbSize, 1, 
            (pPager->fullSync ? pPager->sync_flags : 0)
        );
      }
      sqlite3PcacheCleanAll(pPager->pPCache);
    }else{
      /* The following block updates the change-counter. Exactly how it
      ** does this depends on whether or not the atomic-update optimization
................................................................................
**   the journal file. It needs to be left in the file-system so that
**   some other process can use it to restore the database state (by
**   hot-journal rollback).
*/
int sqlite3PagerRollback(Pager *pPager){
  int rc = SQLITE_OK;                  /* Return code */
  PAGERTRACE(("ROLLBACK %d\n", PAGERID(pPager)));
  if( pagerUseLog(pPager) ){
    int rc2;

    rc = sqlite3PagerSavepoint(pPager, SAVEPOINT_ROLLBACK, -1);
    rc2 = pager_end_transaction(pPager, pPager->setMaster);
    if( rc==SQLITE_OK ) rc = rc2;
  }else if( !pPager->dbModified || !isOpen(pPager->jfd) ){
    rc = pager_end_transaction(pPager, pPager->setMaster);
................................................................................
        aNew[ii].iOffset = JOURNAL_HDR_SZ(pPager);
      }
      aNew[ii].iSubRec = pPager->nSubRec;
      aNew[ii].pInSavepoint = sqlite3BitvecCreate(nPage);
      if( !aNew[ii].pInSavepoint ){
        return SQLITE_NOMEM;
      }
      if( pagerUseLog(pPager) ){
        aNew[ii].iFrame = sqlite3WalSavepoint(pPager->pLog);
      }
    }

    /* Open the sub-journal, if it is not already opened. */
    rc = openSubJournal(pPager);
    assertTruncateConstraint(pPager);
  }
................................................................................
      }
    }
    /* Else this is a rollback operation, playback the specified savepoint.
    ** If this is a temp-file, it is possible that the journal file has
    ** not yet been opened. In this case there have been no changes to
    ** the database file, so the playback operation can be skipped.
    */
    else if( pagerUseLog(pPager) || isOpen(pPager->jfd) ){
      PagerSavepoint *pSavepoint = (nNew==0)?0:&pPager->aSavepoint[nNew-1];
      rc = pagerPlaybackSavepoint(pPager, pSavepoint);
      assert(rc!=SQLITE_DONE);
    }
  
  }
  return rc;
................................................................................
}

/*
** This function is called when the user invokes "PRAGMA checkpoint".
*/
int sqlite3PagerCheckpoint(Pager *pPager){
  int rc = SQLITE_OK;
  if( pPager->pLog ){
    u8 *zBuf = (u8 *)pPager->pTmpSpace;
    rc = sqlite3WalCheckpoint(pPager->pLog, pPager->fd, 
        (pPager->noSync ? 0 : pPager->sync_flags),
        zBuf, pPager->xBusyHandler, pPager->pBusyHandlerArg
    );
  }
  return rc;
}

int sqlite3PagerLogCallback(Pager *pPager){
  return sqlite3WalCallback(pPager->pLog);
}

/*
** Open a connection to the write-ahead log file for pager pPager. If
** the log connection is already open, this function is a no-op.
**
** The caller must be holding a SHARED lock on the database file to call
** this function.
*/
int sqlite3PagerOpenLog(Pager *pPager, int *pisOpen){
  int rc = SQLITE_OK;             /* Return code */

  assert( pPager->state>=PAGER_SHARED );
  if( !pPager->pLog ){

    /* Open the connection to the log file. If this operation fails, 
    ** (e.g. due to malloc() failure), unlock the database file and 
    ** return an error code.
    */
    rc = sqlite3WalOpen(pPager->pVfs, pPager->zFilename, &pPager->pLog);
    if( rc==SQLITE_OK ){
      pPager->journalMode = PAGER_JOURNALMODE_WAL;
    }
  }else{
    *pisOpen = 1;
  }

................................................................................
** to switching from WAL to rollback mode.
**
** Before closing the log file, this function attempts to take an 
** EXCLUSIVE lock on the database file. If this cannot be obtained, an
** error (SQLITE_BUSY) is returned and the log connection is not closed.
** If successful, the EXCLUSIVE lock is not released before returning.
*/
int sqlite3PagerCloseLog(Pager *pPager){
  int rc = SQLITE_OK;

  assert( pPager->journalMode==PAGER_JOURNALMODE_WAL );

  /* If the log file is not already open, but does exist in the file-system,
  ** it may need to be checkpointed before the connection can switch to
  ** rollback mode. Open it now so this can happen.
  */
  if( !pPager->pLog ){
    int logexists = 0;
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_SHARED);
    if( rc==SQLITE_OK ){
      rc = pagerHasWAL(pPager, &logexists);
    }
    if( rc==SQLITE_OK && logexists ){
      rc = sqlite3WalOpen(pPager->pVfs, pPager->zFilename, &pPager->pLog);
    }
  }
    
  /* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
  ** the database file, the log and log-summary files will be deleted.
  */
  if( rc==SQLITE_OK && pPager->pLog ){
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_EXCLUSIVE);
    if( rc==SQLITE_OK ){
      rc = sqlite3WalClose(pPager->pLog, pPager->fd,
        (pPager->noSync ? 0 : pPager->sync_flags), 
        (u8*)pPager->pTmpSpace
      );
      pPager->pLog = 0;
    }
  }
  return rc;
}

#endif /* SQLITE_OMIT_DISKIO */







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2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
....
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
....
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
....
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
....
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
....
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
....
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
....
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
....
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
....
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
....
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
....
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
....
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
....
4328
4329
4330
4331
4332
4333
4334
4335
4336
4337
4338
4339
4340
4341
4342
....
4464
4465
4466
4467
4468
4469
4470
4471
4472
4473
4474
4475
4476
4477
4478
4479
4480
4481
4482
4483
4484
....
4503
4504
4505
4506
4507
4508
4509
4510
4511
4512
4513
4514
4515
4516
4517
....
4558
4559
4560
4561
4562
4563
4564
4565
4566
4567
4568
4569
4570
4571
4572
....
4576
4577
4578
4579
4580
4581
4582
4583
4584
4585
4586
4587
4588
4589
4590
4591
4592
4593
4594
4595
4596
4597
4598
4599
4600
4601
4602
4603
....
4970
4971
4972
4973
4974
4975
4976
4977
4978
4979
4980
4981
4982
4983
4984
4985
4986
4987
....
5209
5210
5211
5212
5213
5214
5215
5216
5217
5218
5219
5220
5221
5222
5223
....
5356
5357
5358
5359
5360
5361
5362
5363
5364
5365
5366
5367
5368
5369
5370
5371
....
5436
5437
5438
5439
5440
5441
5442
5443
5444
5445
5446
5447
5448
5449
5450
....
5792
5793
5794
5795
5796
5797
5798
5799
5800
5801
5802
5803
5804
5805
5806
5807
5808
5809
5810
5811
5812
5813
5814
5815
5816
5817
5818
5819
5820
5821
5822
5823
5824
5825
5826
5827
5828
5829
5830
5831
5832
5833
5834
5835
5836
5837
....
5844
5845
5846
5847
5848
5849
5850
5851
5852
5853
5854
5855
5856
5857
5858
5859
5860
5861
5862
5863
5864
5865
5866
5867
5868
5869
5870
5871
5872
5873
5874
5875
5876
5877
5878
5879
5880
5881
5882
5883
5884
5885
5886
5887
  void (*xCodecSizeChng)(void*,int,int); /* Notify of page size changes */
  void (*xCodecFree)(void*);             /* Destructor for the codec */
  void *pCodec;               /* First argument to xCodec... methods */
#endif
  char *pTmpSpace;            /* Pager.pageSize bytes of space for tmp use */
  PCache *pPCache;            /* Pointer to page cache object */
  sqlite3_backup *pBackup;    /* Pointer to list of ongoing backup processes */
  Wal *pWal;                  /* Write-ahead log used by "journal_mode=wal" */
};

/*
** The following global variables hold counters used for
** testing purposes only.  These variables do not exist in
** a non-testing build.  These variables are not thread-safe.
*/
................................................................................
  return rc;
}

/*
** Return true if this pager uses a write-ahead log instead of the usual
** rollback journal. Otherwise false.
*/
static int pagerUseWal(Pager *pPager){
  return (pPager->pWal!=0);
}

/*
** Unlock the database file. This function is a no-op if the pager
** is in exclusive mode.
**
** If the pager is currently in error state, discard the contents of 
................................................................................
    ** values stored in Pager.dbSize etc. might become invalid if
    ** this happens.  One can argue that this doesn't need to be cleared
    ** until the change-counter check fails in PagerSharedLock().
    ** Clearing the page size cache here is being conservative.
    */
    pPager->dbSizeValid = 0;

    if( pagerUseWal(pPager) ){
      sqlite3WalCloseSnapshot(pPager->pWal);
    }else{
      rc = osUnlock(pPager->fd, NO_LOCK);
    }
    if( rc ){
      pPager->errCode = rc;
    }
    IOTRACE(("UNLOCK %p\n", pPager))
................................................................................
  if( pPager->state<PAGER_RESERVED ){
    return SQLITE_OK;
  }
  releaseAllSavepoints(pPager);

  assert( isOpen(pPager->jfd) || pPager->pInJournal==0 );
  if( isOpen(pPager->jfd) ){
    assert( !pagerUseWal(pPager) );

    /* Finalize the journal file. */
    if( sqlite3IsMemJournal(pPager->jfd) ){
      assert( pPager->journalMode==PAGER_JOURNALMODE_MEMORY );
      sqlite3OsClose(pPager->jfd);
    }else if( pPager->journalMode==PAGER_JOURNALMODE_TRUNCATE ){
      if( pPager->journalOff==0 ){
................................................................................
#endif
  }
  sqlite3BitvecDestroy(pPager->pInJournal);
  pPager->pInJournal = 0;
  pPager->nRec = 0;
  sqlite3PcacheCleanAll(pPager->pPCache);

  if( pagerUseWal(pPager) ){
    rc2 = sqlite3WalWriteLock(pPager->pWal, 0);
    pPager->state = PAGER_SHARED;
  }else if( !pPager->exclusiveMode ){
    rc2 = osUnlock(pPager->fd, SHARED_LOCK);
    pPager->state = PAGER_SHARED;
    pPager->changeCountDone = 0;
  }else if( pPager->state==PAGER_SYNCED ){
    pPager->state = PAGER_EXCLUSIVE;
................................................................................
  assert( (isMainJrnl&~1)==0 );      /* isMainJrnl is 0 or 1 */
  assert( (isSavepnt&~1)==0 );       /* isSavepnt is 0 or 1 */
  assert( isMainJrnl || pDone );     /* pDone always used on sub-journals */
  assert( isSavepnt || pDone==0 );   /* pDone never used on non-savepoint */

  aData = pPager->pTmpSpace;
  assert( aData );         /* Temp storage must have already been allocated */
  assert( pagerUseWal(pPager)==0 || (!isMainJrnl && isSavepnt) );

  /* Read the page number and page data from the journal or sub-journal
  ** file. Return an error code to the caller if an IO error occurs.
  */
  jfd = isMainJrnl ? pPager->jfd : pPager->sjfd;
  rc = read32bits(jfd, *pOffset, &pgno);
  if( rc!=SQLITE_OK ) return rc;
................................................................................
  ** in the main journal either because the page is not in cache or else
  ** the page is marked as needSync==0.
  **
  ** 2008-04-14:  When attempting to vacuum a corrupt database file, it
  ** is possible to fail a statement on a database that does not yet exist.
  ** Do not attempt to write if database file has never been opened.
  */
  if( pagerUseWal(pPager) ){
    pPg = 0;
  }else{
    pPg = pager_lookup(pPager, pgno);
  }
  assert( pPg || !MEMDB );
  PAGERTRACE(("PLAYBACK %d page %d hash(%08x) %s\n",
           PAGERID(pPager), pgno, pager_datahash(pPager->pageSize, (u8*)aData),
................................................................................
  }
  if( (pPager->state>=PAGER_EXCLUSIVE)
   && isOpen(pPager->fd)
   && isSynced
  ){
    i64 ofst = (pgno-1)*(i64)pPager->pageSize;
    testcase( !isSavepnt && pPg!=0 && (pPg->flags&PGHDR_NEED_SYNC)!=0 );
    assert( !pagerUseWal(pPager) );
    rc = sqlite3OsWrite(pPager->fd, (u8*)aData, pPager->pageSize, ofst);
    if( pgno>pPager->dbFileSize ){
      pPager->dbFileSize = pgno;
    }
    if( pPager->pBackup ){
      CODEC1(pPager, aData, pgno, 3, rc=SQLITE_NOMEM);
      sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)aData);
................................................................................
      ** the PGHDR_NEED_SYNC flag is cleared, if the page is written to
      ** again within this transaction, it will be marked as dirty but
      ** the PGHDR_NEED_SYNC flag will not be set. It could then potentially
      ** be written out into the database file before its journal file
      ** segment is synced. If a crash occurs during or following this,
      ** database corruption may ensue.
      */
      assert( !pagerUseWal(pPager) );
      sqlite3PcacheMakeClean(pPg);
    }
#ifdef SQLITE_CHECK_PAGES
    pPg->pageHash = pager_pagehash(pPg);
#endif
    /* If this was page 1, then restore the value of Pager.dbFileVers.
    ** Do this before any decoding. */
................................................................................
** Otherwise, SQLITE_OK is returned.
*/
static int readDbPage(PgHdr *pPg){
  Pager *pPager = pPg->pPager; /* Pager object associated with page pPg */
  Pgno pgno = pPg->pgno;       /* Page number to read */
  int rc = SQLITE_OK;          /* Return code */
  i64 iOffset;                 /* Byte offset of file to read from */
  int isInWal = 0;             /* True if page is in log file */

  assert( pPager->state>=PAGER_SHARED && !MEMDB );
  assert( isOpen(pPager->fd) );

  if( NEVER(!isOpen(pPager->fd)) ){
    assert( pPager->tempFile );
    memset(pPg->pData, 0, pPager->pageSize);
    return SQLITE_OK;
  }

  if( pagerUseWal(pPager) ){
    /* Try to pull the page from the write-ahead log. */
    rc = sqlite3WalRead(pPager->pWal, pgno, &isInWal, pPg->pData);
  }
  if( rc==SQLITE_OK && !isInWal ){
    iOffset = (pgno-1)*(i64)pPager->pageSize;
    rc = sqlite3OsRead(pPager->fd, pPg->pData, pPager->pageSize, iOffset);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
    }
  }

................................................................................

  return rc;
}

/*
** This function is called to rollback a transaction on a WAL database.
*/
static int pagerRollbackWal(Pager *pPager){
  int rc;                         /* Return Code */
  PgHdr *pList;                   /* List of dirty pages to revert */

  /* Normally, if a transaction is rolled back, any backup processes are
  ** updated as data is copied out of the rollback journal and into the
  ** database. This is not generally possible with a WAL database, as
  ** rollback involves simply truncating the log file. Therefore, if one
  ** or more frames have already been written to the log (and therefore 
  ** also copied into the backup databases) as part of this transaction,
  ** the backups must be restarted.
  */
  if( sqlite3WalDirty(pPager->pWal) ){
    sqlite3BackupRestart(pPager->pBackup);
  }

  /* For all pages in the cache that are currently dirty or have already
  ** been written (but not committed) to the log file, do one of the 
  ** following:
  **
  **   + Discard the cached page (if refcount==0), or
  **   + Reload page content from the database (if refcount>0).
  */
  pPager->dbSize = pPager->dbOrigSize;
  rc = sqlite3WalUndo(pPager->pWal, pagerUndoCallback, (void *)pPager);
  pList = sqlite3PcacheDirtyList(pPager->pPCache);
  while( pList && rc==SQLITE_OK ){
    PgHdr *pNext = pList->pDirty;
    rc = pagerUndoCallback((void *)pPager, pList->pgno);
    pList = pNext;
  }

................................................................................
  }

  /* Set the database size back to the value it was before the savepoint 
  ** being reverted was opened.
  */
  pPager->dbSize = pSavepoint ? pSavepoint->nOrig : pPager->dbOrigSize;

  if( !pSavepoint && pagerUseWal(pPager) ){
    return pagerRollbackWal(pPager);
  }

  /* Use pPager->journalOff as the effective size of the main rollback
  ** journal.  The actual file might be larger than this in
  ** PAGER_JOURNALMODE_TRUNCATE or PAGER_JOURNALMODE_PERSIST.  But anything
  ** past pPager->journalOff is off-limits to us.
  */
  szJ = pPager->journalOff;
  assert( pagerUseWal(pPager)==0 || szJ==0 );

  /* Begin by rolling back records from the main journal starting at
  ** PagerSavepoint.iOffset and continuing to the next journal header.
  ** There might be records in the main journal that have a page number
  ** greater than the current database size (pPager->dbSize) but those
  ** will be skipped automatically.  Pages are added to pDone as they
  ** are played back.
  */
  if( pSavepoint && !pagerUseWal(pPager) ){
    iHdrOff = pSavepoint->iHdrOffset ? pSavepoint->iHdrOffset : szJ;
    pPager->journalOff = pSavepoint->iOffset;
    while( rc==SQLITE_OK && pPager->journalOff<iHdrOff ){
      rc = pager_playback_one_page(pPager, &pPager->journalOff, pDone, 1, 1);
    }
    assert( rc!=SQLITE_DONE );
  }else{
................................................................................
  ** previously rolled back out of the main journal (and are hence in pDone)
  ** will be skipped.  Out-of-range pages are also skipped.
  */
  if( pSavepoint ){
    u32 ii;            /* Loop counter */
    i64 offset = pSavepoint->iSubRec*(4+pPager->pageSize);

    if( pagerUseWal(pPager) ){
      rc = sqlite3WalSavepointUndo(pPager->pWal, pSavepoint->iFrame);
    }
    for(ii=pSavepoint->iSubRec; rc==SQLITE_OK && ii<pPager->nSubRec; ii++){
      assert( offset==ii*(4+pPager->pageSize) );
      rc = pager_playback_one_page(pPager, &offset, pDone, 0, 1);
    }
    assert( rc!=SQLITE_DONE );
  }
................................................................................
  /* Determine the number of pages in the file. Store this in nPage. */
  if( pPager->dbSizeValid ){
    nPage = pPager->dbSize;
  }else{
    int rc;                 /* Error returned by OsFileSize() */
    i64 n = 0;              /* File size in bytes returned by OsFileSize() */

    if( pagerUseWal(pPager) ){
      sqlite3WalDbsize(pPager->pWal, &nPage);
    }

    if( nPage==0 ){
      assert( isOpen(pPager->fd) || pPager->tempFile );
      if( isOpen(pPager->fd) ){
        if( SQLITE_OK!=(rc = sqlite3OsFileSize(pPager->fd, &n)) ){
          pager_error(pPager, rc);
................................................................................
int sqlite3PagerClose(Pager *pPager){
  u8 *pTmp = (u8 *)pPager->pTmpSpace;

  disable_simulated_io_errors();
  sqlite3BeginBenignMalloc();
  pPager->errCode = 0;
  pPager->exclusiveMode = 0;
  sqlite3WalClose(pPager->pWal, pPager->fd, 
    (pPager->noSync ? 0 : pPager->sync_flags), pTmp
  );
  pPager->pWal = 0;
  pager_reset(pPager);
  if( MEMDB ){
    pager_unlock(pPager);
  }else{
    /* Set Pager.journalHdr to -1 for the benefit of the pager_playback() 
    ** call which may be made from within pagerUnlockAndRollback(). If it
    ** is not -1, then the unsynced portion of an open journal file may
................................................................................
  **
  ** While the pager is in the RESERVED state, the original database file
  ** is unchanged and we can rollback without having to playback the
  ** journal into the original database file.  Once we transition to
  ** EXCLUSIVE, it means the database file has been changed and any rollback
  ** will require a journal playback.
  */
  assert( !pagerUseWal(pList->pPager) );
  assert( pPager->state>=PAGER_RESERVED );
  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);

  /* If the file is a temp-file has not yet been opened, open it now. It
  ** is not possible for rc to be other than SQLITE_OK if this branch
  ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
  */
................................................................................
    void *pData = pPg->pData;
    i64 offset = pPager->nSubRec*(4+pPager->pageSize);
    char *pData2;

    CODEC2(pPager, pData, pPg->pgno, 7, return SQLITE_NOMEM, pData2);
    PAGERTRACE(("STMT-JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno));
  
    assert( pagerUseWal(pPager) 
         || pageInJournal(pPg) 
         || pPg->pgno>pPager->dbOrigSize 
    );
    rc = write32bits(pPager->sjfd, offset, pPg->pgno);
    if( rc==SQLITE_OK ){
      rc = sqlite3OsWrite(pPager->sjfd, pData2, pPager->pageSize, offset+4);
    }
................................................................................

