**
** Parameter eMode is one of SQLITE_CHECKPOINT_PASSIVE, FULL or RESTART.
*/
int sqlite3PagerCheckpoint(Pager *pPager, int eMode, int *pnLog, int *pnCkpt){
  int rc = SQLITE_OK;
  if( pPager->pWal ){
    rc = sqlite3WalCheckpoint(pPager->pWal, eMode,

        pPager->xBusyHandler, pPager->pBusyHandlerArg,
        pPager->ckptSyncFlags, pPager->pageSize, (u8 *)pPager->pTmpSpace,
        pnLog, pnCkpt
    );
  }
  return rc;
}

**
** Parameter eMode is one of SQLITE_CHECKPOINT_PASSIVE, FULL or RESTART.
*/
int sqlite3PagerCheckpoint(Pager *pPager, int eMode, int *pnLog, int *pnCkpt){
  int rc = SQLITE_OK;
  if( pPager->pWal ){
    rc = sqlite3WalCheckpoint(pPager->pWal, eMode,
        (eMode==SQLITE_CHECKPOINT_PASSIVE ? 0 : pPager->xBusyHandler),
        pPager->pBusyHandlerArg,
        pPager->ckptSyncFlags, pPager->pageSize, (u8 *)pPager->pTmpSpace,
        pnLog, pnCkpt
    );
  }
  return rc;
}

** The caller must be holding sufficient locks to ensure that no other
** checkpoint is running (in any other thread or process) at the same
** time.
*/
static int walCheckpoint(
  Wal *pWal,                      /* Wal connection */
  int eMode,                      /* One of PASSIVE, FULL or RESTART */
  int (*xBusyCall)(void*),        /* Function to call when busy */
  void *pBusyArg,                 /* Context argument for xBusyHandler */
  int sync_flags,                 /* Flags for OsSync() (or 0) */
  u8 *zBuf                        /* Temporary buffer to use */
){
  int rc;                         /* Return code */
  int szPage;                     /* Database page-size */
  WalIterator *pIter = 0;         /* Wal iterator context */
  u32 iDbpage = 0;                /* Next database page to write */
  u32 iFrame = 0;                 /* Wal frame containing data for iDbpage */
  u32 mxSafeFrame;                /* Max frame that can be backfilled */
  u32 mxPage;                     /* Max database page to write */
  int i;                          /* Loop counter */
  volatile WalCkptInfo *pInfo;    /* The checkpoint status information */
  int (*xBusy)(void*) = 0;        /* Function to call when waiting for locks */

  szPage = walPagesize(pWal);
  testcase( szPage<=32768 );
  testcase( szPage>=65536 );
  pInfo = walCkptInfo(pWal);
  if( pInfo->nBackfill>=pWal->hdr.mxFrame ) return SQLITE_OK;

................................................................................
  /* Allocate the iterator */
  rc = walIteratorInit(pWal, &pIter);
  if( rc!=SQLITE_OK ){
    return rc;
  }
  assert( pIter );



  if( eMode!=SQLITE_CHECKPOINT_PASSIVE ) xBusy = xBusyCall;

  /* Compute in mxSafeFrame the index of the last frame of the WAL that is
  ** safe to write into the database.  Frames beyond mxSafeFrame might
  ** overwrite database pages that are in use by active readers and thus
  ** cannot be backfilled from the WAL.
  */
  mxSafeFrame = pWal->hdr.mxFrame;
................................................................................
** we can from WAL into the database.
**
** If parameter xBusy is not NULL, it is a pointer to a busy-handler
** callback. In this case this function runs a blocking checkpoint.
*/
int sqlite3WalCheckpoint(
  Wal *pWal,                      /* Wal connection */
  int eMode,                      /* PASSIVE, FULL or RESTART */
  int (*xBusy)(void*),            /* Function to call when busy */
  void *pBusyArg,                 /* Context argument for xBusyHandler */
  int sync_flags,                 /* Flags to sync db file with (or 0) */
  int nBuf,                       /* Size of temporary buffer */
  u8 *zBuf,                       /* Temporary buffer to use */
  int *pnLog,                     /* OUT: Number of frames in WAL */
  int *pnCkpt                     /* OUT: Number of backfilled frames in WAL */
){
  int rc;                         /* Return code */
  int isChanged = 0;              /* True if a new wal-index header is loaded */
  int eMode2 = eMode;             /* Mode to pass to walCheckpoint() */


