SQLite

** to be updated with the new data before the backup operation is
** complete.
**
** It is assumed that the mutex associated with the BtShared object
** corresponding to the source database is held when this function is
** called.
*/
void sqlite3BackupUpdate(sqlite3_backup *pBackup, Pgno iPage, const u8 *aData){
  sqlite3_backup *p;                   /* Iterator variable */
  for(p=pBackup; p; p=p->pNext){
static SQLITE_NOINLINE void backupUpdate(
  sqlite3_backup *p,
  Pgno iPage,
  const u8 *aData
){
  assert( p!=0 );
  do{
    assert( sqlite3_mutex_held(p->pSrc->pBt->mutex) );
    if( !isFatalError(p->rc) && iPage<p->iNext ){
      /* The backup process p has already copied page iPage. But now it
      ** has been modified by a transaction on the source pager. Copy
      ** the new data into the backup.
      */
      int rc;
      assert( p->pDestDb );
      sqlite3_mutex_enter(p->pDestDb->mutex);
      rc = backupOnePage(p, iPage, aData, 1);
      sqlite3_mutex_leave(p->pDestDb->mutex);
      assert( rc!=SQLITE_BUSY && rc!=SQLITE_LOCKED );
      if( rc!=SQLITE_OK ){
        p->rc = rc;
      }
    }
  }while( (p = p->pNext)!=0 );
  }
}
void sqlite3BackupUpdate(sqlite3_backup *pBackup, Pgno iPage, const u8 *aData){
  if( pBackup ) backupUpdate(pBackup, iPage, aData);
}

/*
** Restart the backup process. This is called when the pager layer
** detects that the database has been modified by an external database
** connection. In this case there is no way of knowing which of the
** pages that have been copied into the destination database are still