/*
** This function is a wrapper around sqlite3WalFrames(). As well as logging
** the contents of the list of pages headed by pList (connected by pDirty),
** this function notifies any active backup processes that the pages have
** changed. 
*/ 
static int pagerWalFrames(
  Pager *pPager,                  /* Pager object */
  PgHdr *pList,                   /* List of frames to log */
  Pgno nTruncate,                 /* Database size after this commit */
  int isCommit,                   /* True if this is a commit */
  int sync_flags                  /* Flags to pass to OsSync() (or 0) */
){
  int rc;                         /* Return code */

  assert( pPager->pWal );
  rc = sqlite3WalFrames(pPager->pWal, 
      pPager->pageSize, pList, nTruncate, isCommit, sync_flags
  );
  if( rc==SQLITE_OK && pPager->pBackup ){
    PgHdr *p;
    for(p=pList; p; p=p->pDirty){
      sqlite3BackupUpdate(pPager->pBackup, p->pgno, (u8 *)p->pData);
    }
................................................................................
  Pager *pPager = (Pager *)p;
  int rc = SQLITE_OK;

  assert( pPg->pPager==pPager );
  assert( pPg->flags&PGHDR_DIRTY );

  pPg->pDirty = 0;
  if( pagerUseWal(pPager) ){
    /* Write a single frame for this page to the log. */
    if( subjRequiresPage(pPg) ){ 
      rc = subjournalPage(pPg); 
    }
    if( rc==SQLITE_OK ){
      rc = pagerWalFrames(pPager, pPg, 0, 0, 0);
    }
  }else{
    /* The doNotSync flag is set by the sqlite3PagerWrite() function while it
    ** is journalling a set of two or more database pages that are stored
    ** on the same disk sector. Syncing the journal is not allowed while
    ** this is happening as it is important that all members of such a
    ** set of pages are synced to disk together. So, if the page this function
................................................................................
** a WAL, this ensures there is no race condition between the xAccess() 
** below and an xDelete() being executed by some other connection.
*/
static int pagerHasWAL(Pager *pPager, int *pExists){
  int rc;                         /* Return code */

  if( !pPager->tempFile ){
    char *zWal = sqlite3_mprintf("%s-wal", pPager->zFilename);
    if( !zWal ){
      rc = SQLITE_NOMEM;
    }else{
      rc = sqlite3OsAccess(pPager->pVfs, zWal, SQLITE_ACCESS_EXISTS, pExists);
      sqlite3_free(zWal);
    }
  }else{
    rc = SQLITE_OK;
    *pExists = 0;
  }
  return rc;
}

static int pagerOpenSnapshot(Pager *pPager){
  int rc;                         /* Return code */
  int changed = 0;                /* True if cache must be reset */

  assert( pagerUseWal(pPager) );

  rc = sqlite3WalOpenSnapshot(pPager->pWal, &changed);
  if( rc==SQLITE_OK ){
    int dummy;
    if( changed ){
      pager_reset(pPager);
      assert( pPager->errCode || pPager->dbSizeValid==0 );
    }
    rc = sqlite3PagerPagecount(pPager, &dummy);
................................................................................
    if( isOpen(pPager->jfd) || pPager->zJournal ){
      isErrorReset = 1;
    }
    pPager->errCode = SQLITE_OK;
    pager_reset(pPager);
  }

  if( pagerUseWal(pPager) ){
    rc = pagerOpenSnapshot(pPager);
  }else if( pPager->state==PAGER_UNLOCK || isErrorReset ){
    sqlite3_vfs * const pVfs = pPager->pVfs;
    int isHotJournal = 0;
    int isWal = 0;
    assert( !MEMDB );
    assert( sqlite3PcacheRefCount(pPager->pPCache)==0 );
................................................................................

    rc = pagerHasWAL(pPager, &isWal);
    if( rc!=SQLITE_OK ){
      goto failed;
    }
    if( isWal ){
      pager_reset(pPager);
      rc = sqlite3PagerOpenWal(pPager, 0);
      if( rc==SQLITE_OK ){
        rc = pagerOpenSnapshot(pPager);
      }
    }else if( pPager->journalMode==PAGER_JOURNALMODE_WAL ){
      pPager->journalMode = PAGER_JOURNALMODE_DELETE;
    }
  }
................................................................................
**
** SQLITE_OK is returned if everything goes according to plan. 
** An SQLITE_IOERR_XXX error code is returned if a call to 
** sqlite3OsOpen() fails.
*/
static int openSubJournal(Pager *pPager){
  int rc = SQLITE_OK;
  if( (pagerUseWal(pPager) || isOpen(pPager->jfd)) && !isOpen(pPager->sjfd) ){
    if( pPager->journalMode==PAGER_JOURNALMODE_MEMORY || pPager->subjInMemory ){
      sqlite3MemJournalOpen(pPager->sjfd);
    }else{
      rc = pagerOpentemp(pPager, pPager->sjfd, SQLITE_OPEN_SUBJOURNAL);
    }
  }
  return rc;
................................................................................
  int rc = SQLITE_OK;
  assert( pPager->state!=PAGER_UNLOCK );
  pPager->subjInMemory = (u8)subjInMemory;
  if( pPager->state==PAGER_SHARED ){
    assert( pPager->pInJournal==0 );
    assert( !MEMDB && !pPager->tempFile );

    if( pagerUseWal(pPager) ){
      /* Grab the write lock on the log file. If successful, upgrade to
      ** PAGER_EXCLUSIVE state. Otherwise, return an error code to the caller.
      ** The busy-handler is not invoked if another connection already
      ** holds the write-lock. If possible, the upper layer will call it.
      */
      rc = sqlite3WalWriteLock(pPager->pWal, 1);
      if( rc==SQLITE_OK ){
        pPager->dbOrigSize = pPager->dbSize;
        pPager->state = PAGER_RESERVED;
        pPager->journalOff = 0;
      }
    }else{
      /* Obtain a RESERVED lock on the database file. If the exFlag parameter
................................................................................
  }else if( isOpen(pPager->jfd) && pPager->journalOff==0 ){
    /* This happens when the pager was in exclusive-access mode the last
    ** time a (read or write) transaction was successfully concluded
    ** by this connection. Instead of deleting the journal file it was 
    ** kept open and either was truncated to 0 bytes or its header was
    ** overwritten with zeros.
    */
    assert( pagerUseWal(pPager)==0 );
    assert( pPager->nRec==0 );
    assert( pPager->dbOrigSize==0 );
    assert( pPager->pInJournal==0 );
    rc = pager_open_journal(pPager);
  }

  PAGERTRACE(("TRANSACTION %d\n", PAGERID(pPager)));
................................................................................
  CHECK_PAGE(pPg);

  /* Mark the page as dirty.  If the page has already been written
  ** to the journal then we can return right away.
  */
  sqlite3PcacheMakeDirty(pPg);
  if( pageInJournal(pPg) && !subjRequiresPage(pPg) ){
    assert( !pagerUseWal(pPager) );
    pPager->dbModified = 1;
  }else{

    /* If we get this far, it means that the page needs to be
    ** written to the transaction journal or the ckeckpoint journal
    ** or both.
    **
................................................................................
    */
    rc = sqlite3PagerBegin(pPager, 0, pPager->subjInMemory);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    if( !isOpen(pPager->jfd) 
     && pPager->journalMode!=PAGER_JOURNALMODE_OFF 
     && !pagerUseWal(pPager)
    ){
      assert( pPager->useJournal );
      rc = pager_open_journal(pPager);
      if( rc!=SQLITE_OK ) return rc;
    }
    pPager->dbModified = 1;
  
    /* The transaction journal now exists and we have a RESERVED or an
    ** EXCLUSIVE lock on the main database file.  Write the current page to
    ** the transaction journal if it is not there already.
    */
    if( !pageInJournal(pPg) && isOpen(pPager->jfd) ){
      assert( !pagerUseWal(pPager) );
      if( pPg->pgno<=pPager->dbOrigSize ){
        u32 cksum;
        char *pData2;

        /* We should never write to the journal file the page that
        ** contains the database locks.  The following assert verifies
        ** that we do not. */
................................................................................
  if( MEMDB && pPager->dbModified ){
    /* If this is an in-memory db, or no pages have been written to, or this
    ** function has already been called, it is mostly a no-op.  However, any
    ** backup in progress needs to be restarted.
    */
    sqlite3BackupRestart(pPager->pBackup);
  }else if( pPager->state!=PAGER_SYNCED && pPager->dbModified ){
    if( pagerUseWal(pPager) ){
      PgHdr *pList = sqlite3PcacheDirtyList(pPager->pPCache);
      if( pList ){
        rc = pagerWalFrames(pPager, pList, pPager->dbSize, 1, 
            (pPager->fullSync ? pPager->sync_flags : 0)
        );
      }
      sqlite3PcacheCleanAll(pPager->pPCache);
    }else{
      /* The following block updates the change-counter. Exactly how it
      ** does this depends on whether or not the atomic-update optimization
................................................................................
**   the journal file. It needs to be left in the file-system so that
**   some other process can use it to restore the database state (by
**   hot-journal rollback).
*/
int sqlite3PagerRollback(Pager *pPager){
  int rc = SQLITE_OK;                  /* Return code */
  PAGERTRACE(("ROLLBACK %d\n", PAGERID(pPager)));
  if( pagerUseWal(pPager) ){
    int rc2;

    rc = sqlite3PagerSavepoint(pPager, SAVEPOINT_ROLLBACK, -1);
    rc2 = pager_end_transaction(pPager, pPager->setMaster);
    if( rc==SQLITE_OK ) rc = rc2;
  }else if( !pPager->dbModified || !isOpen(pPager->jfd) ){
    rc = pager_end_transaction(pPager, pPager->setMaster);
................................................................................
        aNew[ii].iOffset = JOURNAL_HDR_SZ(pPager);
      }
      aNew[ii].iSubRec = pPager->nSubRec;
      aNew[ii].pInSavepoint = sqlite3BitvecCreate(nPage);
      if( !aNew[ii].pInSavepoint ){
        return SQLITE_NOMEM;
      }
      if( pagerUseWal(pPager) ){
        aNew[ii].iFrame = sqlite3WalSavepoint(pPager->pWal);
      }
    }

    /* Open the sub-journal, if it is not already opened. */
    rc = openSubJournal(pPager);
    assertTruncateConstraint(pPager);
  }
................................................................................
      }
    }
    /* Else this is a rollback operation, playback the specified savepoint.
    ** If this is a temp-file, it is possible that the journal file has
    ** not yet been opened. In this case there have been no changes to
    ** the database file, so the playback operation can be skipped.
    */
    else if( pagerUseWal(pPager) || isOpen(pPager->jfd) ){
      PagerSavepoint *pSavepoint = (nNew==0)?0:&pPager->aSavepoint[nNew-1];
      rc = pagerPlaybackSavepoint(pPager, pSavepoint);
      assert(rc!=SQLITE_DONE);
    }
  
  }
  return rc;
................................................................................
}

/*
** This function is called when the user invokes "PRAGMA checkpoint".
*/
int sqlite3PagerCheckpoint(Pager *pPager){
  int rc = SQLITE_OK;
  if( pPager->pWal ){
    u8 *zBuf = (u8 *)pPager->pTmpSpace;
    rc = sqlite3WalCheckpoint(pPager->pWal, pPager->fd, 
        (pPager->noSync ? 0 : pPager->sync_flags),
        zBuf, pPager->xBusyHandler, pPager->pBusyHandlerArg
    );
  }
  return rc;
}

int sqlite3PagerWalCallback(Pager *pPager){
  return sqlite3WalCallback(pPager->pWal);
}

/*
** Open a connection to the write-ahead log file for pager pPager. If
** the log connection is already open, this function is a no-op.
**
** The caller must be holding a SHARED lock on the database file to call
** this function.
*/
int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen){
  int rc = SQLITE_OK;             /* Return code */

  assert( pPager->state>=PAGER_SHARED );
  if( !pPager->pWal ){

    /* Open the connection to the log file. If this operation fails, 
    ** (e.g. due to malloc() failure), unlock the database file and 
    ** return an error code.
    */
    rc = sqlite3WalOpen(pPager->pVfs, pPager->zFilename, &pPager->pWal);
    if( rc==SQLITE_OK ){
      pPager->journalMode = PAGER_JOURNALMODE_WAL;
    }
  }else{
    *pisOpen = 1;
  }

................................................................................
** to switching from WAL to rollback mode.
**
** Before closing the log file, this function attempts to take an 
** EXCLUSIVE lock on the database file. If this cannot be obtained, an
** error (SQLITE_BUSY) is returned and the log connection is not closed.
** If successful, the EXCLUSIVE lock is not released before returning.
*/
int sqlite3PagerCloseWal(Pager *pPager){
  int rc = SQLITE_OK;

  assert( pPager->journalMode==PAGER_JOURNALMODE_WAL );

  /* If the log file is not already open, but does exist in the file-system,
  ** it may need to be checkpointed before the connection can switch to
  ** rollback mode. Open it now so this can happen.
  */
  if( !pPager->pWal ){
    int logexists = 0;
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_SHARED);
    if( rc==SQLITE_OK ){
      rc = pagerHasWAL(pPager, &logexists);
    }
    if( rc==SQLITE_OK && logexists ){
      rc = sqlite3WalOpen(pPager->pVfs, pPager->zFilename, &pPager->pWal);
    }
  }
    
  /* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
  ** the database file, the log and log-summary files will be deleted.
  */
  if( rc==SQLITE_OK && pPager->pWal ){
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_EXCLUSIVE);
    if( rc==SQLITE_OK ){
      rc = sqlite3WalClose(pPager->pWal, pPager->fd,
        (pPager->noSync ? 0 : pPager->sync_flags), 
        (u8*)pPager->pTmpSpace
      );
      pPager->pWal = 0;
    }
  }
  return rc;
}

#endif /* SQLITE_OMIT_DISKIO */

Changes to src/pager.h.

131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
int sqlite3PagerCommitPhaseTwo(Pager*);
int sqlite3PagerRollback(Pager*);
int sqlite3PagerOpenSavepoint(Pager *pPager, int n);
int sqlite3PagerSavepoint(Pager *pPager, int op, int iSavepoint);
int sqlite3PagerSharedLock(Pager *pPager);

int sqlite3PagerCheckpoint(Pager *pPager);
int sqlite3PagerLogCallback(Pager *pPager);
int sqlite3PagerOpenLog(Pager *pPager, int *pisOpen);
int sqlite3PagerCloseLog(Pager *pPager);

/* Functions used to query pager state and configuration. */
u8 sqlite3PagerIsreadonly(Pager*);
int sqlite3PagerRefcount(Pager*);
int sqlite3PagerMemUsed(Pager*);
const char *sqlite3PagerFilename(Pager*);
const sqlite3_vfs *sqlite3PagerVfs(Pager*);







|
|
|







131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
int sqlite3PagerCommitPhaseTwo(Pager*);
int sqlite3PagerRollback(Pager*);
int sqlite3PagerOpenSavepoint(Pager *pPager, int n);
int sqlite3PagerSavepoint(Pager *pPager, int op, int iSavepoint);
int sqlite3PagerSharedLock(Pager *pPager);

int sqlite3PagerCheckpoint(Pager *pPager);
int sqlite3PagerWalCallback(Pager *pPager);
int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
int sqlite3PagerCloseWal(Pager *pPager);

/* Functions used to query pager state and configuration. */
u8 sqlite3PagerIsreadonly(Pager*);
int sqlite3PagerRefcount(Pager*);
int sqlite3PagerMemUsed(Pager*);
const char *sqlite3PagerFilename(Pager*);
const sqlite3_vfs *sqlite3PagerVfs(Pager*);

Changes to src/sqlite.h.in.

840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
  int (*xCurrentTime)(sqlite3_vfs*, double*);
  int (*xGetLastError)(sqlite3_vfs*, int, char *);
  /*
  ** The methods above are in version 1 of the sqlite_vfs object
  ** definition.  Those that follow are added in version 2 or later
  */
  int (*xShmOpen)(sqlite3_vfs*, const char *zName, sqlite3_shm**);
  int (*xShmSize)(sqlite3_shm*, int reqSize, int *pNewSize, char**);
  int (*xShmRelease)(sqlite3_shm*);
  int (*xShmPush)(sqlite3_shm*);
  int (*xShmPull)(sqlite3_shm*);
  int (*xShmLock)(sqlite3_shm*, int desiredLock, int *gotLock);
  int (*xShmClose)(sqlite3_shm*);
  int (*xShmDelete)(sqlite3_vfs*, const char *zName);
  int (*xRename)(sqlite3_vfs*, const char *zOld, const char *zNew, int dirSync);







|







840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
  int (*xCurrentTime)(sqlite3_vfs*, double*);
  int (*xGetLastError)(sqlite3_vfs*, int, char *);
  /*
  ** The methods above are in version 1 of the sqlite_vfs object
  ** definition.  Those that follow are added in version 2 or later
  */
  int (*xShmOpen)(sqlite3_vfs*, const char *zName, sqlite3_shm**);
  int (*xShmSize)(sqlite3_shm*, int reqSize, int *pNewSize, void**);
  int (*xShmRelease)(sqlite3_shm*);
  int (*xShmPush)(sqlite3_shm*);
  int (*xShmPull)(sqlite3_shm*);
  int (*xShmLock)(sqlite3_shm*, int desiredLock, int *gotLock);
  int (*xShmClose)(sqlite3_shm*);
  int (*xShmDelete)(sqlite3_vfs*, const char *zName);
  int (*xRename)(sqlite3_vfs*, const char *zOld, const char *zNew, int dirSync);

Changes to src/sqliteInt.h.

819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
  void *pProfileArg;                        /* Argument to profile function */
  void *pCommitArg;                 /* Argument to xCommitCallback() */   
  int (*xCommitCallback)(void*);    /* Invoked at every commit. */
  void *pRollbackArg;               /* Argument to xRollbackCallback() */   
  void (*xRollbackCallback)(void*); /* Invoked at every commit. */
  void *pUpdateArg;
  void (*xUpdateCallback)(void*,int, const char*,const char*,sqlite_int64);
  int (*xLogCallback)(void *, sqlite3 *, const char *, int);
  void *pLogArg;
  void(*xCollNeeded)(void*,sqlite3*,int eTextRep,const char*);
  void(*xCollNeeded16)(void*,sqlite3*,int eTextRep,const void*);
  void *pCollNeededArg;
  sqlite3_value *pErr;          /* Most recent error message */
  char *zErrMsg;                /* Most recent error message (UTF-8 encoded) */
  char *zErrMsg16;              /* Most recent error message (UTF-16 encoded) */
  union {







|
|







819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
  void *pProfileArg;                        /* Argument to profile function */
  void *pCommitArg;                 /* Argument to xCommitCallback() */   
  int (*xCommitCallback)(void*);    /* Invoked at every commit. */
  void *pRollbackArg;               /* Argument to xRollbackCallback() */   
  void (*xRollbackCallback)(void*); /* Invoked at every commit. */
  void *pUpdateArg;
  void (*xUpdateCallback)(void*,int, const char*,const char*,sqlite_int64);
  int (*xWalCallback)(void *, sqlite3 *, const char *, int);
  void *pWalArg;
  void(*xCollNeeded)(void*,sqlite3*,int eTextRep,const char*);
  void(*xCollNeeded16)(void*,sqlite3*,int eTextRep,const void*);
  void *pCollNeededArg;
  sqlite3_value *pErr;          /* Most recent error message */
  char *zErrMsg;                /* Most recent error message (UTF-8 encoded) */
  char *zErrMsg16;              /* Most recent error message (UTF-16 encoded) */
  union {

Changes to src/test1.c.

