** thread to write as doing so would cause a fork.  So this routine
** returns SQLITE_BUSY in that case and no write transaction is started.
**
** There can only be a single writer active at a time.
*/
int sqlite3WalBeginWriteTransaction(Wal *pWal){
  int rc;
  volatile WalCkptInfo *pInfo;

  /* Cannot start a write transaction without first holding a read
  ** transaction. */
  assert( pWal->readLock>=0 );

  /* Only one writer allowed at a time.  Get the write lock.  Return
  ** SQLITE_BUSY if unable.
................................................................................
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
    pWal->writeLock = 0;
    return rc;
  }
  if( memcmp(&pWal->hdr, (void*)pWal->pWiData, sizeof(WalIndexHdr))!=0 ){
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
    pWal->writeLock = 0;
    walIndexUnmap(pWal);
    return SQLITE_BUSY;
  }

  pInfo = walCkptInfo(pWal);
  if( pWal->readLock==0 ){
    assert( pInfo->nBackfill==pWal->hdr.mxFrame );
    if( pInfo->nBackfill>0 ){
      rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      if( rc==SQLITE_OK ){
        /* If all readers are using WAL_READ_LOCK(0) (in other words if no
        ** readers are currently using the WAL) */
        pWal->nCkpt++;
        pWal->hdr.mxFrame = 0;
        sqlite3Put4byte((u8*)pWal->hdr.aSalt,
                         1 + sqlite3Get4byte((u8*)pWal->hdr.aSalt));
        sqlite3_randomness(4, &pWal->hdr.aSalt[1]);
        walIndexWriteHdr(pWal);
        pInfo->nBackfill = 0;
        memset((void*)&pInfo->aReadMark[1], 0,
               sizeof(pInfo->aReadMark)-sizeof(u32));
        rc = sqlite3OsTruncate(pWal->pDbFd, 
                               ((i64)pWal->hdr.nPage*(i64)pWal->szPage));
        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }
    }
    walUnlockShared(pWal, WAL_READ_LOCK(0));
    pWal->readLock = -1;
    do{
      int notUsed;
      rc = walTryBeginRead(pWal, &notUsed, 1);
    }while( rc==WAL_RETRY );
  }
  walIndexUnmap(pWal);
  return rc;
}

/*
** End a write transaction.  The commit has already been done.  This
** routine merely releases the lock.
................................................................................
    if( rc==SQLITE_OK ){
      walCleanupHash(pWal);
      walIndexUnmap(pWal);
    }
  }
  return rc;
}

/* 

























































** 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 szPage,                     /* Database page-size in bytes */
  PgHdr *pList,                   /* List of dirty pages to write */
  Pgno nTruncate,                 /* Database size after this commit */
  int isCommit,                   /* True if this is a commit */
................................................................................

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif










  /* If this is the first frame written into the log, write the WAL
  ** header to the start of the WAL file. See comments at the top of
  ** this source file for a description of the WAL header format.
  */
  iFrame = pWal->hdr.mxFrame;
  if( iFrame==0 ){
................................................................................
    if( rc!=SQLITE_OK ){
      return rc;
    }
    walChecksumBytes(1, aWalHdr, sizeof(aWalHdr), 0, pWal->hdr.aFrameCksum);
  }
  assert( pWal->szPage==szPage );

    /* 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, szPage);
    
    /* Populate and write the frame header */

** thread to write as doing so would cause a fork.  So this routine
** returns SQLITE_BUSY in that case and no write transaction is started.
**
** There can only be a single writer active at a time.
*/
int sqlite3WalBeginWriteTransaction(Wal *pWal){
  int rc;


  /* Cannot start a write transaction without first holding a read
  ** transaction. */
  assert( pWal->readLock>=0 );

