SQLite

/*
** Try to read the wal-index header. Attempt to verify the header
** checksum. If the checksum can be verified, copy the wal-index
** header into structure pWal->hdr. If the contents of pWal->hdr are
** modified by this and pChanged is not NULL, set *pChanged to 1. 
** Otherwise leave *pChanged unmodified.
**
** If the checksum cannot be verified return non-zero. If the header
** is read successfully and the checksum verified, return zero.
*/
int walIndexTryHdr(Wal *pWal, int *pChanged){
  u32 aCksum[2] = {1, 1};
  u32 aHdr[WALINDEX_HDR_NFIELD+2];

  assert( pWal->pWiData );
  if( pWal->szWIndex==0 ){
    /* The wal-index is of size 0 bytes. This is handled in the same way
    ** as an invalid header. The caller will run recovery to construct
    ** a valid wal-index file before accessing the database.
    */
    return 1;
  }

  /* Read the header. The caller may or may not have an exclusive 
  ** (WRITE, PENDING, CHECKPOINT or RECOVER) lock on the wal-index
  ** file, meaning it is possible that an inconsistent snapshot is read
  ** from the file. If this happens, return non-zero.
  */
  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 1;
  }


  if( memcmp(&pWal->hdr, aHdr, sizeof(WalIndexHdr)) ){
    *pChanged = 1;
    memcpy(&pWal->hdr, aHdr, sizeof(WalIndexHdr));
  }

  /* The header was successfully read. Return zero. */
  return 0;
}

/*
** Read the wal-index header from the wal-index file into structure 
** pWal->hdr. If attempting to verify the header checksum fails, try
** to recover the log before returning.
**
** If the wal-index header is successfully read, return SQLITE_OK. 
** Otherwise an SQLite error code.
*/
static int walIndexReadHdr(Wal *pWal, int *pChanged){
  int rc;                         /* Return code */

  int lockState;                  /* pWal->lockState before running recovery */

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

  /* First attempt to read the wal-index header. This may fail for one
  ** of two reasons: (a) the wal-index does not yet exist or has been
  ** corrupted and needs to be constructed by running recovery, or (b)
  ** the caller is only holding a READ lock and made a dirty read of
  ** the wal-index header.
  **
  ** A dirty read of the wal-index header occurs if another thread or
  ** process happens to be writing to the wal-index header at roughly
  ** the same time as this thread is reading it. In this case it is 
  ** possible that an inconsistent header is read (which is detected
  ** using the header checksum mechanism).
  */
  if( walIndexTryHdr(pWal, pChanged)==0 ){

    return SQLITE_OK;
  }

  /* If the first attempt to read the header failed, lock the wal-index
  ** file with an exclusive lock and try again. If the header checksum 
  ** verification fails again, we can be sure that it is not simply a
  ** dirty read, but that the wal-index really does need to be 
  ** reconstructed by running log recovery.
  **
  ** In the paragraph above, an "exclusive lock" may be any of WRITE,
  ** PENDING, CHECKPOINT or RECOVER. If any of these are already held,
  ** no locking operations are required. If the caller currently holds
  ** a READ lock, then upgrade to a RECOVER lock before re-reading the
  ** wal-index header and revert to a READ lock before returning.
  */
  lockState = pWal->lockState;
  if( lockState>SQLITE_SHM_READ
   || SQLITE_OK==(rc = walSetLock(pWal, SQLITE_SHM_RECOVER)) 
  ){




    if( walIndexTryHdr(pWal, pChanged) ){


      *pChanged = 1;
      rc = walIndexRecover(pWal);
    }
    if( lockState==SQLITE_SHM_READ ){
      walSetLock(pWal, SQLITE_SHM_READ);
    }
  }