5109
5110
5111
5112
5113
5114
5115
5116
5117
5118
5119
5120
5121
5122
5123
....
5169
5170
5171
5172
5173
5174
5175
5176
5177
5178
5179
5180
5181
5182
5183
5184
  extern int sqlite3WhereTrace;
  extern int sqlite3OSTrace;
  extern int sqlite3VdbeAddopTrace;
#endif
#ifdef SQLITE_TEST
  extern char sqlite3_query_plan[];
  static char *query_plan = sqlite3_query_plan;
  extern int sqlite3_walsummary_mmap_incr;                 /* In wal.c */
#ifdef SQLITE_ENABLE_FTS3
  extern int sqlite3_fts3_enable_parentheses;
#endif
#endif

  for(i=0; i<sizeof(aCmd)/sizeof(aCmd[0]); i++){
    Tcl_CreateCommand(interp, aCmd[i].zName, aCmd[i].xProc, 0, 0);
................................................................................
#if SQLITE_OS_WIN
  Tcl_LinkVar(interp, "sqlite_os_type",
      (char*)&sqlite3_os_type, TCL_LINK_INT);
#endif
#ifdef SQLITE_TEST
  Tcl_LinkVar(interp, "sqlite_query_plan",
      (char*)&query_plan, TCL_LINK_STRING|TCL_LINK_READ_ONLY);
  Tcl_LinkVar(interp, "sqlite_walsummary_mmap_incr",
      (char*)&sqlite3_walsummary_mmap_incr, TCL_LINK_INT);
#endif
#ifdef SQLITE_DEBUG
  Tcl_LinkVar(interp, "sqlite_addop_trace",
      (char*)&sqlite3VdbeAddopTrace, TCL_LINK_INT);
  Tcl_LinkVar(interp, "sqlite_where_trace",
      (char*)&sqlite3WhereTrace, TCL_LINK_INT);
  Tcl_LinkVar(interp, "sqlite_os_trace",







|







 







|
|







5109
5110
5111
5112
5113
5114
5115
5116
5117
5118
5119
5120
5121
5122
5123
....
5169
5170
5171
5172
5173
5174
5175
5176
5177
5178
5179
5180
5181
5182
5183
5184
  extern int sqlite3WhereTrace;
  extern int sqlite3OSTrace;
  extern int sqlite3VdbeAddopTrace;
#endif
#ifdef SQLITE_TEST
  extern char sqlite3_query_plan[];
  static char *query_plan = sqlite3_query_plan;
/*  extern int sqlite3_walsummary_mmap_incr;                 // In wal.c */
#ifdef SQLITE_ENABLE_FTS3
  extern int sqlite3_fts3_enable_parentheses;
#endif
#endif

  for(i=0; i<sizeof(aCmd)/sizeof(aCmd[0]); i++){
    Tcl_CreateCommand(interp, aCmd[i].zName, aCmd[i].xProc, 0, 0);
................................................................................
#if SQLITE_OS_WIN
  Tcl_LinkVar(interp, "sqlite_os_type",
      (char*)&sqlite3_os_type, TCL_LINK_INT);
#endif
#ifdef SQLITE_TEST
  Tcl_LinkVar(interp, "sqlite_query_plan",
      (char*)&query_plan, TCL_LINK_STRING|TCL_LINK_READ_ONLY);
/*  Tcl_LinkVar(interp, "sqlite_walsummary_mmap_incr",
      (char*)&sqlite3_walsummary_mmap_incr, TCL_LINK_INT); */
#endif
#ifdef SQLITE_DEBUG
  Tcl_LinkVar(interp, "sqlite_addop_trace",
      (char*)&sqlite3VdbeAddopTrace, TCL_LINK_INT);
  Tcl_LinkVar(interp, "sqlite_where_trace",
      (char*)&sqlite3WhereTrace, TCL_LINK_INT);
  Tcl_LinkVar(interp, "sqlite_os_trace",

Changes to src/vdbe.c.

5248
5249
5250
5251
5252
5253
5254
5255
5256
5257
5258
5259
5260
5261
5262
5263
5264
5265
5266
            "cannot change %s wal mode from within a transaction",
            (eNew==PAGER_JOURNALMODE_WAL ? "into" : "out of")
        );
      }else{
  
        if( eOld==PAGER_JOURNALMODE_WAL ){
          /* If leaving WAL mode, close the log file. If successful, the call
          ** to PagerCloseLog() checkpoints and deletes the write-ahead-log 
          ** file. An EXCLUSIVE lock may still be held on the database file 
          ** after a successful return. 
          */
          rc = sqlite3PagerCloseLog(pPager);
          if( rc!=SQLITE_OK ) goto abort_due_to_error;
          sqlite3PagerJournalMode(pPager, eNew);
        }else{
          sqlite3PagerJournalMode(pPager, PAGER_JOURNALMODE_DELETE);
        }
  
        /* Open a transaction on the database file. Regardless of the journal







|



|







5248
5249
5250
5251
5252
5253
5254
5255
5256
5257
5258
5259
5260
5261
5262
5263
5264
5265
5266
            "cannot change %s wal mode from within a transaction",
            (eNew==PAGER_JOURNALMODE_WAL ? "into" : "out of")
        );
      }else{
  
        if( eOld==PAGER_JOURNALMODE_WAL ){
          /* If leaving WAL mode, close the log file. If successful, the call
          ** to PagerCloseWal() checkpoints and deletes the write-ahead-log 
          ** file. An EXCLUSIVE lock may still be held on the database file 
          ** after a successful return. 
          */
          rc = sqlite3PagerCloseWal(pPager);
          if( rc!=SQLITE_OK ) goto abort_due_to_error;
          sqlite3PagerJournalMode(pPager, eNew);
        }else{
          sqlite3PagerJournalMode(pPager, PAGER_JOURNALMODE_DELETE);
        }
  
        /* Open a transaction on the database file. Regardless of the journal

Changes to src/vdbeapi.c.

302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
...
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
void sqlite3_result_error_nomem(sqlite3_context *pCtx){
  assert( sqlite3_mutex_held(pCtx->s.db->mutex) );
  sqlite3VdbeMemSetNull(&pCtx->s);
  pCtx->isError = SQLITE_NOMEM;
  pCtx->s.db->mallocFailed = 1;
}

static int doLogCallbacks(sqlite3 *db){
  int i;
  int rc = SQLITE_OK;
  for(i=0; i<db->nDb; i++){
    Btree *pBt = db->aDb[i].pBt;
    if( pBt ){
      int nEntry = sqlite3PagerLogCallback(sqlite3BtreePager(pBt));
      if( db->xLogCallback && nEntry>0 && rc==SQLITE_OK
       && db->xLogCallback(db->pLogArg, db, db->aDb[i].zName, nEntry)
      ){
        rc = sqlite3PagerCheckpoint(sqlite3BtreePager(pBt));
      }
    }
  }
  return rc;
}
................................................................................
    elapseTime -= p->startTime;
    db->xProfile(db->pProfileArg, p->zSql, elapseTime);
  }
#endif

  if( rc==SQLITE_DONE ){
    assert( p->rc==SQLITE_OK );
    p->rc = doLogCallbacks(db);
    if( p->rc!=SQLITE_OK ){
      rc = SQLITE_ERROR;
    }
  }

  db->errCode = rc;
  if( SQLITE_NOMEM==sqlite3ApiExit(p->db, p->rc) ){







|





|
|
|







 







|







302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
...
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
void sqlite3_result_error_nomem(sqlite3_context *pCtx){
  assert( sqlite3_mutex_held(pCtx->s.db->mutex) );
  sqlite3VdbeMemSetNull(&pCtx->s);
  pCtx->isError = SQLITE_NOMEM;
  pCtx->s.db->mallocFailed = 1;
}

static int doWalCallbacks(sqlite3 *db){
  int i;
  int rc = SQLITE_OK;
  for(i=0; i<db->nDb; i++){
    Btree *pBt = db->aDb[i].pBt;
    if( pBt ){
      int nEntry = sqlite3PagerWalCallback(sqlite3BtreePager(pBt));
      if( db->xWalCallback && nEntry>0 && rc==SQLITE_OK
       && db->xWalCallback(db->pWalArg, db, db->aDb[i].zName, nEntry)
      ){
        rc = sqlite3PagerCheckpoint(sqlite3BtreePager(pBt));
      }
    }
  }
  return rc;
}
................................................................................
    elapseTime -= p->startTime;
    db->xProfile(db->pProfileArg, p->zSql, elapseTime);
  }
#endif

  if( rc==SQLITE_DONE ){
    assert( p->rc==SQLITE_OK );
    p->rc = doWalCallbacks(db);
    if( p->rc!=SQLITE_OK ){
      rc = SQLITE_ERROR;
    }
  }

  db->errCode = rc;
  if( SQLITE_NOMEM==sqlite3ApiExit(p->db, p->rc) ){

Changes to src/wal.c.

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/*











** This file contains the implementation of a log file used in 
** "journal_mode=wal" mode.
*/



/*
** LOG FILE FORMAT
**
** A log file consists of a header followed by zero or more log frames.
** The log header is 12 bytes in size and consists of the following three
** big-endian 32-bit unsigned integer values:
**
**     0: Database page size,
**     4: Randomly selected salt value 1,
**     8: Randomly selected salt value 2.
**
** Immediately following the log header are zero or more log frames. Each
** frame itself consists of a 16-byte header followed by a <page-size> bytes
** of page data. The header is broken into 4 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: Checksum value 1.
**    12: Checksum value 2.
*/

/* 
** LOG SUMMARY FILE FORMAT
**
** The log-summary file consists of a header region, followed by an 
** region that contains no useful data (used to apply byte-range locks
** to), followed by the data region. 
**
** The contents of both the header and data region are specified in terms
** of 1, 2 and 4 byte unsigned integers. All integers are stored in 
** machine-endian order.

**
** A log-summary file is essentially a shadow-pager map. It contains a
** mapping from database page number to the set of locations in the log 
** file that contain versions of the database page. When a database 
** client needs to read a page of data, it first queries the log-summary
** file to determine if the required version of the page is stored in
** the log. If so, it is read from the log file. If not, it is read from

** the database file.
**
** Whenever a transaction is appended to the log or a checkpoint transfers
** data from the log file into the database file, the log-summary is 
** updated accordingly.
**
** The fields in the log-summary file header are described in the comment 
** directly above the definition of struct LogSummaryHdr (see below). 
** Immediately following the fields in the LogSummaryHdr structure is
** an 8 byte checksum based on the contents of the header. This field is
** not the same as the iCheck1 and iCheck2 fields of the LogSummaryHdr.
*/

#include "wal.h"

#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

typedef struct LogSummaryHdr LogSummaryHdr;

typedef struct LogSummary LogSummary;
typedef struct LogIterator LogIterator;
typedef struct LogLock LogLock;


/*
** The following structure may be used to store the same data that
** is stored in the log-summary header.
**
** Member variables iCheck1 and iCheck2 contain the checksum for the
** last frame written to the log, or 2 and 3 respectively if the log 
** is currently empty.
*/
struct LogSummaryHdr {

  u32 iChange;                    /* Counter incremented each transaction */
  u32 pgsz;                       /* Database page size in bytes */
  u32 iLastPg;                    /* Address of last valid frame in log */
  u32 nPage;                      /* Size of database in pages */
  u32 iCheck1;                    /* Checkpoint value 1 */
  u32 iCheck2;                    /* Checkpoint value 2 */
};

/* Size of serialized LogSummaryHdr object. */
#define LOGSUMMARY_HDR_NFIELD (sizeof(LogSummaryHdr) / sizeof(u32))

/* A block of 16 bytes beginning at LOGSUMMARY_LOCK_OFFSET is reserved
** for locks. Since some systems only feature mandatory file-locks, we
** do not read or write data from the region of the file on which locks
** are applied.
*/
#define LOGSUMMARY_LOCK_OFFSET   ((sizeof(LogSummaryHdr))+2*sizeof(u32))
#define LOGSUMMARY_LOCK_RESERVED 16

/* Size of header before each frame in log file */
#define LOG_FRAME_HDRSIZE 16

/* Size of log header */
#define LOG_HDRSIZE 12

/*
** Return the offset of frame iFrame in the log file, assuming a database
** page size of pgsz bytes. The offset returned is to the start of the
** log frame-header.
*/
#define logFrameOffset(iFrame, pgsz) (                               \
  LOG_HDRSIZE + ((iFrame)-1)*((pgsz)+LOG_FRAME_HDRSIZE)        \
)

/*
** If using mmap() to access a shared (or otherwise) log-summary file, then
** the mapping size is incremented in units of the following size.
**
** A 64 KB log-summary mapping corresponds to a log file containing over
** 13000 frames, so the mapping size does not need to be increased often.
*/
#ifdef SQLITE_TEST
int sqlite3_walsummary_mmap_incr = 128;
# define LOGSUMMARY_MMAP_INCREMENT sqlite3_walsummary_mmap_incr
#else
# define LOGSUMMARY_MMAP_INCREMENT (64*1024)
#endif

/*
** There is one instance of this structure for each log-summary object
** that this process has a connection to. They are stored in a linked
** list starting at pLogSummary (global variable).
**
** TODO: LogSummary.fd is a unix file descriptor. Unix APIs are used 
**       directly in this implementation because the VFS does not support
**       the required blocking file-locks.
*/
struct LogSummary {
  sqlite3_mutex *mutex;           /* Mutex used to protect this object */
  int nRef;                       /* Number of pointers to this structure */
  int fd;                         /* File descriptor open on log-summary */
  char *zPath;                    /* Path to associated WAL file */
  LogLock *pLock;                 /* Linked list of locks on this object */
  LogSummary *pNext;              /* Next in global list */

  int nData;                      /* Size of aData allocation/mapping */
  u32 *aData;                     /* File body */
};

/*
** This module uses three different types of file-locks. All are taken
** on the log-summary file. The three types of locks are as follows:
**
** MUTEX:  The MUTEX lock is used as a robust inter-process mutex. It
**         is held while the log-summary header is modified, and 
**         sometimes when it is read. It is also held while a new client
**         obtains the DMH lock (see below), and while log recovery is
**         being run.
**
** DMS:    The DMS (Dead Mans Switch mechanism) lock is used to ensure
**         that log-recovery is always run following a system restart.
**         When it first opens a log-summary file, a process takes a
**         SHARED lock on the DMH region. This lock is not released until
**         the log-summary file is closed. 
**
**         The process then attempts to upgrade to an EXCLUSIVE lock. If 
**         successful, then the contents of the log-summary file are deemed 
**         suspect and the log-summary header zeroed. This forces the
**         first process that reads the log-summary file to run log 
**         recovery. After zeroing the log-summary header, the process
**         downgrades to a SHARED lock on the DMH region.
**
**         If the attempt to obtain the EXCLUSIVE lock fails, then the
**         process concludes that some other process is already using the
**         log-summary file, and it can therefore be trusted.
**
**         The procedure described in the previous three paragraphs (taking
**         a SHARED lock and then upgrading to an EXCLUSIVE lock to check
**         if the process is the only one to have an open connection to the 
**         log file) is protected by holding the MUTEX lock. This avoids the
**         race condition wherein the first two clients connect almost 
**         simultaneously following a system restart and each prevents 
**         the other from obtaining the EXCLUSIVE lock.
**
**
** REGION: There are 4 different region locks, regions A, B, C and D.
**         Various EXCLUSIVE and SHARED locks on these regions are obtained
**         when a client reads, writes or checkpoints the database.
**
**    To obtain a reader lock:
**
**         1. Attempt a SHARED lock on regions A and B.
**         2. If step 1 is successful, drop the lock on region B. Or, if
**            it is unsuccessful, attempt a SHARED lock on region D.
**         3. Repeat the above until the lock attempt in step 1 or 2 is 
**            successful.
**
**         The reader lock is released when the read transaction is finished.
**
**    To obtain a writer lock:
**
**         1. Take (wait for) an EXCLUSIVE lock on regions C and D.
**
**         The locks are released after the write transaction is finished
**         and, if any frames were committed to the log, the log-summary
**         file updated.
**
**    To obtain a checkpointer lock:
**
**         1. Take (wait for) an EXCLUSIVE lock on regions B and C.
**         2. Take (wait for) an EXCLUSIVE lock on region A.
**
**         Step 1 waits until any existing writer has finished. And forces
**         all new readers to become "region D" readers.
**
**         Step 2 causes the checkpointer to wait until all existing region A
**         readers have finished their transactions. Once the exclusive lock
**         on region A has been obtained, only "region D" readers exist.
**         These readers are operating on the snapshot at the head of the
**         log. As such, the log can be safely copied into the database file
**         without interfering with the readers.
**
**         Once the checkpoint has finished and the log-summary header
**         updated (to indicate the log contents can now be ignored), all
**         locks are released.
**
**         However, there may still exist region D readers using data in 
**         the body of the log file, so the log file itself cannot be 
**         truncated or overwritten until all region D readers have finished.
**         That requirement is satisfied, because writers (the clients that
**         write to the log file) require an exclusive lock on region D.
**         Which they cannot get until all region D readers have finished.
*/
#define LOG_LOCK_MUTEX  (LOGSUMMARY_LOCK_OFFSET)
#define LOG_LOCK_DMH    (LOG_LOCK_MUTEX+1)
#define LOG_LOCK_REGION (LOG_LOCK_DMH+1)

/*
** The four lockable regions associated with each log-summary. A connection
** may take either a SHARED or EXCLUSIVE lock on each. An ORed combination
** of the following bitmasks is passed as the second argument to the
** logLockRegion() function.
*/
#define LOG_REGION_A 0x01
#define LOG_REGION_B 0x02
#define LOG_REGION_C 0x04
#define LOG_REGION_D 0x08

/*
** Values for the third parameter to logLockRegion().
*/
#define LOG_UNLOCK  0             /* Unlock a range of bytes */
#define LOG_RDLOCK  1             /* Put a SHARED lock on a range of bytes */
#define LOG_WRLOCK  2             /* Put an EXCLUSIVE lock on a byte-range */
#define LOG_WRLOCKW 3             /* Block on EXCLUSIVE lock on a byte-range */

/*
** A single instance of this structure is allocated as part of each 
** connection to a database log. All structures associated with the 
** same log file are linked together into a list using LogLock.pNext
** starting at LogSummary.pLock.
**
** The mLock field of the structure describes the locks (if any) 
** currently held by the connection. If a SHARED lock is held on
** any of the four locking regions, then the associated LOG_REGION_X
** bit (see above) is set. If an EXCLUSIVE lock is held on the region,
** then the (LOG_REGION_X << 8) bit is set.
*/
struct LogLock {
  LogLock *pNext;                 /* Next lock on the same log */
  u32 mLock;                      /* Mask of locks */
};

struct Log {
  LogSummary *pSummary;           /* Log file summary data */
  sqlite3_vfs *pVfs;              /* The VFS used to create pFd */
  sqlite3_file *pFd;              /* File handle for log file */
  int isLocked;                   /* Non-zero if a snapshot is held open */
  int isWriteLocked;              /* True if this is the writer connection */
  u32 iCallback;                  /* Value to pass to log callback (or 0) */
  LogSummaryHdr hdr;              /* Log summary header for current snapshot */
  LogLock lock;                   /* Lock held by this connection (if any) */






};


/*
** This structure is used to implement an iterator that iterates through
** all frames in the log 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 log.
**
** The internals of this structure are only accessed by:
**
**   logIteratorInit() - Create a new iterator,
**   logIteratorNext() - Step an iterator,
**   logIteratorFree() - Free an iterator.
**
** This functionality is used by the checkpoint code (see logCheckpoint()).
*/
struct LogIterator {
  int nSegment;                   /* Size of LogIterator.aSegment[] array */
  int nFinal;                     /* Elements in segment nSegment-1 */
  struct LogSegment {
    int iNext;                    /* Next aIndex index */
    u8 *aIndex;                   /* Pointer to index array */
    u32 *aDbPage;                 /* Pointer to db page array */
  } aSegment[1];
};



/*
** List of all LogSummary objects created by this process. Protected by
** static mutex LOG_SUMMARY_MUTEX. TODO: Should have a dedicated mutex
** here instead of borrowing the LRU mutex.
*/
#define LOG_SUMMARY_MUTEX SQLITE_MUTEX_STATIC_LRU
static LogSummary *pLogSummary = 0;

/*
** Generate an 8 byte checksum based on the data in array aByte[] and the
** initial values of aCksum[0] and aCksum[1]. The checksum is written into
** aCksum[] before returning.
**
** The range of bytes to checksum is treated as an array of 32-bit 
** little-endian unsigned integers. For each integer X in the array, from
................................................................................
**
** For the calculation above, use 64-bit unsigned accumulators. Before
** returning, truncate the values to 32-bits as follows: 
**
**   aCksum[0] = (u32)(aCksum[0] + (aCksum[0]>>24));
**   aCksum[1] = (u32)(aCksum[1] + (aCksum[1]>>24));
*/
static void logChecksumBytes(u8 *aByte, int nByte, u32 *aCksum){
  u64 sum1 = aCksum[0];
  u64 sum2 = aCksum[1];
  u32 *a32 = (u32 *)aByte;
  u32 *aEnd = (u32 *)&aByte[nByte];

  assert( (nByte&0x00000003)==0 );

................................................................................
  }

  aCksum[0] = sum1 + (sum1>>24);
  aCksum[1] = sum2 + (sum2>>24);
}

/*
** Argument zPath must be a nul-terminated string containing a path-name.
** This function modifies the string in-place by removing any "./" or "../" 
** elements in the path. For example, the following input:
**


**   "/home/user/plans/good/../evil/./world_domination.txt"
**
** is overwritten with the 'normalized' version:
**
**   "/home/user/plans/evil/world_domination.txt"
*/
static void logNormalizePath(char *zPath){

  int i, j;




  char *z = zPath;
  int n = strlen(z);

  while( n>1 && z[n-1]=='/' ){ n--; }
  for(i=j=0; i<n; i++){
    if( z[i]=='/' ){
      if( z[i+1]=='/' ) continue;
      if( z[i+1]=='.' && i+2<n && z[i+2]=='/' ){
        i += 1;
        continue;
      }
      if( z[i+1]=='.' && i+3<n && z[i+2]=='.' && z[i+3]=='/' ){
        while( j>0 && z[j-1]!='/' ){ j--; }
        if( j>0 ){ j--; }
        i += 2;
        continue;

      }
    }
    z[j++] = z[i];
  }
  z[j] = 0;
}

/*
** Unmap the log-summary mapping and close the file-descriptor. If
** the isTruncate argument is non-zero, truncate the log-summary file
** region to zero bytes.
**
** Regardless of the value of isTruncate, close the file-descriptor
** opened on the log-summary file.