  /* Only one writer allowed at a time.  Get the write lock.  Return
  ** SQLITE_BUSY if unable.
................................................................................
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
    pWal->writeLock = 0;
    return rc;
  }
  if( memcmp(&pWal->hdr, (void*)pWal->pWiData, sizeof(WalIndexHdr))!=0 ){
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
    pWal->writeLock = 0;

    rc = SQLITE_BUSY;
  }






























  walIndexUnmap(pWal);
  return rc;
}

/*
** End a write transaction.  The commit has already been done.  This
** routine merely releases the lock.
................................................................................
    if( rc==SQLITE_OK ){
      walCleanupHash(pWal);
      walIndexUnmap(pWal);
    }
  }
  return rc;
}

/*
** This function is called just before writing a set of frames to the log
** file (see sqlite3WalFrames()). It checks to see if, instead of appending
** to the current log file, it is possible to overwrite the start of the
** existing log file with the new frames (i.e. "reset" the log). If so,
** it sets pWal->hdr.mxFrame to 0. Otherwise, pWal->hdr.mxFrame is left
** unchanged.
**
** SQLITE_OK is returned if no error is encountered (regardless of whether
** or not pWal->hdr.mxFrame is modified). An SQLite error code is returned
** if some error 
*/
static int walRestartLog(Wal *pWal){
  int rc = SQLITE_OK;
  if( pWal->readLock==0 
   && SQLITE_OK==(rc = walIndexMap(pWal, walMappingSize(pWal->hdr.mxFrame)))
  ){
    volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
    assert( pInfo->nBackfill==pWal->hdr.mxFrame );
    if( pInfo->nBackfill>0 ){
      rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      if( rc==SQLITE_OK ){
        /* If all readers are using WAL_READ_LOCK(0) (in other words if no
        ** readers are currently using the WAL), then the transactions
        ** frames will overwrite the start of the existing log. Update the
        ** wal-index header to reflect this.
        **
        ** In theory it would be Ok to update the cache of the header only
        ** at this point. But updating the actual wal-index header is also
        ** safe and means there is no special case for sqlite3WalUndo()
        ** to handle if this transaction is rolled back.
        */
        u32 *aSalt = pWal->hdr.aSalt;       /* Big-endian salt values */
        pWal->nCkpt++;
        pWal->hdr.mxFrame = 0;
        sqlite3Put4byte((u8*)&aSalt[0], 1 + sqlite3Get4byte((u8*)&aSalt[0]));
        sqlite3_randomness(4, &aSalt[1]);
        walIndexWriteHdr(pWal);
        memset((void*)pInfo, 0, sizeof(*pInfo));
        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }
    }
    walUnlockShared(pWal, WAL_READ_LOCK(0));
    pWal->readLock = -1;
    do{
      int notUsed;
      rc = walTryBeginRead(pWal, &notUsed, 1);
    }while( rc==WAL_RETRY );

    /* Unmap the wal-index before returning. Otherwise the VFS layer may
    ** hold a mutex for the duration of the IO performed by WalFrames().
    */
    walIndexUnmap(pWal);
  }
  return rc;
}

/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
*/
int sqlite3WalFrames(
  Wal *pWal,                      /* Wal handle to write to */
  int szPage,                     /* Database page-size in bytes */
  PgHdr *pList,                   /* List of dirty pages to write */
  Pgno nTruncate,                 /* Database size after this commit */
  int isCommit,                   /* True if this is a commit */
................................................................................

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif

  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
    assert( pWal->pWiData==0 );
    return rc;
  }
  assert( pWal->pWiData==0 && pWal->readLock>0 );

  /* If this is the first frame written into the log, write the WAL
  ** header to the start of the WAL file. See comments at the top of
  ** this source file for a description of the WAL header format.
  */
  iFrame = pWal->hdr.mxFrame;
  if( iFrame==0 ){
................................................................................
    if( rc!=SQLITE_OK ){
      return rc;
    }
    walChecksumBytes(1, aWalHdr, sizeof(aWalHdr), 0, pWal->hdr.aFrameCksum);
  }
  assert( pWal->szPage==szPage );

  /* 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, szPage);
    
    /* Populate and write the frame header */