** Check if the *-wal file that corresponds to the database opened by pPager
** exists. Assuming no error occurs, set *pExists to 1 if the file exists,
** or 0 otherwise and return SQLITE_OK. If an IO or OOM error occurs, return
** an SQLite error code.
*/
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;
}

/*
** Check if the *-wal file that corresponds to the database opened by pPager
** exists. If it does, open the pager in WAL mode. Otherwise, if no error
................................................................................
** output buffer undefined.
*/
int sqlite3PagerReadFileheader(Pager *pPager, int N, unsigned char *pDest){
  int rc = SQLITE_OK;
  memset(pDest, 0, N);
  assert( isOpen(pPager->fd) || pPager->tempFile );







  if( pagerUseWal(pPager) ){
    int isInWal = 0;
    rc = sqlite3WalRead(pPager->pWal, 1, &isInWal, N, pDest);
    if( rc!=SQLITE_OK || isInWal ){
      return rc;
    }
  }


  if( isOpen(pPager->fd) ){
    IOTRACE(("DBHDR %p 0 %d\n", pPager, N))
    rc = sqlite3OsRead(pPager->fd, pDest, N, 0);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
    }

** Check if the *-wal file that corresponds to the database opened by pPager
** exists. Assuming no error occurs, set *pExists to 1 if the file exists,
** or 0 otherwise and return SQLITE_OK. If an IO or OOM error occurs, return
** an SQLite error code.
*/
static int pagerHasWAL(Pager *pPager, int *pExists){
  int rc;                         /* Return code */
  char *zWal;                     /* Name of the WAL file */

  assert( !pPager->tempFile );
  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);




  }
  return rc;
}

/*
** Check if the *-wal file that corresponds to the database opened by pPager
** exists. If it does, open the pager in WAL mode. Otherwise, if no error
................................................................................
** output buffer undefined.
*/
int sqlite3PagerReadFileheader(Pager *pPager, int N, unsigned char *pDest){
  int rc = SQLITE_OK;
  memset(pDest, 0, N);
  assert( isOpen(pPager->fd) || pPager->tempFile );

  /* This routine is only called by btree immediately after creating
  ** the Pager object.  There has not been an opportunity to transition
  ** to WAL mode yet.
  */
  assert( !pagerUseWal(pPager) );
#if 0
  if( pagerUseWal(pPager) ){
    int isInWal = 0;
    rc = sqlite3WalRead(pPager->pWal, 1, &isInWal, N, pDest);
    if( rc!=SQLITE_OK || isInWal ){
      return rc;
    }
  }
#endif

  if( isOpen(pPager->fd) ){
    IOTRACE(("DBHDR %p 0 %d\n", pPager, N))
    rc = sqlite3OsRead(pPager->fd, pDest, N, 0);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
    }