*/
static int logSummaryUnmap(LogSummary *pSummary, int isUnlink){
  int rc = SQLITE_OK;
  if( pSummary->aData ){
    assert( pSummary->fd>0 );
    munmap(pSummary->aData, pSummary->nData);
    pSummary->aData = 0;
    if( isUnlink ){
      char *zFile = sqlite3_mprintf("%s-summary", pSummary->zPath);
      if( !zFile ){
        rc = SQLITE_NOMEM;
      }
      unlink(zFile);
      sqlite3_free(zFile);
    }
  }
  if( pSummary->fd>0 ){
    close(pSummary->fd);
    pSummary->fd = -1;
  }
  return rc;
}

static void logSummaryWriteHdr(LogSummary *pSummary, LogSummaryHdr *pHdr){

  u32 *aHdr = pSummary->aData;                   /* Write header here */
  u32 *aCksum = &aHdr[LOGSUMMARY_HDR_NFIELD];    /* Write header cksum here */

  assert( LOGSUMMARY_HDR_NFIELD==sizeof(LogSummaryHdr)/4 );

  memcpy(aHdr, pHdr, sizeof(LogSummaryHdr));
  aCksum[0] = aCksum[1] = 1;
  logChecksumBytes((u8 *)aHdr, sizeof(LogSummaryHdr), aCksum);
}

/*
** This function encodes a single frame header and writes it to a buffer
** supplied by the caller. A log frame-header is made up of a series of 
** 4-byte big-endian integers, as follows:
**
**     0: Database page size in bytes.
**     4: Page number.
**     8: New database size (for commit frames, otherwise zero).
**    12: Frame checksum 1.
**    16: Frame checksum 2.
*/
static void logEncodeFrame(
  u32 *aCksum,                    /* IN/OUT: Checksum values */
  u32 iPage,                      /* Database page number for frame */
  u32 nTruncate,                  /* New db size (or 0 for non-commit frames) */
  int nData,                      /* Database page size (size of aData[]) */
  u8 *aData,                      /* Pointer to page data (for checksum) */
  u8 *aFrame                      /* OUT: Write encoded frame here */
){
  assert( LOG_FRAME_HDRSIZE==16 );

  sqlite3Put4byte(&aFrame[0], iPage);
  sqlite3Put4byte(&aFrame[4], nTruncate);

  logChecksumBytes(aFrame, 8, aCksum);
  logChecksumBytes(aData, nData, aCksum);

  sqlite3Put4byte(&aFrame[8], aCksum[0]);
  sqlite3Put4byte(&aFrame[12], aCksum[1]);
}

/*
** Return 1 and populate *piPage, *pnTruncate and aCksum if the 
** frame checksum looks Ok. Otherwise return 0.
*/
static int logDecodeFrame(
  u32 *aCksum,                    /* IN/OUT: Checksum values */
  u32 *piPage,                    /* OUT: Database page number for frame */
  u32 *pnTruncate,                /* OUT: New db size (or 0 if not commit) */
  int nData,                      /* Database page size (size of aData[]) */
  u8 *aData,                      /* Pointer to page data (for checksum) */
  u8 *aFrame                      /* Frame data */
){
  assert( LOG_FRAME_HDRSIZE==16 );

  logChecksumBytes(aFrame, 8, aCksum);
  logChecksumBytes(aData, nData, aCksum);

  if( aCksum[0]!=sqlite3Get4byte(&aFrame[8]) 
   || aCksum[1]!=sqlite3Get4byte(&aFrame[12]) 
  ){
    /* Checksum failed. */
    return 0;
  }

  *piPage = sqlite3Get4byte(&aFrame[0]);
  *pnTruncate = sqlite3Get4byte(&aFrame[4]);
  return 1;
}

static void logMergesort8(
  Pgno *aContent,                 /* Pages in log */
  u8 *aBuffer,                    /* Buffer of at least *pnList items to use */
  u8 *aList,                      /* IN/OUT: List to sort */
  int *pnList                     /* IN/OUT: Number of elements in aList[] */
){
  int nList = *pnList;
  if( nList>1 ){
    int nLeft = nList / 2;        /* Elements in left list */
................................................................................
    u8 *aLeft = aList;            /* Left list */
    u8 *aRight = &aList[nLeft];   /* Right list */
    int iLeft = 0;                /* Current index in aLeft */
    int iRight = 0;               /* Current index in aright */
    int iOut = 0;                 /* Current index in output buffer */

    /* TODO: Change to non-recursive version. */
    logMergesort8(aContent, aBuffer, aLeft, &nLeft);
    logMergesort8(aContent, aBuffer, aRight, &nRight);

    while( iRight<nRight || iLeft<nLeft ){
      u8 logpage;
      Pgno dbpage;

      if( (iLeft<nLeft) 
       && (iRight>=nRight || aContent[aLeft[iLeft]]<aContent[aRight[iRight]])
................................................................................
    }
  }
#endif
}


/*
** Memory map the first nByte bytes of the summary file opened with 
** pSummary->fd at pSummary->aData. If the summary file is smaller than
** nByte bytes in size when this function is called, ftruncate() is
** used to expand it before it is mapped.
**
** It is assumed that an exclusive lock is held on the summary file
** by the caller (to protect the ftruncate()).
*/
static int logSummaryMap(LogSummary *pSummary, int nByte){
  struct stat sStat;
  int rc;
  int fd = pSummary->fd;
  void *pMap;

  assert( pSummary->aData==0 );

  /* If the file is less than nByte bytes in size, cause it to grow. */
  rc = fstat(fd, &sStat);
  if( rc!=0 ) return SQLITE_IOERR;
  if( sStat.st_size<nByte ){
    rc = ftruncate(fd, nByte);
    if( rc!=0 ) return SQLITE_IOERR;
  }else{
    nByte = sStat.st_size;
  }

  /* Map the file. */
  pMap = mmap(0, nByte, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
  if( pMap==MAP_FAILED ){
    return SQLITE_IOERR;
  }
  pSummary->aData = (u32 *)pMap;
  pSummary->nData = nByte/4;

  return SQLITE_OK;
}

/*
** The log-summary file is already mapped to pSummary->aData[], but the
** mapping needs to be resized. Unmap and remap the file so that the mapping 
** is at least nByte bytes in size, or the size of the entire file if it 
** is larger than nByte bytes.
*/
static int logSummaryRemap(LogSummary *pSummary, int nByte){
  int rc;
  sqlite3_mutex_enter(pSummary->mutex);
  munmap(pSummary->aData, pSummary->nData*4);
  pSummary->aData = 0;
  rc = logSummaryMap(pSummary, nByte);
  sqlite3_mutex_leave(pSummary->mutex);
  return rc;
}

/*
** Return the index in the LogSummary.aData array that corresponds to 
** frame iFrame. The log-summary file consists of a header, followed by
** alternating "map" and "index" blocks.
*/
static int logSummaryEntry(u32 iFrame){
  return (
      (LOGSUMMARY_LOCK_OFFSET+LOGSUMMARY_LOCK_RESERVED)/sizeof(u32)

    + (((iFrame-1)>>8)<<6)        /* Indexes that occur before iFrame */
    + iFrame-1                    /* Db page numbers that occur before iFrame */
  );
}











/*




























** Set an entry in the log-summary map to map log frame iFrame to db 
** page iPage. Values are always appended to the log-summary (i.e. the
** value of iFrame is always exactly one more than the value passed to
** the previous call), but that restriction is not enforced or asserted
** here.
*/
static int logSummaryAppend(LogSummary *pSummary, u32 iFrame, u32 iPage){
  u32 iSlot = logSummaryEntry(iFrame);


  while( (iSlot+128)>=pSummary->nData ){
    int rc;
    int nByte = pSummary->nData*4 + LOGSUMMARY_MMAP_INCREMENT;


    /* Unmap and remap the log-summary file. */
    rc = logSummaryRemap(pSummary, nByte);

    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  /* Set the log-summary entry itself */
  pSummary->aData[iSlot] = iPage;

  /* If the frame number is a multiple of 256 (frames are numbered starting
  ** at 1), build an index of the most recently added 256 frames.
  */
  if( (iFrame&0x000000FF)==0 ){
    int i;                        /* Iterator used while initializing aIndex */
    u32 *aFrame;                  /* Pointer to array of 256 frames */
    int nIndex;                   /* Number of entries in index */
    u8 *aIndex;                   /* 256 bytes to build index in */
    u8 *aTmp;                     /* Scratch space to use while sorting */

    aFrame = &pSummary->aData[iSlot-255];
    aIndex = (u8 *)&pSummary->aData[iSlot+1];
    aTmp = &aIndex[256];

    nIndex = 256;
    for(i=0; i<256; i++) aIndex[i] = (u8)i;
    logMergesort8(aFrame, aTmp, aIndex, &nIndex);
    memset(&aIndex[nIndex], aIndex[nIndex-1], 256-nIndex);
  }

  return SQLITE_OK;
}


/*
** Recover the log-summary by reading the log file. The caller must hold 
** an exclusive lock on the log-summary file.
*/
static int logSummaryRecover(LogSummary *pSummary, sqlite3_file *pFd){
  int rc;                         /* Return Code */
  i64 nSize;                      /* Size of log file */
  LogSummaryHdr hdr;              /* Recovered log-summary header */


  memset(&hdr, 0, sizeof(hdr));

  rc = sqlite3OsFileSize(pFd, &nSize);
  if( rc!=SQLITE_OK ){
    return rc;
  }

  if( nSize>LOG_FRAME_HDRSIZE ){
    u8 aBuf[LOG_FRAME_HDRSIZE];   /* Buffer to load first frame header into */
    u8 *aFrame = 0;               /* Malloc'd buffer to load entire frame */
    int nFrame;                   /* Number of bytes at 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 nPgsz;                    /* Page size according to the log */
    u32 aCksum[2];                /* Running checksum */

    /* Read in the first frame header in the file (to determine the 
    ** database page size).
    */
    rc = sqlite3OsRead(pFd, aBuf, LOG_HDRSIZE, 0);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* If the database page size is not a power of two, or is greater than
    ** SQLITE_MAX_PAGE_SIZE, conclude that the log file contains no valid data.
    */
................................................................................
    if( nPgsz&(nPgsz-1) || nPgsz>SQLITE_MAX_PAGE_SIZE || nPgsz<512 ){
      goto finished;
    }
    aCksum[0] = sqlite3Get4byte(&aBuf[4]);
    aCksum[1] = sqlite3Get4byte(&aBuf[8]);

    /* Malloc a buffer to read frames into. */
    nFrame = nPgsz + LOG_FRAME_HDRSIZE;
    aFrame = (u8 *)sqlite3_malloc(nFrame);
    if( !aFrame ){
      return SQLITE_NOMEM;
    }
    aData = &aFrame[LOG_FRAME_HDRSIZE];

    /* Read all frames from the log file. */
    iFrame = 0;
    for(iOffset=LOG_HDRSIZE; (iOffset+nFrame)<=nSize; iOffset+=nFrame){
      u32 pgno;                   /* Database page number for frame */
      u32 nTruncate;              /* dbsize field from frame header */
      int isValid;                /* True if this frame is valid */

      /* Read and decode the next log frame. */
      rc = sqlite3OsRead(pFd, aFrame, nFrame, iOffset);
      if( rc!=SQLITE_OK ) break;
      isValid = logDecodeFrame(aCksum, &pgno, &nTruncate, nPgsz, aData, aFrame);
      if( !isValid ) break;
      logSummaryAppend(pSummary, ++iFrame, pgno);

      /* If nTruncate is non-zero, this is a commit record. */
      if( nTruncate ){
        hdr.iCheck1 = aCksum[0];
        hdr.iCheck2 = aCksum[1];
        hdr.iLastPg = iFrame;
        hdr.nPage = nTruncate;
................................................................................
    sqlite3_free(aFrame);
  }else{
    hdr.iCheck1 = 2;
    hdr.iCheck2 = 3;
  }

finished:
  logSummaryWriteHdr(pSummary, &hdr);
  return rc;
}

/*
** Place, modify or remove a lock on the log-summary file associated 
** with pSummary.
**
** The locked byte-range should be inside the region dedicated to 
** locking. This region of the log-summary file is never read or written.
*/
static int logLockFd(
  LogSummary *pSummary,           /* The log-summary object to lock */
  int iStart,                     /* First byte to lock */
  int nByte,                      /* Number of bytes to lock */
  int op                          /* LOG_UNLOCK, RDLOCK, WRLOCK or WRLOCKW */
){
  int aType[4] = { 
    F_UNLCK,                      /* LOG_UNLOCK */
    F_RDLCK,                      /* LOG_RDLOCK */
    F_WRLCK,                      /* LOG_WRLOCK */
    F_WRLCK                       /* LOG_WRLOCKW */
  };
  int aOp[4] = { 
    F_SETLK,                      /* LOG_UNLOCK */
    F_SETLK,                      /* LOG_RDLOCK */
    F_SETLK,                      /* LOG_WRLOCK */
    F_SETLKW                      /* LOG_WRLOCKW */
  };
  struct flock f;                 /* Locking operation */
  int rc;                         /* Value returned by fcntl() */

  assert( ArraySize(aType)==ArraySize(aOp) );
  assert( op>=0 && op<ArraySize(aType) );
  assert( nByte>0 );
  assert( iStart>=LOGSUMMARY_LOCK_OFFSET 
       && iStart+nByte<=LOGSUMMARY_LOCK_OFFSET+LOGSUMMARY_LOCK_RESERVED
  );
#if defined(SQLITE_DEBUG) && defined(SQLITE_OS_UNIX)
  if( pSummary->aData ) memset(&((u8*)pSummary->aData)[iStart], op, nByte);
#endif

  memset(&f, 0, sizeof(f));
  f.l_type = aType[op];
  f.l_whence = SEEK_SET;
  f.l_start = iStart;
  f.l_len = nByte;
  rc = fcntl(pSummary->fd, aOp[op], &f);
  return (rc==0) ? SQLITE_OK : SQLITE_BUSY;
}

static int logLockRegion(Log *pLog, u32 mRegion, int op){
  LogSummary *pSummary = pLog->pSummary;
  LogLock *p;                     /* Used to iterate through in-process locks */
  u32 mOther;                     /* Locks held by other connections */
  u32 mNew;                       /* New mask for pLog */

  assert( 
       /* Writer lock operations */
          (op==LOG_WRLOCK && mRegion==(LOG_REGION_C|LOG_REGION_D))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_C|LOG_REGION_D))

       /* Normal reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B))

       /* Region D reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_D))
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_D))

       /* Checkpointer lock operations */
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B|LOG_REGION_C))
  );

  /* Assert that a connection never tries to go from an EXCLUSIVE to a 
  ** SHARED lock on a region. Moving from SHARED to EXCLUSIVE sometimes
  ** happens though (when a region D reader upgrades to a writer).
  */
  assert( op!=LOG_RDLOCK || 0==(pLog->lock.mLock & (mRegion<<8)) );

  sqlite3_mutex_enter(pSummary->mutex);

  /* Calculate a mask of logs held by all connections in this process apart
  ** from this one. The least significant byte of the mask contains a mask
  ** of the SHARED logs held. The next least significant byte of the mask
  ** indicates the EXCLUSIVE locks held. For example, to test if some other
  ** connection is holding a SHARED lock on region A, or an EXCLUSIVE lock
  ** on region C, do:
  **
  **   hasSharedOnA    = (mOther & (LOG_REGION_A<<0));
  **   hasExclusiveOnC = (mOther & (LOG_REGION_C<<8));
  **
  ** In all masks, if the bit in the EXCLUSIVE byte mask is set, so is the 
  ** corresponding bit in the SHARED mask.
  */
  mOther = 0;
  for(p=pSummary->pLock; p; p=p->pNext){
    assert( (p->mLock & (p->mLock<<8))==(p->mLock&0x0000FF00) );
    if( p!=&pLog->lock ){
      mOther |= p->mLock;
    }
  }

  /* If this call is to lock a region (not to unlock one), test if locks held
  ** by any other connection in this process prevent the new locks from
  ** begin granted. If so, exit the summary mutex and return SQLITE_BUSY.
  */
  if( op && (mOther & (mRegion << (op==LOG_RDLOCK ? 8 : 0))) ){
    sqlite3_mutex_leave(pSummary->mutex);
    return SQLITE_BUSY;
  }

  /* Figure out the new log mask for this connection. */
  switch( op ){
    case LOG_UNLOCK: 
      mNew = (pLog->lock.mLock & ~(mRegion|(mRegion<<8)));
      break;
    case LOG_RDLOCK:
      mNew = (pLog->lock.mLock | mRegion);
      break;
    default:
      assert( op==LOG_WRLOCK );
      mNew = (pLog->lock.mLock | (mRegion<<8) | mRegion);
      break;
  }

  /* Now modify the locks held on the log-summary file descriptor. This
  ** file descriptor is shared by all log connections in this process. 
  ** Therefore:
  **
  **   + If one or more log connections in this process hold a SHARED lock
  **     on a region, the file-descriptor should hold a SHARED lock on
  **     the file region.
  **
  **   + If a log connection in this process holds an EXCLUSIVE lock on a
  **     region, the file-descriptor should also hold an EXCLUSIVE lock on
  **     the region in question.
  **
  ** If this is an LOG_UNLOCK operation, only regions for which no other
  ** connection holds a lock should actually be unlocked. And if this
  ** is a LOG_RDLOCK operation and other connections already hold all
  ** the required SHARED locks, then no system call is required.
  */
  if( op==LOG_UNLOCK ){
    mRegion = (mRegion & ~mOther);
  }
  if( (op==LOG_WRLOCK)
   || (op==LOG_UNLOCK && mRegion) 
   || (op==LOG_RDLOCK && (mOther&mRegion)!=mRegion)
  ){
    struct LockMap {
      int iStart;                 /* Byte offset to start locking operation */
      int iLen;                   /* Length field for locking operation */
    } aMap[] = {
      /* 0000 */ {0, 0},                    /* 0001 */ {3+LOG_LOCK_REGION, 1}, 
      /* 0010 */ {2+LOG_LOCK_REGION, 1},    /* 0011 */ {2+LOG_LOCK_REGION, 2},
      /* 0100 */ {1+LOG_LOCK_REGION, 1},    /* 0101 */ {0, 0}, 
      /* 0110 */ {1+LOG_LOCK_REGION, 2},    /* 0111 */ {1+LOG_LOCK_REGION, 3},
      /* 1000 */ {0+LOG_LOCK_REGION, 1},    /* 1001 */ {0, 0}, 
      /* 1010 */ {0, 0},                    /* 1011 */ {0, 0},
      /* 1100 */ {0+LOG_LOCK_REGION, 2},    /* 1101 */ {0, 0}, 
      /* 1110 */ {0, 0},                    /* 1111 */ {0, 0}
    };
    int rc;                       /* Return code of logLockFd() */

    assert( mRegion<ArraySize(aMap) && aMap[mRegion].iStart!=0 );

    rc = logLockFd(pSummary, aMap[mRegion].iStart, aMap[mRegion].iLen, op);
    if( rc!=0 ){
      sqlite3_mutex_leave(pSummary->mutex);
      return rc;
    }
  }

  pLog->lock.mLock = mNew;
  sqlite3_mutex_leave(pSummary->mutex);
  return SQLITE_OK;
}

