SQLite

/*
** Generate an 8 byte checksum based on the data in array aByte[] and the
** initial values of aCksum[0] and aCksum[1]. The checksum is written into
** aCksum[] before returning.
*/
#define LOG_CKSM_BYTES 8
static void logChecksumBytes(u8 *aByte, int nByte, u32 *aCksum){
  u64 sum1 = aCksum[0];
  u64 sum2 = aCksum[1];
  u32 *a32 = (u32 *)aByte;
  u32 *aEnd = (u32 *)&aByte[nByte];


  assert( LOG_CKSM_BYTES==2*sizeof(u32) );
  assert( (nByte&0x00000003)==0 );

  do {
    sum1 += (*a32++);
    sum2 += sum1;
  } while( a32<aEnd );




  aCksum[0] = sum1 + (sum1>>24);
  aCksum[1] = sum2 + (sum2>>24);
}

/*
** Argument zPath must be a nul-terminated string containing a path-name.
** This function modifies the string in-place by removing any "./" or "../" 
** elements in the path. For example, the following input:
**

  if( pLog->isLocked ){
    assert( pLog->isLocked==LOG_REGION_A || pLog->isLocked==LOG_REGION_D );
    logLockRegion(pLog, pLog->isLocked, LOG_UNLOCK);
  }
  pLog->isLocked = 0;
}



/* 
** Read a page from the log, if it is present. 
*/
int sqlite3LogRead(Log *pLog, Pgno pgno, int *pInLog, u8 *pOut){
  u32 iRead = 0;
  u32 *aData = pLog->pSummary->aData;
  int iFrame = (pLog->hdr.iLastPg & 0xFFFFFF00);

  assert( pLog->isLocked );

  /* Do a linear search of the unindexed block of page-numbers (if any) 
  ** at the end of the log-summary. An alternative to this would be to
  ** build an index in private memory each time a read transaction is
  ** opened on a new snapshot.
  */
  if( pLog->hdr.iLastPg ){

    /* Obtain the writer lock */
    int rc = logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_WRLOCK);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* If this is connection is a region D reader, then the SHARED lock on 
    ** region D has just been upgraded to EXCLUSIVE. But no lock at all is 
    ** held on region A. This means that if the write-transaction is committed
    ** and this connection downgrades to a reader, it will be left with no
    ** lock at all. And so its snapshot could get clobbered by a checkpoint
    ** operation. 
    **
    ** To stop this from happening, grab a SHARED lock on region A now.
    ** This should always be successful, as the only time a client holds
    ** an EXCLUSIVE lock on region A, it must also be holding an EXCLUSIVE
    ** lock on region C (a checkpointer does this). This is not possible,
    ** as this connection currently has the EXCLUSIVE lock on region C.
    */
    if( pLog->isLocked==LOG_REGION_D ){
      logLockRegion(pLog, LOG_REGION_A, LOG_RDLOCK);
      pLog->isLocked = LOG_REGION_A;
    }

    /* If this connection is not reading the most recent database snapshot,
    ** it is not possible to write to the database. In this case release
    ** the write locks and return SQLITE_BUSY.
    */
    if( memcmp(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr)) ){
      logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_UNLOCK);
      return SQLITE_BUSY;
    }
    pLog->isWriteLocked = 1;

  }else if( pLog->isWriteLocked ){

  sqlite3_file *pFd,              /* File descriptor open on db file */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
){
  int rc;                         /* Return code */

  assert( !pLog->isLocked );

  /* Wait for an EXCLUSIVE lock on regions B and C. */
  do {
    rc = logLockRegion(pLog, LOG_REGION_B|LOG_REGION_C, LOG_WRLOCK);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );
  if( rc!=SQLITE_OK ) return rc;

  /* Wait for an EXCLUSIVE lock on region A. */
  do {
    rc = logLockRegion(pLog, LOG_REGION_A, LOG_WRLOCK);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );
  if( rc!=SQLITE_OK ){
    logLockRegion(pLog, LOG_REGION_B|LOG_REGION_C, LOG_UNLOCK);
    return rc;
  }