  assert( pWal->ckptLock==0 );
  assert( pWal->writeLock==0 );





  if( pWal->readOnly ) return SQLITE_READONLY;
  WALTRACE(("WAL%p: checkpoint begins\n", pWal));



  rc = walLockExclusive(pWal, WAL_CKPT_LOCK, 1);
  if( rc ){
    /* Usually this is SQLITE_BUSY meaning that another thread or process
    ** is already running a checkpoint, or maybe a recovery.  But it might
    ** also be SQLITE_IOERR. */








    return rc;
  }
  pWal->ckptLock = 1;

  /* If this is a blocking-checkpoint, then obtain the write-lock as well
  ** to prevent any writers from running while the checkpoint is underway.
  ** This has to be done before the call to walIndexReadHdr() below.
  **
  ** If the writer lock cannot be obtained, then a passive checkpoint is
  ** run instead. Since the checkpointer is not holding the writer lock,
  ** there is no point in blocking waiting for any readers. Assuming no 
  ** other error occurs, this function will return SQLITE_BUSY to the caller.
  */
  if( eMode!=SQLITE_CHECKPOINT_PASSIVE ){
    rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_WRITE_LOCK, 1);
    if( rc==SQLITE_OK ){
      pWal->writeLock = 1;
    }else if( rc==SQLITE_BUSY ){
      eMode2 = SQLITE_CHECKPOINT_PASSIVE;

      rc = SQLITE_OK;
    }
  }

  /* Read the wal-index header. */
  if( rc==SQLITE_OK ){
    rc = walIndexReadHdr(pWal, &isChanged);
................................................................................
  }

  /* Copy data from the log to the database file. */
  if( rc==SQLITE_OK ){
    if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){
      rc = SQLITE_CORRUPT_BKPT;
    }else{
      rc = walCheckpoint(pWal, eMode2, xBusy, pBusyArg, sync_flags, zBuf);
    }

    /* If no error occurred, set the output variables. */
    if( rc==SQLITE_OK || rc==SQLITE_BUSY ){
      if( pnLog ) *pnLog = (int)pWal->hdr.mxFrame;
      if( pnCkpt ) *pnCkpt = (int)(walCkptInfo(pWal)->nBackfill);
    }

** The caller must be holding sufficient locks to ensure that no other
** checkpoint is running (in any other thread or process) at the same
** time.
*/
static int walCheckpoint(
  Wal *pWal,                      /* Wal connection */
  int eMode,                      /* One of PASSIVE, FULL or RESTART */
  int (*xBusy)(void*),            /* Function to call when busy */
  void *pBusyArg,                 /* Context argument for xBusyHandler */
  int sync_flags,                 /* Flags for OsSync() (or 0) */
  u8 *zBuf                        /* Temporary buffer to use */
){
  int rc;                         /* Return code */
  int szPage;                     /* Database page-size */
  WalIterator *pIter = 0;         /* Wal iterator context */
  u32 iDbpage = 0;                /* Next database page to write */
  u32 iFrame = 0;                 /* Wal frame containing data for iDbpage */
  u32 mxSafeFrame;                /* Max frame that can be backfilled */
  u32 mxPage;                     /* Max database page to write */
  int i;                          /* Loop counter */
  volatile WalCkptInfo *pInfo;    /* The checkpoint status information */


  szPage = walPagesize(pWal);
  testcase( szPage<=32768 );
  testcase( szPage>=65536 );
  pInfo = walCkptInfo(pWal);
  if( pInfo->nBackfill>=pWal->hdr.mxFrame ) return SQLITE_OK;

................................................................................
  /* Allocate the iterator */
  rc = walIteratorInit(pWal, &pIter);
  if( rc!=SQLITE_OK ){
    return rc;
  }
  assert( pIter );