/*
** Lock the DMH region, either with an EXCLUSIVE or SHARED lock. This
** function is never called with LOG_UNLOCK - the only way the DMH region
** is every completely unlocked is by by closing the file descriptor.
*/
static int logLockDMH(LogSummary *pSummary, int eLock){
  assert( sqlite3_mutex_held(pSummary->mutex) );
  assert( eLock==LOG_RDLOCK || eLock==LOG_WRLOCK );
  return logLockFd(pSummary, LOG_LOCK_DMH, 1, eLock);
}

/*
** Lock (or unlock) the MUTEX region. It is always locked using an
** EXCLUSIVE, blocking lock.
*/
static int logLockMutex(LogSummary *pSummary, int eLock){
  assert( sqlite3_mutex_held(pSummary->mutex) );
  assert( eLock==LOG_WRLOCKW || eLock==LOG_UNLOCK );
  logLockFd(pSummary, LOG_LOCK_MUTEX, 1, eLock);
  return SQLITE_OK;
}

/*
** This function intializes the connection to the log-summary identified
** by struct pSummary.
*/
static int logSummaryInit(
  LogSummary *pSummary,           /* Log summary object to initialize */
  sqlite3_file *pFd               /* File descriptor open on log file */
){
  int rc;                         /* Return Code */
  char *zFile;                    /* File name for summary file */

  assert( pSummary->fd<0 );
  assert( pSummary->aData==0 );
  assert( pSummary->nRef>0 );
  assert( pSummary->zPath );

  /* Open a file descriptor on the summary file. */
  zFile = sqlite3_mprintf("%s-summary", pSummary->zPath);
  if( !zFile ){
    return SQLITE_NOMEM;
  }
  pSummary->fd = open(zFile, O_RDWR|O_CREAT, S_IWUSR|S_IRUSR);
  sqlite3_free(zFile);
  if( pSummary->fd<0 ){
    return SQLITE_IOERR;
  }

  /* Grab an exclusive lock the summary file. Then mmap() it. 
  **
  ** TODO: This code needs to be enhanced to support a growable mapping. 
  ** For now, just make the mapping very large to start with. The 
  ** pages should not be allocated until they are first accessed anyhow,
  ** so using a large mapping consumes no more resources than a smaller
  ** one would.
  */
  assert( sqlite3_mutex_held(pSummary->mutex) );
  rc = logLockMutex(pSummary, LOG_WRLOCKW);
  if( rc!=SQLITE_OK ) return rc;
  rc = logSummaryMap(pSummary, LOGSUMMARY_MMAP_INCREMENT);
  if( rc!=SQLITE_OK ) goto out;

  /* Try to obtain an EXCLUSIVE lock on the dead-mans-hand region. If this
  ** is possible, the contents of the log-summary file (if any) may not
  ** be trusted. Zero the log-summary header before continuing.
  */
  rc = logLockDMH(pSummary, LOG_WRLOCK);
  if( rc==SQLITE_OK ){
    memset(pSummary->aData, 0, (LOGSUMMARY_HDR_NFIELD+2)*sizeof(u32) );
  }
  rc = logLockDMH(pSummary, LOG_RDLOCK);
  if( rc!=SQLITE_OK ){
    rc = SQLITE_IOERR;
  }

 out:
  logLockMutex(pSummary, LOG_UNLOCK);
  return rc;
}

/* 
** Open a connection to the log file associated with database zDb. The
** database file does not actually have to exist. zDb is used only to
** figure out the name of the log file to open. If the log file does not 
** exist it is created by this call.
**
** 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 log or log-summary file. If another process
** were to do this just after this client opened one of these files, the
** system would be badly broken.
*/
int sqlite3WalOpen(
  sqlite3_vfs *pVfs,              /* vfs module to open log file with */
  const char *zDb,                /* Name of database file */
  Log **ppLog                     /* OUT: Allocated Log handle */
){
  int rc = SQLITE_OK;             /* Return Code */
  Log *pRet;                      /* Object to allocate and return */
  LogSummary *pSummary = 0;       /* Summary object */
  sqlite3_mutex *mutex = 0;       /* LOG_SUMMARY_MUTEX mutex */
  int flags;                      /* Flags passed to OsOpen() */
  char *zWal = 0;                 /* Path to WAL file */
  int nWal;                       /* Length of zWal in bytes */

  assert( zDb );


  /* Allocate an instance of struct Log to return. */
  *ppLog = 0;

  pRet = (Log *)sqlite3MallocZero(sizeof(Log) + pVfs->szOsFile);
  if( !pRet ) goto out;
  pRet->pVfs = pVfs;
  pRet->pFd = (sqlite3_file *)&pRet[1];

  /* Normalize the path name. */
  zWal = sqlite3_mprintf("%s-wal", zDb);
  if( !zWal ) goto out;
  logNormalizePath(zWal);
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_MAIN_JOURNAL);
  nWal = sqlite3Strlen30(zWal);

  /* Enter the mutex that protects the linked-list of LogSummary structures */
  if( sqlite3GlobalConfig.bCoreMutex ){
    mutex = sqlite3_mutex_alloc(LOG_SUMMARY_MUTEX);
  }
  sqlite3_mutex_enter(mutex);

  /* Search for an existing log summary object in the linked list. If one 
  ** cannot be found, allocate and initialize a new object.
  */
  for(pSummary=pLogSummary; pSummary; pSummary=pSummary->pNext){
    int nPath = sqlite3Strlen30(pSummary->zPath);
    if( nWal==nPath && 0==memcmp(pSummary->zPath, zWal, nPath) ) break;
  }
  if( !pSummary ){
    int nByte = sizeof(LogSummary) + nWal + 1;
    pSummary = (LogSummary *)sqlite3MallocZero(nByte);
    if( !pSummary ){
      rc = SQLITE_NOMEM;
      goto out;
    }
    if( sqlite3GlobalConfig.bCoreMutex ){
      pSummary->mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_RECURSIVE);
    }
    pSummary->zPath = (char *)&pSummary[1];
    pSummary->fd = -1;
    memcpy(pSummary->zPath, zWal, nWal);
    pSummary->pNext = pLogSummary;
    pLogSummary = pSummary;
  }
  pSummary->nRef++;
  pRet->pSummary = pSummary;

  /* Exit the mutex protecting the linked-list of LogSummary objects. */
  sqlite3_mutex_leave(mutex);
  mutex = 0;

  /* Open file handle on the log file. */


  rc = sqlite3OsOpen(pVfs, pSummary->zPath, pRet->pFd, flags, &flags);
  if( rc!=SQLITE_OK ) goto out;

  /* Object pSummary is shared between all connections to the database made
  ** by this process. So at this point it may or may not be connected to
  ** the log-summary. If it is not, connect it.
  */
  sqlite3_mutex_enter(pSummary->mutex);
  mutex = pSummary->mutex;
  if( pSummary->fd<0 ){
    rc = logSummaryInit(pSummary, pRet->pFd);
  }

  pRet->lock.pNext = pSummary->pLock;
  pSummary->pLock = &pRet->lock;

 out:
  sqlite3_mutex_leave(mutex);
  sqlite3_free(zWal);
  if( rc!=SQLITE_OK ){
    assert(0);
    if( pRet ){

      sqlite3OsClose(pRet->pFd);
      sqlite3_free(pRet);
    }
    assert( !pSummary || pSummary->nRef==0 );
    sqlite3_free(pSummary);
  }
  *ppLog = pRet;
  return rc;
}

static int logIteratorNext(
  LogIterator *p,               /* Iterator */
  u32 *piPage,                    /* OUT: Next db page to write */
  u32 *piFrame                    /* OUT: Log frame to read from */
){
  u32 iMin = *piPage;
  u32 iRet = 0xFFFFFFFF;
  int i;
  int nBlock = p->nFinal;

  for(i=p->nSegment-1; i>=0; i--){
    struct LogSegment *pSegment = &p->aSegment[i];
    while( pSegment->iNext<nBlock ){
      u32 iPg = pSegment->aDbPage[pSegment->aIndex[pSegment->iNext]];
      if( iPg>iMin ){
        if( iPg<iRet ){
          iRet = iPg;
          *piFrame = i*256 + 1 + pSegment->aIndex[pSegment->iNext];
        }
................................................................................
    nBlock = 256;
  }

  *piPage = iRet;
  return (iRet==0xFFFFFFFF);
}

static LogIterator *logIteratorInit(Log *pLog){
  u32 *aData = pLog->pSummary->aData;
  LogIterator *p;                 /* Return value */
  int nSegment;                   /* Number of segments to merge */
  u32 iLast;                      /* Last frame in log */
  int nByte;                      /* Number of bytes to allocate */
  int i;                          /* Iterator variable */
  int nFinal;                     /* Number of unindexed entries */
  struct LogSegment *pFinal;      /* Final (unindexed) segment */
  u8 *aTmp;                       /* Temp space used by merge-sort */



  iLast = pLog->hdr.iLastPg;
  nSegment = (iLast >> 8) + 1;
  nFinal = (iLast & 0x000000FF);

  nByte = sizeof(LogIterator) + (nSegment-1)*sizeof(struct LogSegment) + 512;
  p = (LogIterator *)sqlite3_malloc(nByte);
  if( p ){
    memset(p, 0, nByte);
    p->nSegment = nSegment;
    p->nFinal = nFinal;
  }

  for(i=0; i<nSegment-1; i++){
    p->aSegment[i].aDbPage = &aData[logSummaryEntry(i*256+1)];
    p->aSegment[i].aIndex = (u8 *)&aData[logSummaryEntry(i*256+1)+256];
  }
  pFinal = &p->aSegment[nSegment-1];

  pFinal->aDbPage = &aData[logSummaryEntry((nSegment-1)*256+1)];
  pFinal->aIndex = (u8 *)&pFinal[1];
  aTmp = &pFinal->aIndex[256];
  for(i=0; i<nFinal; i++){
    pFinal->aIndex[i] = i;
  }
  logMergesort8(pFinal->aDbPage, aTmp, pFinal->aIndex, &nFinal);
  p->nFinal = nFinal;

  return p;
}

/* 
** Free a log iterator allocated by logIteratorInit().
*/
static void logIteratorFree(LogIterator *p){
  sqlite3_free(p);
}

/*
** Checkpoint the contents of the log file.
*/
static int logCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  int sync_flags,                 /* Flags for OsSync() (or 0) */
  u8 *zBuf                        /* Temporary buffer to use */
){
  int rc;                         /* Return code */
  int pgsz = pLog->hdr.pgsz;      /* Database page-size */
  LogIterator *pIter = 0;         /* Log iterator context */
  u32 iDbpage = 0;                /* Next database page to write */
  u32 iFrame = 0;                 /* Log frame containing data for iDbpage */

  if( pLog->hdr.iLastPg==0 ){
    return SQLITE_OK;
  }

  /* Allocate the iterator */
  pIter = logIteratorInit(pLog);
  if( !pIter ) return SQLITE_NOMEM;

  /* Sync the log file to disk */
  if( sync_flags ){
    rc = sqlite3OsSync(pLog->pFd, sync_flags);
    if( rc!=SQLITE_OK ) goto out;
  }

  /* Iterate through the contents of the log, copying data to the db file. */
  while( 0==logIteratorNext(pIter, &iDbpage, &iFrame) ){
    rc = sqlite3OsRead(pLog->pFd, zBuf, pgsz, 
        logFrameOffset(iFrame, pgsz) + LOG_FRAME_HDRSIZE
    );
    if( rc!=SQLITE_OK ) goto out;
    rc = sqlite3OsWrite(pFd, zBuf, pgsz, (iDbpage-1)*pgsz);
    if( rc!=SQLITE_OK ) goto out;
  }

  /* Truncate the database file */
  rc = sqlite3OsTruncate(pFd, ((i64)pLog->hdr.nPage*(i64)pgsz));
  if( rc!=SQLITE_OK ) goto out;

  /* Sync the database file. If successful, update the log-summary. */
  if( sync_flags ){
    rc = sqlite3OsSync(pFd, sync_flags);
    if( rc!=SQLITE_OK ) goto out;
  }
  pLog->hdr.iLastPg = 0;
  pLog->hdr.iCheck1 = 2;
  pLog->hdr.iCheck2 = 3;
  logSummaryWriteHdr(pLog->pSummary, &pLog->hdr);

  /* TODO: If a crash occurs and the current log is copied into the 
  ** database there is no problem. However, if a crash occurs while
  ** writing the next transaction into the start of the log, such that:
  **
  **   * The first transaction currently in the log is left intact, but
  **   * The second (or subsequent) transaction is damaged,
................................................................................
  ** then the database could become corrupt.
  **
  ** The easiest thing to do would be to write and sync a dummy header
  ** into the log at this point. Unfortunately, that turns out to be
  ** an unwelcome performance hit. Alternatives are...
  */
#if 0 
  memset(zBuf, 0, LOG_FRAME_HDRSIZE);
  rc = sqlite3OsWrite(pLog->pFd, zBuf, LOG_FRAME_HDRSIZE, 0);
  if( rc!=SQLITE_OK ) goto out;
  rc = sqlite3OsSync(pLog->pFd, pLog->sync_flags);
#endif

 out:
  logIteratorFree(pIter);
  return rc;
}

/*
** Close a connection to a log file.
*/
int sqlite3WalClose(
  Log *pLog,                      /* Log to close */
  sqlite3_file *pFd,              /* Database file */
  int sync_flags,                 /* Flags to pass to OsSync() (or 0) */
  u8 *zBuf                        /* Buffer of at least page-size bytes */
){
  int rc = SQLITE_OK;
  if( pLog ){
    LogLock **ppL;
    LogSummary *pSummary = pLog->pSummary;
    sqlite3_mutex *mutex = 0;

    sqlite3_mutex_enter(pSummary->mutex);
    for(ppL=&pSummary->pLock; *ppL!=&pLog->lock; ppL=&(*ppL)->pNext);
    *ppL = pLog->lock.pNext;
    sqlite3_mutex_leave(pSummary->mutex);

    if( sqlite3GlobalConfig.bCoreMutex ){
      mutex = sqlite3_mutex_alloc(LOG_SUMMARY_MUTEX);
    }
    sqlite3_mutex_enter(mutex);

    /* Decrement the reference count on the log summary. If this is the last
    ** reference to the log summary object in this process, the object will
    ** be freed. If this is also the last connection to the database, then
    ** checkpoint the database and truncate the log and log-summary files
    ** to zero bytes in size.
    **/
    pSummary->nRef--;
    if( pSummary->nRef==0 ){
      int rc;
      LogSummary **pp;
      for(pp=&pLogSummary; *pp!=pSummary; pp=&(*pp)->pNext);
      *pp = (*pp)->pNext;

      sqlite3_mutex_leave(mutex);

      rc = sqlite3OsLock(pFd, SQLITE_LOCK_EXCLUSIVE);
      if( rc==SQLITE_OK ){

        /* This is the last connection to the database (including other
        ** processes). Do three things:
        **
        **   1. Checkpoint the db.
        **   2. Truncate the log file.
        **   3. Unlink the log-summary file.
        */
        rc = logCheckpoint(pLog, pFd, sync_flags, zBuf);
        if( rc==SQLITE_OK ){
          rc = sqlite3OsDelete(pLog->pVfs, pSummary->zPath, 0);
        }

        logSummaryUnmap(pSummary, 1);
      }else{
        if( rc==SQLITE_BUSY ){
          rc = SQLITE_OK;
        }
        logSummaryUnmap(pSummary, 0);
      }

      sqlite3_mutex_free(pSummary->mutex);
      sqlite3_free(pSummary);
    }else{
      sqlite3_mutex_leave(mutex);
    }

    /* Close the connection to the log file and free the Log handle. */
    sqlite3OsClose(pLog->pFd);
    sqlite3_free(pLog);
  }
  return rc;
}

/*
** Enter and leave the log-summary mutex. In this context, entering the
** log-summary mutex means:
**
**   1. Obtaining mutex pLog->pSummary->mutex, and
**   2. Taking an exclusive lock on the log-summary file.
**
** i.e. this mutex locks out other processes as well as other threads
** hosted in this address space.
*/
static int logEnterMutex(Log *pLog){
  LogSummary *pSummary = pLog->pSummary;
  int rc;

  sqlite3_mutex_enter(pSummary->mutex);
  rc = logLockMutex(pSummary, LOG_WRLOCKW);
  if( rc!=SQLITE_OK ){
    sqlite3_mutex_leave(pSummary->mutex);
  }
  return rc;
}
static void logLeaveMutex(Log *pLog){
  LogSummary *pSummary = pLog->pSummary;
  logLockMutex(pSummary, LOG_UNLOCK);
  sqlite3_mutex_leave(pSummary->mutex);
}

/*
** Try to read the log-summary header. Attempt to verify the header
** checksum. If the checksum can be verified, copy the log-summary
** header into structure pLog->hdr. If the contents of pLog->hdr are
** modified by this and pChanged is not NULL, set *pChanged to 1. 
** Otherwise leave *pChanged unmodified.
**
** If the checksum cannot be verified return SQLITE_ERROR.
*/
int logSummaryTryHdr(Log *pLog, int *pChanged){
  u32 aCksum[2] = {1, 1};
  u32 aHdr[LOGSUMMARY_HDR_NFIELD+2];

  /* Read the header. The caller may or may not have locked the log-summary
  ** file, meaning it is possible that an inconsistent snapshot is read
  ** from the file. If this happens, return SQLITE_ERROR. The caller will
  ** retry. Or, if the caller has already locked the file and the header
  ** still looks inconsistent, it will run recovery.
  */
  memcpy(aHdr, pLog->pSummary->aData, sizeof(aHdr));
  logChecksumBytes((u8*)aHdr, sizeof(u32)*LOGSUMMARY_HDR_NFIELD, aCksum);
  if( aCksum[0]!=aHdr[LOGSUMMARY_HDR_NFIELD]
   || aCksum[1]!=aHdr[LOGSUMMARY_HDR_NFIELD+1]
  ){
    return SQLITE_ERROR;
  }

  if( memcmp(&pLog->hdr, aHdr, sizeof(LogSummaryHdr)) ){
    if( pChanged ){
      *pChanged = 1;
    }
    memcpy(&pLog->hdr, aHdr, sizeof(LogSummaryHdr));
  }
  return SQLITE_OK;
}

/*
** Read the log-summary header from the log-summary file into structure 
** pLog->hdr. If attempting to verify the header checksum fails, try
** to recover the log before returning.
**
** If the log-summary header is successfully read, return SQLITE_OK. 
** Otherwise an SQLite error code.
*/
int logSummaryReadHdr(Log *pLog, int *pChanged){
  int rc;




