}else{
    hdr.iCheck1 = 2;
    hdr.iCheck2 = 3;
  }

finished:
  walIndexWriteHdr(pWal, &hdr);

  return rc;
}

/*
** Close an open wal-index.
*/
static void walIndexClose(Wal *pWal, int isDelete){
................................................................................
    int rc;
    rc = walIndexRemap(pWal, WALINDEX_MMAP_INCREMENT);
    if( rc ) return rc;
  }

  /* 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.
  **
  ** FIX-ME:  It is no longer possible to have not locked the wal-index.
  */
  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_OK;
  }
  *pisValid = 1;

  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 
................................................................................
** Otherwise an SQLite error code.
*/
static int walIndexReadHdr(Wal *pWal, int *pChanged){
  int rc;
  int isValid = 0;

  assert( pWal->lockState>=SQLITE_SHM_READ );

  rc = walIndexMap(pWal, -1);
  if( rc!=SQLITE_OK ){
    return rc;
  }

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

  /* 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)) ){




    rc = walIndexTryHdr(pWal, &isValid, pChanged);
    if( rc==SQLITE_OK && isValid==0 ){
      if( pChanged ){

        *pChanged = 1;
      }
      rc = walIndexRecover(pWal);
      if( rc==SQLITE_OK ){
        rc = walIndexTryHdr(pWal, &isValid, 0);
      }
    }
    walSetLock(pWal, SQLITE_SHM_READ);
  }

  if( rc==SQLITE_OK && isValid==0 ){
    rc = SQLITE_ERROR;
  }
  return rc;
}

/*
** Lock a snapshot.
**
** If this call obtains a new read-lock and the database contents have been
................................................................................
** 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;

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

  }else{
    hdr.iCheck1 = 2;
    hdr.iCheck2 = 3;
  }

finished:
  walIndexWriteHdr(pWal, &hdr);
  memcpy(&pWal->hdr, &hdr, sizeof(hdr));
  return rc;
}

/*
** Close an open wal-index.
*/
static void walIndexClose(Wal *pWal, int isDelete){
................................................................................
    int rc;
    rc = walIndexRemap(pWal, WALINDEX_MMAP_INCREMENT);
    if( rc ) return rc;
  }

  /* 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.




  */
  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_OK;
  }
  *pisValid = 1;

  if( memcmp(&pWal->hdr, aHdr, sizeof(WalIndexHdr)) ){

    *pChanged = 1;

    memcpy(&pWal->hdr, aHdr, sizeof(WalIndexHdr));
  }
  return SQLITE_OK;
}

/*
** Read the wal-index header from the wal-index file into structure 
................................................................................
** Otherwise an SQLite error code.
*/
static int walIndexReadHdr(Wal *pWal, int *pChanged){
  int rc;
  int isValid = 0;

  assert( pWal->lockState>=SQLITE_SHM_READ );
  assert( pChanged );
  rc = walIndexMap(pWal, -1);
  if( rc!=SQLITE_OK ){
    return rc;
  }

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

  /* 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)) ){
    /* This call to walIndexTryHdr() may not return an error code, as the
    ** wal-index is already mapped. It may find that the header is invalid,
    ** but there is no chance of hitting an actual error.  */
    assert( pWal->szWIndex );
    rc = walIndexTryHdr(pWal, &isValid, pChanged);
    assert( rc==SQLITE_OK );

    if( isValid==0 ){
      *pChanged = 1;

      rc = walIndexRecover(pWal);



    }
    walSetLock(pWal, SQLITE_SHM_READ);
  }




  return rc;
}

/*
** Lock a snapshot.
**
** If this call obtains a new read-lock and the database contents have been
................................................................................
** 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 unused;
  int rc = SQLITE_OK;
  Pgno iMax = pWal->hdr.iLastPg;
  Pgno iFrame;

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

  unset ::shmfault_ioerr_methods(xShmGet)
  if {[file exists test.db-wal]==0} {error "Failed to create WAL file!"}

  sqlite3 db test.db -vfs shmfault
} -sqlbody {
  SELECT count(*) FROM t1;
}


























finish_test

  unset ::shmfault_ioerr_methods(xShmGet)
  if {[file exists test.db-wal]==0} {error "Failed to create WAL file!"}

  sqlite3 db test.db -vfs shmfault
} -sqlbody {
  SELECT count(*) FROM t1;
}

do_shmfault_test walfault-shm-4 -tclprep {
  sqlite3 db test.db -vfs shmfault
  unset -nocomplain ::shmfault_ioerr_countdown
  db eval {
    PRAGMA page_size = 512;
    PRAGMA journal_mode = WAL;
    PRAGMA wal_autocheckpoint = 0;
    CREATE TABLE t1(x);
    BEGIN;
      INSERT INTO t1 VALUES(randomblob(400));           /* 1 */
      INSERT INTO t1 SELECT randomblob(400) FROM t1;    /* 2 */
      INSERT INTO t1 SELECT randomblob(400) FROM t1;    /* 4 */
    COMMIT;
  }

  set ::shmfault_ioerr_countdown 1
  set ::shmfault_ioerr_methods(xShmGet) 1
  db close
  unset ::shmfault_ioerr_methods(xShmGet)
  if {[file exists test.db-wal]==0} {error "Failed to create WAL file!"}
  sqlite3 db test.db -vfs shmfault
} -sqlbody {
  SELECT count(*) FROM t1;
}

finish_test