  /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
  ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
  assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );

  /* Compute in mxSafeFrame the index of the last frame of the WAL that is
  ** safe to write into the database.  Frames beyond mxSafeFrame might
  ** overwrite database pages that are in use by active readers and thus
  ** cannot be backfilled from the WAL.
  */
  mxSafeFrame = pWal->hdr.mxFrame;
................................................................................
** we can from WAL into the database.
**
** If parameter xBusy is not NULL, it is a pointer to a busy-handler
** callback. In this case this function runs a blocking checkpoint.
*/
int sqlite3WalCheckpoint(
  Wal *pWal,                      /* Wal connection */
  int eMode,                      /* PASSIVE, FULL, RESTART, or TRUNCATE */
  int (*xBusy)(void*),            /* Function to call when busy */
  void *pBusyArg,                 /* Context argument for xBusyHandler */
  int sync_flags,                 /* Flags to sync db file with (or 0) */
  int nBuf,                       /* Size of temporary buffer */
  u8 *zBuf,                       /* Temporary buffer to use */
  int *pnLog,                     /* OUT: Number of frames in WAL */
  int *pnCkpt                     /* OUT: Number of backfilled frames in WAL */
){
  int rc;                         /* Return code */
  int isChanged = 0;              /* True if a new wal-index header is loaded */
  int eMode2 = eMode;             /* Mode to pass to walCheckpoint() */
  int (*xBusy2)(void*) = xBusy;   /* Busy handler for eMode2 */

  assert( pWal->ckptLock==0 );
  assert( pWal->writeLock==0 );

  /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
  ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
  assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );

  if( pWal->readOnly ) return SQLITE_READONLY;
  WALTRACE(("WAL%p: checkpoint begins\n", pWal));

  /* IMPLEMENTATION-OF: R-62028-47212 All calls obtain an exclusive 
  ** "checkpoint" lock on the database file. */
  rc = walLockExclusive(pWal, WAL_CKPT_LOCK, 1);
  if( rc ){



    /* EVIDENCE-OF: R-10421-19736 If any other process is running a
    ** checkpoint operation at the same time, the lock cannot be obtained and
    ** SQLITE_BUSY is returned.
    ** EVIDENCE-OF: R-53820-33897 Even if there is a busy-handler configured,
    ** it will not be invoked in this case.
    */
    testcase( rc==SQLITE_BUSY );
    testcase( xBusy!=0 );
    return rc;
  }
  pWal->ckptLock = 1;

  /* IMPLEMENTATION-OF: R-59782-36818 The SQLITE_CHECKPOINT_FULL, RESTART and
  ** TRUNCATE modes also obtain the exclusive "writer" lock on the database
  ** file.
  **
  ** EVIDENCE-OF: R-60642-04082 If the writer lock cannot be obtained
  ** immediately, and a busy-handler is configured, it is invoked and the
  ** writer lock retried until either the busy-handler returns 0 or the
  ** lock is successfully obtained.
  */
  if( eMode!=SQLITE_CHECKPOINT_PASSIVE ){
    rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_WRITE_LOCK, 1);
    if( rc==SQLITE_OK ){
      pWal->writeLock = 1;
    }else if( rc==SQLITE_BUSY ){
      eMode2 = SQLITE_CHECKPOINT_PASSIVE;
      xBusy2 = 0;
      rc = SQLITE_OK;
    }
  }

  /* Read the wal-index header. */
  if( rc==SQLITE_OK ){
    rc = walIndexReadHdr(pWal, &isChanged);
................................................................................
  }

  /* Copy data from the log to the database file. */
  if( rc==SQLITE_OK ){
    if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){
      rc = SQLITE_CORRUPT_BKPT;
    }else{
      rc = walCheckpoint(pWal, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
    }

    /* If no error occurred, set the output variables. */
    if( rc==SQLITE_OK || rc==SQLITE_BUSY ){
      if( pnLog ) *pnLog = (int)pWal->hdr.mxFrame;
      if( pnCkpt ) *pnCkpt = (int)(walCkptInfo(pWal)->nBackfill);
    }