  /* First try to read the header without a lock. Verify the checksum
  ** before returning. This will almost always work.  
  **
  ** TODO: Doing this causes a race-condition with the code that resizes
  ** the mapping. Unless Log.pSummary->mutex is held, it is possible that 
  ** LogSummary.aData is invalid.
  */
#if 0
  if( SQLITE_OK==logSummaryTryHdr(pLog, pChanged) ){

    return SQLITE_OK;
  }
#endif

  /* If the first attempt to read the header failed, lock the log-summary
  ** file and try again. If the header checksum verification fails this
  ** time as well, run log recovery.
  */
  if( SQLITE_OK==(rc = logEnterMutex(pLog)) ){
    if( SQLITE_OK!=logSummaryTryHdr(pLog, pChanged) ){
      if( pChanged ){
        *pChanged = 1;
      }
      rc = logSummaryRecover(pLog->pSummary, pLog->pFd);
      if( rc==SQLITE_OK ){
        rc = logSummaryTryHdr(pLog, 0);
      }
    }
    logLeaveMutex(pLog);
  }

  return rc;
}

/*
** Lock a snapshot.
**
** If this call obtains a new read-lock and the database contents have been
** modified since the most recent call to LogCloseSnapshot() on this Log
** connection, then *pChanged is set to 1 before returning. Otherwise, it 
** is left unmodified. This is used by the pager layer to determine whether 
** or not any cached pages may be safely reused.
*/
int sqlite3WalOpenSnapshot(Log *pLog, int *pChanged){
  int rc = SQLITE_OK;
  if( pLog->isLocked==0 ){
    int nAttempt;

    /* Obtain a snapshot-lock on the log-summary file. The procedure
    ** for obtaining the snapshot log is:
    **
    **    1. Attempt a SHARED lock on regions A and B.
    **    2a. If step 1 is successful, drop the lock on region B.
    **    2b. If step 1 is unsuccessful, attempt a SHARED lock on region D.
    **    3. Repeat the above until the lock attempt in step 1 or 2b is 
    **       successful.
    **
    ** If neither of the locks can be obtained after 5 tries, presumably
    ** something is wrong (i.e. a process not following the locking protocol). 
    ** Return an error code in this case.
    */
    rc = SQLITE_BUSY;
    for(nAttempt=0; nAttempt<5 && rc==SQLITE_BUSY; nAttempt++){
      rc = logLockRegion(pLog, LOG_REGION_A|LOG_REGION_B, LOG_RDLOCK);

      if( rc==SQLITE_BUSY ){
        rc = logLockRegion(pLog, LOG_REGION_D, LOG_RDLOCK);
        if( rc==SQLITE_OK ) pLog->isLocked = LOG_REGION_D;
      }else{
        logLockRegion(pLog, LOG_REGION_B, LOG_UNLOCK);
        pLog->isLocked = LOG_REGION_A;

      }
    }

    if( rc!=SQLITE_OK ){
      return rc;
    }

    rc = logSummaryReadHdr(pLog, pChanged);
    if( rc!=SQLITE_OK ){
      /* An error occured while attempting log recovery. */
      sqlite3WalCloseSnapshot(pLog);
    }else{
      /* Check if the mapping needs to grow. */
      LogSummary *pSummary = pLog->pSummary;

      if( pLog->hdr.iLastPg 
       && logSummaryEntry(pLog->hdr.iLastPg)>=pSummary->nData 

      ){
        rc = logSummaryRemap(pSummary, 0);
        assert( rc || logSummaryEntry(pLog->hdr.iLastPg)<pSummary->nData );


      }
    }
  }
  return rc;
}

/*
** Unlock the current snapshot.
*/
void sqlite3WalCloseSnapshot(Log *pLog){
  if( pLog->isLocked ){
    assert( pLog->isLocked==LOG_REGION_A || pLog->isLocked==LOG_REGION_D );
    logLockRegion(pLog, pLog->isLocked, LOG_UNLOCK);
  }
  pLog->isLocked = 0;
}

/*
** Read a page from the log, if it is present. 
*/
int sqlite3WalRead(Log *pLog, Pgno pgno, int *pInLog, u8 *pOut){
  LogSummary *pSummary = pLog->pSummary;
  u32 iRead = 0;
  u32 *aData; 
  int iFrame = (pLog->hdr.iLastPg & 0xFFFFFF00);


  assert( pLog->isLocked );
  sqlite3_mutex_enter(pSummary->mutex);

  /* Do a linear search of the unindexed block of page-numbers (if any) 
  ** at the end of the log-summary. An alternative to this would be to
  ** build an index in private memory each time a read transaction is
  ** opened on a new snapshot.
  */
  aData = pSummary->aData;
  if( pLog->hdr.iLastPg ){
    u32 *pi = &aData[logSummaryEntry(pLog->hdr.iLastPg)];
    u32 *piStop = pi - (pLog->hdr.iLastPg & 0xFF);
    while( *pi!=pgno && pi!=piStop ) pi--;
    if( pi!=piStop ){
      iRead = (pi-piStop) + iFrame;
    }
  }
  assert( iRead==0 || aData[logSummaryEntry(iRead)]==pgno );

  while( iRead==0 && iFrame>0 ){
    int iLow = 0;
    int iHigh = 255;
    u32 *aFrame;
    u8 *aIndex;

    iFrame -= 256;
    aFrame = &aData[logSummaryEntry(iFrame+1)];
    aIndex = (u8 *)&aFrame[256];

    while( iLow<=iHigh ){
      int iTest = (iLow+iHigh)>>1;
      u32 iPg = aFrame[aIndex[iTest]];

      if( iPg==pgno ){
................................................................................
      else if( iPg<pgno ){
        iLow = iTest+1;
      }else{
        iHigh = iTest-1;
      }
    }
  }
  assert( iRead==0 || aData[logSummaryEntry(iRead)]==pgno );

  sqlite3_mutex_leave(pLog->pSummary->mutex);

  /* If iRead is non-zero, then it is the log frame number that contains the
  ** required page. Read and return data from the log file.
  */
  if( iRead ){
    i64 iOffset = logFrameOffset(iRead, pLog->hdr.pgsz) + LOG_FRAME_HDRSIZE;
    *pInLog = 1;
    return sqlite3OsRead(pLog->pFd, pOut, pLog->hdr.pgsz, iOffset);
  }

  *pInLog = 0;
  return SQLITE_OK;
}


/* 
** Set *pPgno to the size of the database file (or zero, if unknown).
*/
void sqlite3WalDbsize(Log *pLog, Pgno *pPgno){
  assert( pLog->isLocked );

  *pPgno = pLog->hdr.nPage;
}

/* 
** This function returns SQLITE_OK if the caller may write to the database.
** Otherwise, if the caller is operating on a snapshot that has already
** been overwritten by another writer, SQLITE_BUSY is returned.
*/
int sqlite3WalWriteLock(Log *pLog, int op){
  assert( pLog->isLocked );
  if( op ){





    /* Obtain the writer lock */
    int rc = logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_WRLOCK);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* If this is connection is a region D reader, then the SHARED lock on 
    ** region D has just been upgraded to EXCLUSIVE. But no lock at all is 
    ** held on region A. This means that if the write-transaction is committed
    ** and this connection downgrades to a reader, it will be left with no
    ** lock at all. And so its snapshot could get clobbered by a checkpoint
    ** operation. 
    **
    ** To stop this from happening, grab a SHARED lock on region A now.
    ** This should always be successful, as the only time a client holds
    ** an EXCLUSIVE lock on region A, it must also be holding an EXCLUSIVE
    ** lock on region C (a checkpointer does this). This is not possible,
    ** as this connection currently has the EXCLUSIVE lock on region C.
    */
    if( pLog->isLocked==LOG_REGION_D ){
      logLockRegion(pLog, LOG_REGION_A, LOG_RDLOCK);
      pLog->isLocked = LOG_REGION_A;
    }

    /* If this connection is not reading the most recent database snapshot,
    ** it is not possible to write to the database. In this case release
    ** the write locks and return SQLITE_BUSY.
    */
    if( memcmp(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr)) ){
      logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_UNLOCK);
      return SQLITE_BUSY;
    }
    pLog->isWriteLocked = 1;

  }else if( pLog->isWriteLocked ){
    logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_UNLOCK);
    memcpy(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr));
    pLog->isWriteLocked = 0;
  }
  return SQLITE_OK;
}

/*
** The log handle passed to this function must be holding the write-lock.
**
** 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 log 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(Log *pLog, int (*xUndo)(void *, Pgno), void *pUndoCtx){
  int rc = SQLITE_OK;
  Pgno iMax = pLog->hdr.iLastPg;
  Pgno iFrame;

  assert( pLog->isWriteLocked );
  logSummaryReadHdr(pLog, 0);
  for(iFrame=pLog->hdr.iLastPg+1; iFrame<=iMax && rc==SQLITE_OK; iFrame++){
    rc = xUndo(pUndoCtx, pLog->pSummary->aData[logSummaryEntry(iFrame)]);
  }

  return rc;
}




u32 sqlite3WalSavepoint(Log *pLog){
  assert( pLog->isWriteLocked );

  return pLog->hdr.iLastPg;
}




int sqlite3WalSavepointUndo(Log *pLog, u32 iFrame){
  int rc = SQLITE_OK;
  u8 aCksum[8];
  assert( pLog->isWriteLocked );


  pLog->hdr.iLastPg = iFrame;
  if( iFrame>0 ){
    i64 iOffset = logFrameOffset(iFrame, pLog->hdr.pgsz) + sizeof(u32)*2;
    rc = sqlite3OsRead(pLog->pFd, aCksum, sizeof(aCksum), iOffset);
    pLog->hdr.iCheck1 = sqlite3Get4byte(&aCksum[0]);
    pLog->hdr.iCheck2 = sqlite3Get4byte(&aCksum[4]);
  }

  return rc;
}

/* 
** Return true if data has been written but not committed to the log file. 
*/
int sqlite3WalDirty(Log *pLog){
  assert( pLog->isWriteLocked );
  return( pLog->hdr.iLastPg!=((LogSummaryHdr*)pLog->pSummary->aData)->iLastPg );
}

/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalWriteLock()).
*/
int sqlite3WalFrames(
  Log *pLog,                      /* Log handle to write to */
  int nPgsz,                      /* 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 */
  u8 aFrame[LOG_FRAME_HDRSIZE];   /* Buffer to assemble frame-header in */
  PgHdr *p;                       /* Iterator to run through pList with. */
  u32 aCksum[2];                  /* Checksums */
  PgHdr *pLast;                   /* Last frame in list */
  int nLast = 0;                  /* Number of extra copies of last page */

  assert( LOG_FRAME_HDRSIZE==(4 * 2 + 2*sizeof(u32)) );
  assert( pList );


  /* If this is the first frame written into the log, write the log 
  ** header to the start of the log file. See comments at the top of
  ** this file for a description of the log-header format.
  */
  assert( LOG_FRAME_HDRSIZE>=LOG_HDRSIZE );
  iFrame = pLog->hdr.iLastPg;
  if( iFrame==0 ){
    sqlite3Put4byte(aFrame, nPgsz);
    sqlite3_randomness(8, &aFrame[4]);
    pLog->hdr.iCheck1 = sqlite3Get4byte(&aFrame[4]);
    pLog->hdr.iCheck2 = sqlite3Get4byte(&aFrame[8]);
    rc = sqlite3OsWrite(pLog->pFd, aFrame, LOG_HDRSIZE, 0);
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  aCksum[0] = pLog->hdr.iCheck1;
  aCksum[1] = pLog->hdr.iCheck2;

  /* Write the log file. */
  for(p=pList; p; p=p->pDirty){
    u32 nDbsize;                  /* Db-size field for frame header */
    i64 iOffset;                  /* Write offset in log file */

    iOffset = logFrameOffset(++iFrame, nPgsz);
    
    /* Populate and write the frame header */
    nDbsize = (isCommit && p->pDirty==0) ? nTruncate : 0;
    logEncodeFrame(aCksum, p->pgno, nDbsize, nPgsz, p->pData, aFrame);
    rc = sqlite3OsWrite(pLog->pFd, aFrame, sizeof(aFrame), iOffset);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* Write the page data */
    rc = sqlite3OsWrite(pLog->pFd, p->pData, nPgsz, iOffset + sizeof(aFrame));
    if( rc!=SQLITE_OK ){
      return rc;
    }
    pLast = p;
  }

  /* Sync the log file if the 'isSync' flag was specified. */
  if( sync_flags ){
    i64 iSegment = sqlite3OsSectorSize(pLog->pFd);
    i64 iOffset = logFrameOffset(iFrame+1, nPgsz);

    assert( isCommit );

    if( iSegment<SQLITE_DEFAULT_SECTOR_SIZE ){
      iSegment = SQLITE_DEFAULT_SECTOR_SIZE;
    }
    iSegment = (((iOffset+iSegment-1)/iSegment) * iSegment);
    while( iOffset<iSegment ){
      logEncodeFrame(aCksum,pLast->pgno,nTruncate,nPgsz,pLast->pData,aFrame);
      rc = sqlite3OsWrite(pLog->pFd, aFrame, sizeof(aFrame), iOffset);
      if( rc!=SQLITE_OK ){
        return rc;
      }

      iOffset += LOG_FRAME_HDRSIZE;
      rc = sqlite3OsWrite(pLog->pFd, pLast->pData, nPgsz, iOffset); 
      if( rc!=SQLITE_OK ){
        return rc;
      }
      nLast++;
      iOffset += nPgsz;
    }

    rc = sqlite3OsSync(pLog->pFd, sync_flags);
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  /* Append data to the log summary. It is not necessary to lock the 
  ** log-summary to do this as the RESERVED lock held on the db file
  ** guarantees that there are no other writers, and no data that may
  ** be in use by existing readers is being overwritten.
  */
  iFrame = pLog->hdr.iLastPg;
  for(p=pList; p; p=p->pDirty){
    iFrame++;
    logSummaryAppend(pLog->pSummary, iFrame, p->pgno);
  }
  while( nLast>0 ){
    iFrame++;
    nLast--;
    logSummaryAppend(pLog->pSummary, iFrame, pLast->pgno);
  }

  /* Update the private copy of the header. */
  pLog->hdr.pgsz = nPgsz;
  pLog->hdr.iLastPg = iFrame;
  if( isCommit ){
    pLog->hdr.iChange++;
    pLog->hdr.nPage = nTruncate;
  }
  pLog->hdr.iCheck1 = aCksum[0];
  pLog->hdr.iCheck2 = aCksum[1];

  /* If this is a commit, update the log-summary header too. */
  if( isCommit && SQLITE_OK==(rc = logEnterMutex(pLog)) ){
    logSummaryWriteHdr(pLog->pSummary, &pLog->hdr);
    logLeaveMutex(pLog);
    pLog->iCallback = iFrame;
  }


  return rc;
}

/* 
** Checkpoint the database:
**
**   1. Wait for an EXCLUSIVE lock on regions B and C.
**   2. Wait for an EXCLUSIVE lock on region A.
**   3. Copy the contents of the log into the database file.
**   4. Zero the log-summary header (so new readers will ignore the log).
**   5. Drop the locks obtained in steps 1 and 2.

*/
int sqlite3WalCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  int sync_flags,                 /* Flags to sync db file with (or 0) */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
){
  int rc;                         /* Return code */
  int isChanged = 0;              /* True if a new wal-index header is loaded */

  assert( !pLog->isLocked );

  /* Wait for an EXCLUSIVE lock on regions B and C. */
  do {
    rc = logLockRegion(pLog, LOG_REGION_B|LOG_REGION_C, LOG_WRLOCK);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );
  if( rc!=SQLITE_OK ) return rc;

  /* Wait for an EXCLUSIVE lock on region A. */
  do {
    rc = logLockRegion(pLog, LOG_REGION_A, LOG_WRLOCK);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );
  if( rc!=SQLITE_OK ){
    logLockRegion(pLog, LOG_REGION_B|LOG_REGION_C, LOG_UNLOCK);

    return rc;
  }

  /* Copy data from the log to the database file. */
  rc = logSummaryReadHdr(pLog, &isChanged);
  if( rc==SQLITE_OK ){
    rc = logCheckpoint(pLog, pFd, sync_flags, zBuf);
  }
  if( isChanged ){
    /* If a new wal-index header was loaded before the checkpoint was 
    ** performed, then the pager-cache associated with log pLog 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(&pLog->hdr, 0, sizeof(LogSummaryHdr));
  }

  /* Release the locks. */
  logLockRegion(pLog, LOG_REGION_A|LOG_REGION_B|LOG_REGION_C, LOG_UNLOCK);
  return rc;
}






int sqlite3WalCallback(Log *pLog){
  u32 ret = 0;
  if( pLog ){
    ret = pLog->iCallback;
    pLog->iCallback = 0;
  }
  return (int)ret;
}
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/*
** 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 file used in 
** "journal_mode=wal" mode.
*/
#include "wal.h"


/*
** WRITE-AHEAD LOG (WAL) FILE FORMAT
**
** A wal file consists of a header followed by zero or more "frames".
** The header is 12 bytes in size and consists of the following three
** big-endian 32-bit unsigned integer values:
**
**     0: Database page size,
**     4: Randomly selected salt value 1,
**     8: Randomly selected salt value 2.
**
** Immediately following the header are zero or more frames. Each
** frame itself consists of a 16-byte header followed by a <page-size> bytes
** of page data. The header is broken into 4 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: Checksum value 1.
**    12: Checksum value 2.
*/

/* 
** WAL-INDEX FILE FORMAT
**
** The wal-index file consists of a 32-byte header region, followed by an 
** 8-byte region that contains no useful data (used to apply byte-range locks
** to), followed by the data region. 
**
** The contents of both the header and data region are specified in terms
** of 1, 2 and 4 byte unsigned integers. All integers are stored in 
** machine-endian order.  The wal-index is not a persistent file and
** so it does not need to be portable across archtectures.
**
** A wal-index file is essentially a shadow-pager map. It contains a
** mapping from database page number to the set of locations in the wal
** file that contain versions of the database page. When a database 
** client needs to read a page of data, it first queries the wal-index
** file to determine if the required version of the page is stored in

** the wal. If so, the page is read from the wal. If not, the page is
** read from the database file.
**
** Whenever a transaction is appended to the wal or a checkpoint transfers
** data from the wal into the database file, the wal-index is 
** updated accordingly.
**
** The fields in the wal-index file header are described in the comment 
** directly above the definition of struct WalIndexHdr (see below). 
** Immediately following the fields in the WalIndexHdr structure is
** an 8 byte checksum based on the contents of the header. This field is
** not the same as the iCheck1 and iCheck2 fields of the WalIndexHdr.
*/








/* Object declarations */
typedef struct WalIndexHdr WalIndexHdr;

typedef struct WalIterator WalIterator;


/*
** The following object stores a copy of the wal-index header.

**
** Member variables iCheck1 and iCheck2 contain the checksum for the
** last frame written to the wal, or 2 and 3 respectively if the log 
** is currently empty.
*/

struct WalIndexHdr {
  u32 iChange;                    /* Counter incremented each transaction */
  u32 pgsz;                       /* Database page size in bytes */
  u32 iLastPg;                    /* Address of last valid frame in log */
  u32 nPage;                      /* Size of database in pages */
  u32 iCheck1;                    /* Checkpoint value 1 */
  u32 iCheck2;                    /* Checkpoint value 2 */
};

/* Size of serialized WalIndexHdr object. */
#define WALINDEX_HDR_NFIELD (sizeof(WalIndexHdr) / sizeof(u32))

/* A block of 16 bytes beginning at WALINDEX_LOCK_OFFSET is reserved
** for locks. Since some systems only feature 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*sizeof(u32))
#define WALINDEX_LOCK_RESERVED 8

/* Size of header before each frame in wal */
#define WAL_FRAME_HDRSIZE 16

/* Size of write ahead log header */
#define WAL_HDRSIZE 12

/*
** Return the offset of frame iFrame in the write-ahead log file, 
** assuming a database page size of pgsz bytes. The offset returned
** is to the start of the write-ahead log frame-header.
*/
#define walFrameOffset(iFrame, pgsz) (                               \
  WAL_HDRSIZE + ((iFrame)-1)*((pgsz)+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 pFd */
  sqlite3_file *pFd;         /* File handle for WAL file */


  u32 iCallback;             /* Value to pass to log callback (or 0) */


  sqlite3_shm *pWIndex;      /* The open wal-index file */
  int szWIndex;              /* Size of the wal-index */
  u32 *pWiData;              /* Pointer to wal-index content in memory */
  u8 lockState;              /* SQLITE_SHM_xxxx constant showing lock state */
  u8 readerType;             /* SQLITE_SHM_READ or SQLITE_SHM_READ_FULL */
  WalIndexHdr hdr;           /* Wal-index for current snapshot */
};


/*
** This structure is used to implement an iterator that iterates through
** all frames in the log 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 log.
**
** 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 nSegment;                   /* Size of WalIterator.aSegment[] array */
  int nFinal;                     /* Elements in segment nSegment-1 */
  struct WalSegment {
    int iNext;                    /* Next aIndex index */
    u8 *aIndex;                   /* Pointer to index array */
    u32 *aDbPage;                 /* Pointer to db page array */
  } aSegment[1];
};











/*
** Generate an 8 byte checksum based on the data in array aByte[] and the
** initial values of aCksum[0] and aCksum[1]. The checksum is written into
** aCksum[] before returning.
**
** The range of bytes to checksum is treated as an array of 32-bit 
** little-endian unsigned integers. For each integer X in the array, from
................................................................................
**
** For the calculation above, use 64-bit unsigned accumulators. Before
** returning, truncate the values to 32-bits as follows: 
**
**   aCksum[0] = (u32)(aCksum[0] + (aCksum[0]>>24));
**   aCksum[1] = (u32)(aCksum[1] + (aCksum[1]>>24));
*/
static void walChecksumBytes(u8 *aByte, int nByte, u32 *aCksum){
  u64 sum1 = aCksum[0];
  u64 sum2 = aCksum[1];
  u32 *a32 = (u32 *)aByte;
  u32 *aEnd = (u32 *)&aByte[nByte];

  assert( (nByte&0x00000003)==0 );

................................................................................
  }

  aCksum[0] = sum1 + (sum1>>24);
  aCksum[1] = sum2 + (sum2>>24);
}

/*
** Attempt to change the lock status.


**
** When changing the lock status to SQLITE_SHM_READ, store the
** type of reader lock (either SQLITE_SHM_READ or SQLITE_SHM_READ_FULL)
** in pWal->readerType.




*/

static int walSetLock(Wal *pWal, int desiredStatus){
  int rc, got;
  if( pWal->lockState==desiredStatus ) return SQLITE_OK;
  rc = pWal->pVfs->xShmLock(pWal->pWIndex, desiredStatus, &got);
  if( rc==SQLITE_OK ){
    pWal->lockState = desiredStatus;
    if( desiredStatus==SQLITE_SHM_READ ){
      pWal->readerType = got;
    }







  }





  return rc;
}






/*






** Update the header of the wal-index file.
*/























static void walIndexWriteHdr(Wal *pWal, WalIndexHdr *pHdr){
  u32 *aHdr = pWal->pWiData;                   /* Write header here */
  u32 *aCksum = &aHdr[WALINDEX_HDR_NFIELD];    /* Write header cksum here */

  assert( WALINDEX_HDR_NFIELD==sizeof(WalIndexHdr)/4 );
  assert( aHdr!=0 );
  memcpy(aHdr, pHdr, sizeof(WalIndexHdr));
  aCksum[0] = aCksum[1] = 1;
  walChecksumBytes((u8 *)aHdr, sizeof(WalIndexHdr), aCksum);
}

/*
** 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: Database page size in bytes.
**     4: Page number.
**     8: New database size (for commit frames, otherwise zero).
**    12: Frame checksum 1.
**    16: Frame checksum 2.
*/
static void walEncodeFrame(
  u32 *aCksum,                    /* IN/OUT: Checksum values */
  u32 iPage,                      /* Database page number for frame */
  u32 nTruncate,                  /* New db size (or 0 for non-commit frames) */
  int nData,                      /* Database page size (size of aData[]) */
  u8 *aData,                      /* Pointer to page data (for checksum) */
  u8 *aFrame                      /* OUT: Write encoded frame here */
){
  assert( WAL_FRAME_HDRSIZE==16 );

  sqlite3Put4byte(&aFrame[0], iPage);
  sqlite3Put4byte(&aFrame[4], nTruncate);

  walChecksumBytes(aFrame, 8, aCksum);
  walChecksumBytes(aData, nData, aCksum);

  sqlite3Put4byte(&aFrame[8], aCksum[0]);
  sqlite3Put4byte(&aFrame[12], aCksum[1]);
}

/*
** Return 1 and populate *piPage, *pnTruncate and aCksum if the 
** frame checksum looks Ok. Otherwise return 0.
*/
static int walDecodeFrame(
  u32 *aCksum,                    /* IN/OUT: Checksum values */
  u32 *piPage,                    /* OUT: Database page number for frame */
  u32 *pnTruncate,                /* OUT: New db size (or 0 if not commit) */
  int nData,                      /* Database page size (size of aData[]) */
  u8 *aData,                      /* Pointer to page data (for checksum) */
  u8 *aFrame                      /* Frame data */
){
  assert( WAL_FRAME_HDRSIZE==16 );

  walChecksumBytes(aFrame, 8, aCksum);
  walChecksumBytes(aData, nData, aCksum);

  if( aCksum[0]!=sqlite3Get4byte(&aFrame[8]) 
   || aCksum[1]!=sqlite3Get4byte(&aFrame[12]) 
  ){
    /* Checksum failed. */
    return 0;
  }

  *piPage = sqlite3Get4byte(&aFrame[0]);
  *pnTruncate = sqlite3Get4byte(&aFrame[4]);
  return 1;
}

static void walMergesort8(
  Pgno *aContent,                 /* Pages in wal */
  u8 *aBuffer,                    /* Buffer of at least *pnList items to use */
  u8 *aList,                      /* IN/OUT: List to sort */
  int *pnList                     /* IN/OUT: Number of elements in aList[] */
){
  int nList = *pnList;
  if( nList>1 ){
    int nLeft = nList / 2;        /* Elements in left list */
................................................................................
    u8 *aLeft = aList;            /* Left list */
    u8 *aRight = &aList[nLeft];   /* Right list */
    int iLeft = 0;                /* Current index in aLeft */
    int iRight = 0;               /* Current index in aright */
    int iOut = 0;                 /* Current index in output buffer */

    /* TODO: Change to non-recursive version. */
    walMergesort8(aContent, aBuffer, aLeft, &nLeft);
    walMergesort8(aContent, aBuffer, aRight, &nRight);

    while( iRight<nRight || iLeft<nLeft ){
      u8 logpage;
      Pgno dbpage;

      if( (iLeft<nLeft) 
       && (iRight>=nRight || aContent[aLeft[iLeft]]<aContent[aRight[iRight]])
................................................................................
    }
  }
#endif
}


/*






















































** Return the index in the WalIndex.aData array that corresponds to 
** frame iFrame. The wal-index file consists of a header, followed by
** alternating "map" and "index" blocks.
*/
static int walIndexEntry(u32 iFrame){
  return (

      (WALINDEX_LOCK_OFFSET+WALINDEX_LOCK_RESERVED)/sizeof(u32)
    + (((iFrame-1)>>8)<<6)        /* Indexes that occur before iFrame */
    + iFrame-1                    /* Db page numbers that occur before iFrame */
  );
}

/*
** Release our reference to the wal-index memory map.
*/
static void walIndexUnmap(Wal *pWal){
  if( pWal->pWiData ){
    pWal->pVfs->xShmRelease(pWal->pWIndex);
    pWal->pWiData = 0;
  }
}

/*
** Map the wal-index file into memory if it isn't already.
*/
static int walIndexMap(Wal *pWal){
  int rc = SQLITE_OK;
  if( pWal->pWiData==0 ){
    rc = pWal->pVfs->xShmSize(pWal->pWIndex, -1,
                              &pWal->szWIndex, (void**)(char*)&pWal->pWiData);
  }
  return rc;
}

/*
** Resize the wal-index file.
*/
static int walIndexRemap(Wal *pWal, int newSize){
  int rc;
  walIndexUnmap(pWal);
  rc = pWal->pVfs->xShmSize(pWal->pWIndex, newSize,
                            &pWal->szWIndex, (void**)(char*)&pWal->pWiData);
  return rc;
}

/*
** Increment by which to increase the wal-index file size.
*/
#define WALINDEX_MMAP_INCREMENT (64*1024)

/*
** Set an entry in the wal-index map to map log frame iFrame to db 
** page iPage. Values are always appended to the wal-index (i.e. the
** value of iFrame is always exactly one more than the value passed to
** the previous call), but that restriction is not enforced or asserted
** here.
*/
static int walIndexAppend(Wal *pWal, u32 iFrame, u32 iPage){
  u32 iSlot = walIndexEntry(iFrame);
  
  walIndexMap(pWal);
  while( (iSlot+128)>=pWal->szWIndex ){
    int rc;

    int nByte = pWal->szWIndex*4 + WALINDEX_MMAP_INCREMENT;

    /* Unmap and remap the wal-index file. */

    rc = walIndexRemap(pWal, nByte);
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  /* Set the wal-index entry itself */
  pWal->pWiData[iSlot] = iPage;

  /* If the frame number is a multiple of 256 (frames are numbered starting
  ** at 1), build an index of the most recently added 256 frames.
  */
  if( (iFrame&0x000000FF)==0 ){
    int i;                        /* Iterator used while initializing aIndex */
    u32 *aFrame;                  /* Pointer to array of 256 frames */
    int nIndex;                   /* Number of entries in index */
    u8 *aIndex;                   /* 256 bytes to build index in */
    u8 *aTmp;                     /* Scratch space to use while sorting */

    aFrame = &pWal->pWiData[iSlot-255];
    aIndex = (u8 *)&pWal->pWiData[iSlot+1];
    aTmp = &aIndex[256];

    nIndex = 256;
    for(i=0; i<256; i++) aIndex[i] = (u8)i;
    walMergesort8(aFrame, aTmp, aIndex, &nIndex);
    memset(&aIndex[nIndex], aIndex[nIndex-1], 256-nIndex);
  }

  return SQLITE_OK;
}


/*
** Recover the wal-index by reading the write-ahead log file. 
** The caller must hold RECOVER lock on the wal-index file.
*/
static int walIndexRecover(Wal *pWal){
  int rc;                         /* Return Code */
  i64 nSize;                      /* Size of log file */
  WalIndexHdr hdr;              /* Recovered wal-index header */

  assert( pWal->lockState==SQLITE_SHM_RECOVER );
  memset(&hdr, 0, sizeof(hdr));

  rc = sqlite3OsFileSize(pWal->pFd, &nSize);
  if( rc!=SQLITE_OK ){
    return rc;
  }

  if( nSize>WAL_FRAME_HDRSIZE ){
    u8 aBuf[WAL_FRAME_HDRSIZE];   /* Buffer to load first frame header into */
    u8 *aFrame = 0;               /* Malloc'd buffer to load entire frame */
    int nFrame;                   /* Number of bytes at 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 nPgsz;                    /* Page size according to the log */
    u32 aCksum[2];                /* Running checksum */

    /* Read in the first frame header in the file (to determine the 
    ** database page size).
    */
    rc = sqlite3OsRead(pWal->pFd, aBuf, WAL_HDRSIZE, 0);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* If the database page size is not a power of two, or is greater than
    ** SQLITE_MAX_PAGE_SIZE, conclude that the log file contains no valid data.
    */
................................................................................
    if( nPgsz&(nPgsz-1) || nPgsz>SQLITE_MAX_PAGE_SIZE || nPgsz<512 ){
      goto finished;
    }
    aCksum[0] = sqlite3Get4byte(&aBuf[4]);
    aCksum[1] = sqlite3Get4byte(&aBuf[8]);

    /* Malloc a buffer to read frames into. */
    nFrame = nPgsz + WAL_FRAME_HDRSIZE;
    aFrame = (u8 *)sqlite3_malloc(nFrame);
    if( !aFrame ){
      return SQLITE_NOMEM;
    }
    aData = &aFrame[WAL_FRAME_HDRSIZE];

    /* Read all frames from the log file. */
    iFrame = 0;
    for(iOffset=WAL_HDRSIZE; (iOffset+nFrame)<=nSize; iOffset+=nFrame){
      u32 pgno;                   /* Database page number for frame */
      u32 nTruncate;              /* dbsize field from frame header */
      int isValid;                /* True if this frame is valid */

      /* Read and decode the next log frame. */
      rc = sqlite3OsRead(pWal->pFd, aFrame, nFrame, iOffset);
      if( rc!=SQLITE_OK ) break;
      isValid = walDecodeFrame(aCksum, &pgno, &nTruncate, nPgsz, aData, aFrame);
      if( !isValid ) break;
      walIndexAppend(pWal, ++iFrame, pgno);

      /* If nTruncate is non-zero, this is a commit record. */
      if( nTruncate ){
        hdr.iCheck1 = aCksum[0];
        hdr.iCheck2 = aCksum[1];
        hdr.iLastPg = iFrame;
        hdr.nPage = nTruncate;
................................................................................
    sqlite3_free(aFrame);
  }else{
    hdr.iCheck1 = 2;
    hdr.iCheck2 = 3;
  }

finished:
  walIndexWriteHdr(pWal, &hdr);
  return rc;
}

/* 





































































































































































































































































** Open a connection to the log file associated with database zDb. The
** database file does not actually have to exist. zDb is used only to
** figure out the name of the log file to open. If the log file does not 
** exist it is created by this call.
**
** 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 log 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.
*/
int sqlite3WalOpen(
  sqlite3_vfs *pVfs,              /* vfs module to open wal and wal-index */
  const char *zDb,                /* Name of database file */
  Wal **ppWal                     /* OUT: Allocated Wal handle */
){
  int rc = SQLITE_OK;             /* Return Code */
  Wal *pRet;                      /* Object to allocate and return */


  int flags;                      /* Flags passed to OsOpen() */
  char *zWal = 0;                 /* Path to WAL file */
  int nWal;                       /* Length of zWal in bytes */

  assert( zDb );
  if( pVfs->xShmOpen==0 ) return SQLITE_CANTOPEN;

  /* Allocate an instance of struct Wal to return. */
  *ppWal = 0;
  nWal = strlen(zDb);
  pRet = (Wal*)sqlite3MallocZero(sizeof(Wal) + pVfs->szOsFile + nWal+11);
  if( !pRet ) goto wal_open_out;
  pRet->pVfs = pVfs;
  pRet->pFd = (sqlite3_file *)&pRet[1];


  zWal = pVfs->szOsFile + (char*)pRet->pFd;
  sqlite3_snprintf(nWal, zWal, "%s-wal-index", zDb);
  rc = pVfs->xShmOpen(pVfs, zWal, &pRet->pWIndex);
  if( rc ) goto wal_open_out;






































  /* Open file handle on the write-ahead log file. */
  zWal[nWal-6] = 0;
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_MAIN_JOURNAL);
  rc = sqlite3OsOpen(pVfs, zWal, pRet->pFd, flags, &flags);















wal_open_out:


  if( rc!=SQLITE_OK ){

    if( pRet ){
      pVfs->xShmClose(pRet->pWIndex);
      sqlite3OsClose(pRet->pFd);
      sqlite3_free(pRet);
    }


  }
  *ppWal = pRet;
  return rc;
}

static int walIteratorNext(
  WalIterator *p,               /* Iterator */
  u32 *piPage,                  /* OUT: Next db page to write */
  u32 *piFrame                  /* OUT: Wal frame to read from */
){
  u32 iMin = *piPage;
  u32 iRet = 0xFFFFFFFF;
  int i;
  int nBlock = p->nFinal;

  for(i=p->nSegment-1; i>=0; i--){
    struct WalSegment *pSegment = &p->aSegment[i];
    while( pSegment->iNext<nBlock ){
      u32 iPg = pSegment->aDbPage[pSegment->aIndex[pSegment->iNext]];
      if( iPg>iMin ){
        if( iPg<iRet ){
          iRet = iPg;
          *piFrame = i*256 + 1 + pSegment->aIndex[pSegment->iNext];
        }
................................................................................
    nBlock = 256;
  }

  *piPage = iRet;
  return (iRet==0xFFFFFFFF);
}

static WalIterator *walIteratorInit(Wal *pWal){
  u32 *aData;                     /* Content of the wal-index file */
  WalIterator *p;                 /* Return value */
  int nSegment;                   /* Number of segments to merge */
  u32 iLast;                      /* Last frame in log */
  int nByte;                      /* Number of bytes to allocate */
  int i;                          /* Iterator variable */
  int nFinal;                     /* Number of unindexed entries */
  struct WalSegment *pFinal;      /* Final (unindexed) segment */
  u8 *aTmp;                       /* Temp space used by merge-sort */

  walIndexMap(pWal);
  aData = pWal->pWiData;
  iLast = pWal->hdr.iLastPg;
  nSegment = (iLast >> 8) + 1;
  nFinal = (iLast & 0x000000FF);

  nByte = sizeof(WalIterator) + (nSegment-1)*sizeof(struct WalSegment) + 512;
  p = (WalIterator *)sqlite3_malloc(nByte);
  if( p ){
    memset(p, 0, nByte);
    p->nSegment = nSegment;
    p->nFinal = nFinal;
  }

  for(i=0; i<nSegment-1; i++){
    p->aSegment[i].aDbPage = &aData[walIndexEntry(i*256+1)];
    p->aSegment[i].aIndex = (u8 *)&aData[walIndexEntry(i*256+1)+256];
  }
  pFinal = &p->aSegment[nSegment-1];

  pFinal->aDbPage = &aData[walIndexEntry((nSegment-1)*256+1)];
  pFinal->aIndex = (u8 *)&pFinal[1];
  aTmp = &pFinal->aIndex[256];
  for(i=0; i<nFinal; i++){
    pFinal->aIndex[i] = i;
  }
  walMergesort8(pFinal->aDbPage, aTmp, pFinal->aIndex, &nFinal);
  p->nFinal = nFinal;

  return p;
}

/* 
** Free a log iterator allocated by walIteratorInit().
*/
static void walIteratorFree(WalIterator *p){
  sqlite3_free(p);
}

/*
** Checkpoint the contents of the log file.
*/
static int walCheckpoint(
  Wal *pWal,                      /* Wal connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  int sync_flags,                 /* Flags for OsSync() (or 0) */
  u8 *zBuf                        /* Temporary buffer to use */
){
  int rc;                         /* Return code */
  int pgsz = pWal->hdr.pgsz;      /* 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 */

  if( pWal->hdr.iLastPg==0 ){
    return SQLITE_OK;
  }

  /* Allocate the iterator */
  pIter = walIteratorInit(pWal);
  if( !pIter ) return SQLITE_NOMEM;

  /* Sync the log file to disk */
  if( sync_flags ){
    rc = sqlite3OsSync(pWal->pFd, sync_flags);
    if( rc!=SQLITE_OK ) goto out;
  }

  /* Iterate through the contents of the log, copying data to the db file. */
  while( 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){
    rc = sqlite3OsRead(pWal->pFd, zBuf, pgsz, 
        walFrameOffset(iFrame, pgsz) + WAL_FRAME_HDRSIZE
    );
    if( rc!=SQLITE_OK ) goto out;
    rc = sqlite3OsWrite(pFd, zBuf, pgsz, (iDbpage-1)*pgsz);
    if( rc!=SQLITE_OK ) goto out;
  }

  /* Truncate the database file */
  rc = sqlite3OsTruncate(pFd, ((i64)pWal->hdr.nPage*(i64)pgsz));
  if( rc!=SQLITE_OK ) goto out;

  /* Sync the database file. If successful, update the wal-index. */
  if( sync_flags ){
    rc = sqlite3OsSync(pFd, sync_flags);
    if( rc!=SQLITE_OK ) goto out;
  }
  pWal->hdr.iLastPg = 0;
  pWal->hdr.iCheck1 = 2;
  pWal->hdr.iCheck2 = 3;
  walIndexWriteHdr(pWal, &pWal->hdr);

  /* TODO: If a crash occurs and the current log is copied into the 
  ** database there is no problem. However, if a crash occurs while
  ** writing the next transaction into the start of the log, such that:
  **
  **   * The first transaction currently in the log is left intact, but
  **   * The second (or subsequent) transaction is damaged,
................................................................................
  ** then the database could become corrupt.
  **
  ** The easiest thing to do would be to write and sync a dummy header
  ** into the log at this point. Unfortunately, that turns out to be
  ** an unwelcome performance hit. Alternatives are...
  */
#if 0 
  memset(zBuf, 0, WAL_FRAME_HDRSIZE);
  rc = sqlite3OsWrite(pWal->pFd, zBuf, WAL_FRAME_HDRSIZE, 0);
  if( rc!=SQLITE_OK ) goto out;
  rc = sqlite3OsSync(pWal->pFd, pWal->sync_flags);
#endif

 out:
  walIteratorFree(pIter);
  return rc;
}

/*
** Close a connection to a log file.
*/
int sqlite3WalClose(
  Wal *pWal,                      /* Wal to close */
  sqlite3_file *pFd,              /* Database file */
  int sync_flags,                 /* Flags to pass to OsSync() (or 0) */
  u8 *zBuf                        /* Buffer of at least page-size bytes */
){
  int rc = SQLITE_OK;
  if( pWal ){
    pWal->pVfs->xShmClose(pWal->pWIndex);


























































    sqlite3OsClose(pWal->pFd);
    sqlite3_free(pWal);
  }
  return rc;
}

/*



























** Try to read the wal-index header. Attempt to verify the header
** checksum. If the checksum can be verified, copy the wal-index
** header into structure pWal->hdr. If the contents of pWal->hdr are
** modified by this and pChanged is not NULL, set *pChanged to 1. 
** Otherwise leave *pChanged unmodified.
**
** If the checksum cannot be verified return SQLITE_ERROR.
*/
int walIndexTryHdr(Wal *pWal, int *pChanged){
  u32 aCksum[2] = {1, 1};
  u32 aHdr[WALINDEX_HDR_NFIELD+2];

  /* Read the header. The caller may or may not have locked the wal-index
  ** file, meaning it is possible that an inconsistent snapshot is read
  ** from the file. If this happens, return SQLITE_ERROR. The caller will
  ** retry. Or, if the caller has already locked the file and the header
  ** still looks inconsistent, it will run recovery.
  */
  memcpy(aHdr, pWal->pWiData, sizeof(aHdr));
  walChecksumBytes((u8*)aHdr, sizeof(u32)*WALINDEX_HDR_NFIELD, aCksum);
  if( aCksum[0]!=aHdr[WALINDEX_HDR_NFIELD]
   || aCksum[1]!=aHdr[WALINDEX_HDR_NFIELD+1]
  ){
    return SQLITE_ERROR;
  }

  if( memcmp(&pWal->hdr, aHdr, sizeof(WalIndexHdr)) ){
    if( pChanged ){
      *pChanged = 1;
    }
    memcpy(&pWal->hdr, aHdr, sizeof(WalIndexHdr));
  }
  return SQLITE_OK;
}

/*
** Read the wal-index header from the wal-index file into structure 
** pWal->hdr. If attempting to verify the header checksum fails, try
** to recover the log before returning.
**
** 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;

  assert( pWal->lockState==SQLITE_SHM_READ );
  walIndexMap(pWal);

  /* First try to read the header without a lock. Verify the checksum
  ** before returning. This will almost always work.  




  */


  if( SQLITE_OK==walIndexTryHdr(pWal, pChanged) ){
    return SQLITE_OK;
  }


  /* If the first attempt to read the header failed, lock the wal-index
  ** file and try again. If the header checksum verification fails this
  ** time as well, run log recovery.
  */
  if( SQLITE_OK==(rc = walSetLock(pWal, SQLITE_SHM_RECOVER)) ){
    if( SQLITE_OK!=walIndexTryHdr(pWal, pChanged) ){
      if( pChanged ){
        *pChanged = 1;
      }
      rc = walIndexRecover(pWal);
      if( rc==SQLITE_OK ){
        rc = walIndexTryHdr(pWal, 0);
      }
    }
    walSetLock(pWal, SQLITE_SHM_READ);
  }

  return rc;
}

/*
** Lock a snapshot.
**
** If this call obtains a new read-lock and the database contents have been
** modified since the most recent call to WalCloseSnapshot() on this Wal
** connection, then *pChanged is set to 1 before returning. Otherwise, it 
** is left unmodified. This is used by the pager layer to determine whether 
** or not any cached pages may be safely reused.
*/
int sqlite3WalOpenSnapshot(Wal *pWal, int *pChanged){
  int rc;



















  rc = walSetLock(pWal, SQLITE_SHM_READ);
  if( rc==SQLITE_OK ){





    pWal->lockState = SQLITE_SHM_READ;


    rc = walIndexReadHdr(pWal, pChanged);
    if( rc!=SQLITE_OK ){





      /* An error occured while attempting log recovery. */
      sqlite3WalCloseSnapshot(pWal);
    }else{
      /* Check if the mapping needs to grow. */


     if( pWal->hdr.iLastPg 

      && walIndexEntry(pWal->hdr.iLastPg)>=pWal->szWIndex
     ){


        rc = walIndexRemap(pWal, 0);
        assert( rc || walIndexEntry(pWal->hdr.iLastPg)<pWal->szWIndex );
      }
    }
  }
  return rc;
}

/*
** Unlock the current snapshot.
*/
void sqlite3WalCloseSnapshot(Wal *pWal){
  if( pWal->lockState!=SQLITE_SHM_UNLOCK ){
    assert( pWal->lockState==SQLITE_SHM_READ );
    walSetLock(pWal, SQLITE_SHM_UNLOCK);
  }

}

/*
** Read a page from the log, if it is present. 
*/
int sqlite3WalRead(Wal *pWal, Pgno pgno, int *pInWal, u8 *pOut){

  u32 iRead = 0;
  u32 *aData; 
  int iFrame = (pWal->hdr.iLastPg & 0xFFFFFF00);

  assert( pWal->lockState==SQLITE_SHM_READ );
  walIndexMap(pWal);


  /* Do a linear search of the unindexed block of page-numbers (if any) 
  ** at the end of the wal-index. An alternative to this would be to
  ** build an index in private memory each time a read transaction is
  ** opened on a new snapshot.
  */
  aData = pWal->pWiData;
  if( pWal->hdr.iLastPg ){
    u32 *pi = &aData[walIndexEntry(pWal->hdr.iLastPg)];
    u32 *piStop = pi - (pWal->hdr.iLastPg & 0xFF);
    while( *pi!=pgno && pi!=piStop ) pi--;
    if( pi!=piStop ){
      iRead = (pi-piStop) + iFrame;
    }
  }
  assert( iRead==0 || aData[walIndexEntry(iRead)]==pgno );

  while( iRead==0 && iFrame>0 ){
    int iLow = 0;
    int iHigh = 255;
    u32 *aFrame;
    u8 *aIndex;

    iFrame -= 256;
    aFrame = &aData[walIndexEntry(iFrame+1)];
    aIndex = (u8 *)&aFrame[256];

    while( iLow<=iHigh ){
      int iTest = (iLow+iHigh)>>1;
      u32 iPg = aFrame[aIndex[iTest]];

      if( iPg==pgno ){
................................................................................
      else if( iPg<pgno ){
        iLow = iTest+1;
      }else{
        iHigh = iTest-1;
      }
    }
  }
  assert( iRead==0 || aData[walIndexEntry(iRead)]==pgno );
  walIndexUnmap(pWal);


  /* If iRead is non-zero, then it is the log frame number that contains the
  ** required page. Read and return data from the log file.
  */
  if( iRead ){
    i64 iOffset = walFrameOffset(iRead, pWal->hdr.pgsz) + WAL_FRAME_HDRSIZE;
    *pInWal = 1;
    return sqlite3OsRead(pWal->pFd, pOut, pWal->hdr.pgsz, iOffset);
  }

  *pInWal = 0;
  return SQLITE_OK;
}


/* 
** Set *pPgno to the size of the database file (or zero, if unknown).
*/
void sqlite3WalDbsize(Wal *pWal, Pgno *pPgno){
  assert( pWal->lockState==SQLITE_SHM_READ
       || pWal->lockState==SQLITE_SHM_WRITE );
  *pPgno = pWal->hdr.nPage;
}

/* 
** This function returns SQLITE_OK if the caller may write to the database.
** Otherwise, if the caller is operating on a snapshot that has already
** been overwritten by another writer, SQLITE_BUSY is returned.
*/
int sqlite3WalWriteLock(Wal *pWal, int op){
  int rc;
  if( op ){
    assert( pWal->lockState == SQLITE_SHM_READ );
    rc = walSetLock(pWal, SQLITE_SHM_WRITE);
  }else if( pWal->lockState==SQLITE_SHM_WRITE ){
    rc = walSetLock(pWal, SQLITE_SHM_READ);
  }



  return rc;




































}

/*
** The Wal object passed to this function must be holding the write-lock.
**
** 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 log 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;
  Pgno iMax = pWal->hdr.iLastPg;
  Pgno iFrame;

  assert( pWal->lockState==SQLITE_SHM_WRITE );
  walIndexReadHdr(pWal, 0);
  for(iFrame=pWal->hdr.iLastPg+1; iFrame<=iMax && rc==SQLITE_OK; iFrame++){
    rc = xUndo(pUndoCtx, pWal->pWiData[walIndexEntry(iFrame)]);
  }
  walIndexUnmap(pWal);
  return rc;
}

/* Return an integer that records the current (uncommitted) write
** position in the WAL
*/
u32 sqlite3WalSavepoint(Wal *pWal){

  assert( pWal->lockState==SQLITE_SHM_WRITE );
  return pWal->hdr.iLastPg;
}

/* Move the write position of the WAL back to iFrame.  Called in
** response to a ROLLBACK TO command.
*/
int sqlite3WalSavepointUndo(Wal *pWal, u32 iFrame){
  int rc = SQLITE_OK;
  u8 aCksum[8];

  assert( pWal->lockState==SQLITE_SHM_WRITE );

  pWal->hdr.iLastPg = iFrame;
  if( iFrame>0 ){
    i64 iOffset = walFrameOffset(iFrame, pWal->hdr.pgsz) + sizeof(u32)*2;
    rc = sqlite3OsRead(pWal->pFd, aCksum, sizeof(aCksum), iOffset);
    pWal->hdr.iCheck1 = sqlite3Get4byte(&aCksum[0]);
    pWal->hdr.iCheck2 = sqlite3Get4byte(&aCksum[4]);
  }

  return rc;
}

/* 
** Return true if data has been written but not committed to the log file. 
*/
int sqlite3WalDirty(Wal *pWal){
  assert( pWal->lockState==SQLITE_SHM_WRITE );
  return( pWal->hdr.iLastPg!=((WalIndexHdr*)pWal->pWiData)->iLastPg );
}

/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalWriteLock()).
*/
int sqlite3WalFrames(
  Wal *pWal,                      /* Wal handle to write to */
  int nPgsz,                      /* 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 */
  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-header in */
  PgHdr *p;                       /* Iterator to run through pList with. */
  u32 aCksum[2];                  /* Checksums */
  PgHdr *pLast;                   /* Last frame in list */
  int nLast = 0;                  /* Number of extra copies of last page */

  assert( WAL_FRAME_HDRSIZE==(4 * 2 + 2*sizeof(u32)) );
  assert( pList );
  assert( pWal->lockState==SQLITE_SHM_WRITE );

  /* If this is the first frame written into the log, write the log 
  ** header to the start of the log file. See comments at the top of
  ** this file for a description of the log-header format.
  */
  assert( WAL_FRAME_HDRSIZE>=WAL_HDRSIZE );
  iFrame = pWal->hdr.iLastPg;
  if( iFrame==0 ){
    sqlite3Put4byte(aFrame, nPgsz);
    sqlite3_randomness(8, &aFrame[4]);
    pWal->hdr.iCheck1 = sqlite3Get4byte(&aFrame[4]);
    pWal->hdr.iCheck2 = sqlite3Get4byte(&aFrame[8]);
    rc = sqlite3OsWrite(pWal->pFd, aFrame, WAL_HDRSIZE, 0);
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  aCksum[0] = pWal->hdr.iCheck1;
  aCksum[1] = pWal->hdr.iCheck2;

  /* Write the log file. */
  for(p=pList; p; p=p->pDirty){
    u32 nDbsize;                  /* Db-size field for frame header */
    i64 iOffset;                  /* Write offset in log file */

    iOffset = walFrameOffset(++iFrame, nPgsz);
    
    /* Populate and write the frame header */
    nDbsize = (isCommit && p->pDirty==0) ? nTruncate : 0;
    walEncodeFrame(aCksum, p->pgno, nDbsize, nPgsz, p->pData, aFrame);
    rc = sqlite3OsWrite(pWal->pFd, aFrame, sizeof(aFrame), iOffset);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* Write the page data */
    rc = sqlite3OsWrite(pWal->pFd, p->pData, nPgsz, iOffset + sizeof(aFrame));
    if( rc!=SQLITE_OK ){
      return rc;
    }
    pLast = p;
  }

  /* Sync the log file if the 'isSync' flag was specified. */
  if( sync_flags ){
    i64 iSegment = sqlite3OsSectorSize(pWal->pFd);
    i64 iOffset = walFrameOffset(iFrame+1, nPgsz);

    assert( isCommit );

    if( iSegment<SQLITE_DEFAULT_SECTOR_SIZE ){
      iSegment = SQLITE_DEFAULT_SECTOR_SIZE;
    }
    iSegment = (((iOffset+iSegment-1)/iSegment) * iSegment);
    while( iOffset<iSegment ){
      walEncodeFrame(aCksum,pLast->pgno,nTruncate,nPgsz,pLast->pData,aFrame);
      rc = sqlite3OsWrite(pWal->pFd, aFrame, sizeof(aFrame), iOffset);
      if( rc!=SQLITE_OK ){
        return rc;
      }

      iOffset += WAL_FRAME_HDRSIZE;
      rc = sqlite3OsWrite(pWal->pFd, pLast->pData, nPgsz, iOffset); 
      if( rc!=SQLITE_OK ){
        return rc;
      }
      nLast++;
      iOffset += nPgsz;
    }

    rc = sqlite3OsSync(pWal->pFd, sync_flags);
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }

  /* Append data to the log summary. It is not necessary to lock the 
  ** wal-index to do this as the RESERVED lock held on the db file
  ** guarantees that there are no other writers, and no data that may
  ** be in use by existing readers is being overwritten.
  */
  iFrame = pWal->hdr.iLastPg;
  for(p=pList; p; p=p->pDirty){
    iFrame++;
    walIndexAppend(pWal, iFrame, p->pgno);
  }
  while( nLast>0 ){
    iFrame++;
    nLast--;
    walIndexAppend(pWal, iFrame, pLast->pgno);
  }

  /* Update the private copy of the header. */
  pWal->hdr.pgsz = nPgsz;
  pWal->hdr.iLastPg = iFrame;
  if( isCommit ){
    pWal->hdr.iChange++;
    pWal->hdr.nPage = nTruncate;
  }
  pWal->hdr.iCheck1 = aCksum[0];
  pWal->hdr.iCheck2 = aCksum[1];

  /* If this is a commit, update the wal-index header too. */
  if( isCommit ){
    walIndexWriteHdr(pWal, &pWal->hdr);

    pWal->iCallback = iFrame;
  }
  walIndexUnmap(pWal);

  return rc;
}

/* 
** Checkpoint the database:
**
**   1. Acquire a CHECKPOINT lock

**   2. Copy the contents of the log into the database file.
**   3. Zero the wal-index header (so new readers will ignore the log).

**   4. Drop the CHECKPOINT lock.
*/
int sqlite3WalCheckpoint(
  Wal *pWal,                      /* Wal connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  int sync_flags,                 /* Flags to sync db file with (or 0) */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
){
  int rc;                         /* Return code */
  int isChanged = 0;              /* True if a new wal-index header is loaded */

  assert( pWal->lockState==SQLITE_SHM_UNLOCK );

  /* Get the CHECKPOINT lock */
  do {
    rc = walSetLock(pWal, SQLITE_SHM_CHECKPOINT);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );






  if( rc!=SQLITE_OK ){

    walSetLock(pWal, SQLITE_SHM_UNLOCK);
    return rc;
  }

  /* Copy data from the log to the database file. */
  rc = walIndexReadHdr(pWal, &isChanged);
  if( rc==SQLITE_OK ){
    rc = walCheckpoint(pWal, pFd, sync_flags, zBuf);
  }
  if( isChanged ){
    /* If a new wal-index header was loaded before the checkpoint was 
    ** performed, then the pager-cache associated with log 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. */
  walSetLock(pWal, SQLITE_SHM_UNLOCK);
  return 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;
}

Changes to src/wal.h.

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#define _WAL_H_

#include "sqliteInt.h"

/* Connection to a write-ahead log (WAL) file. 
** There is one object of this type for each pager. 
*/
typedef struct Log Log;

/* Open and close a connection to a write-ahead log. */
int sqlite3WalOpen(sqlite3_vfs*, const char *zDb, Log **ppLog);
int sqlite3WalClose(Log *pLog, sqlite3_file *pFd, int sync_flags, u8 *zBuf);

/* Used by readers to open (lock) and close (unlock) a snapshot.  A 
** snapshot is like a read-transaction.  It is the state of the database
** at an instant in time.  sqlite3WalOpenSnapshot gets a read lock and
** preserves the current state even if the other threads or processes
** write to or checkpoint the WAL.  sqlite3WalCloseSnapshot() closes the
** transaction and releases the lock.
*/
int sqlite3WalOpenSnapshot(Log *pLog, int *);
void sqlite3WalCloseSnapshot(Log *pLog);

/* Read a page from the write-ahead log, if it is present. */
int sqlite3WalRead(Log *pLog, Pgno pgno, int *pInLog, u8 *pOut);

/* Return the size of the database as it existed at the beginning
** of the snapshot */
void sqlite3WalDbsize(Log *pLog, Pgno *pPgno);

/* Obtain or release the WRITER lock. */
int sqlite3WalWriteLock(Log *pLog, int op);

/* Undo any frames written (but not committed) to the log */
int sqlite3WalUndo(Log *pLog, int (*xUndo)(void *, Pgno), void *pUndoCtx);

/* Return an integer that records the current (uncommitted) write
** position in the WAL */
u32 sqlite3WalSavepoint(Log *pLog);

/* Move the write position of the WAL back to iFrame.  Called in
** response to a ROLLBACK TO command. */
int sqlite3WalSavepointUndo(Log *pLog, u32 iFrame);

/* Return true if data has been written but not committed to the log file. */
int sqlite3WalDirty(Log *pLog);

/* Write a frame or frames to the log. */
int sqlite3WalFrames(Log *pLog, int, PgHdr *, Pgno, int, int);

/* Copy pages from the log to the database file */ 
int sqlite3WalCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  int sync_flags,                 /* Flags to sync db file with (or 0) */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
);

/* 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(Log *pLog);

#endif /* _WAL_H_ */







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#define _WAL_H_

#include "sqliteInt.h"

/* Connection to a write-ahead log (WAL) file. 
** There is one object of this type for each pager. 
*/
typedef struct Wal Wal;

/* Open and close a connection to a write-ahead log. */
int sqlite3WalOpen(sqlite3_vfs*, const char *zDb, Wal **ppWal);
int sqlite3WalClose(Wal *pWal, sqlite3_file *pFd, int sync_flags, u8 *zBuf);

/* Used by readers to open (lock) and close (unlock) a snapshot.  A 
** snapshot is like a read-transaction.  It is the state of the database
** at an instant in time.  sqlite3WalOpenSnapshot gets a read lock and
** preserves the current state even if the other threads or processes
** write to or checkpoint the WAL.  sqlite3WalCloseSnapshot() closes the
** transaction and releases the lock.
*/
int sqlite3WalOpenSnapshot(Wal *pWal, int *);
void sqlite3WalCloseSnapshot(Wal *pWal);

/* Read a page from the write-ahead log, if it is present. */
int sqlite3WalRead(Wal *pWal, Pgno pgno, int *pInWal, u8 *pOut);

/* Return the size of the database as it existed at the beginning
** of the snapshot */
void sqlite3WalDbsize(Wal *pWal, Pgno *pPgno);

/* Obtain or release the WRITER lock. */
int sqlite3WalWriteLock(Wal *pWal, int op);

/* Undo any frames written (but not committed) to the log */
int sqlite3WalUndo(Wal *pWal, int (*xUndo)(void *, Pgno), void *pUndoCtx);

/* Return an integer that records the current (uncommitted) write
** position in the WAL */
u32 sqlite3WalSavepoint(Wal *pWal);

/* Move the write position of the WAL back to iFrame.  Called in
** response to a ROLLBACK TO command. */
int sqlite3WalSavepointUndo(Wal *pWal, u32 iFrame);

/* Return true if data has been written but not committed to the log file. */
int sqlite3WalDirty(Wal *pWal);

/* Write a frame or frames to the log. */
int sqlite3WalFrames(Wal *pWal, int, PgHdr *, Pgno, int, int);

/* Copy pages from the log to the database file */ 
int sqlite3WalCheckpoint(
  Wal *pWal,                      /* Write-ahead log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  int sync_flags,                 /* Flags to sync db file with (or 0) */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
);

/* 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);

#endif /* _WAL_H_ */