SQLite

*/
#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
int sqlite3WalTrace = 0;
# define WALTRACE(X)  if(sqlite3WalTrace) sqlite3DebugPrintf X
#else
# define WALTRACE(X)
#endif

/*
** WAL tracing logic added to search for an SQLITE_PROTOCOL error.
*/
static void walTrace(const char *zFormat, ...){
  va_list ap;
  char zMsg[100];
  va_start(ap, zFormat);
  sqlite3_vsnprintf(sizeof(zMsg), zMsg, zFormat, ap);
  va_end(ap);
#ifdef SQLITE_WAL_TRACE
  fprintf(stdout, "WALTRACE: [%s]\n", zMsg); fflush(stdout);
#endif
  sqlite3_log(99, "%s", zMsg);
}

/*
** The maximum (and only) versions of the wal and wal-index formats
** that may be interpreted by this version of SQLite.
**
** If a client begins recovering a WAL file and finds that (a) the checksum
** values in the wal-header are correct and (b) the version field is not

  /* Enlarge the pWal->apWiData[] array if required */
  if( pWal->nWiData<=iPage ){
    int nByte = sizeof(u32*)*(iPage+1);
    volatile u32 **apNew;
    apNew = (volatile u32 **)sqlite3_realloc((void *)pWal->apWiData, nByte);
    if( !apNew ){
      *ppPage = 0;
      walTrace("realloc(%d) in walIndexPage()", nByte);
      return SQLITE_NOMEM;
    }
    memset((void*)&apNew[pWal->nWiData], 0,
           sizeof(u32*)*(iPage+1-pWal->nWiData));
    pWal->apWiData = apNew;
    pWal->nWiData = iPage+1;
  }

  /* Request a pointer to the required page from the VFS */
  if( pWal->apWiData[iPage]==0 ){
    if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
      pWal->apWiData[iPage] = (u32 volatile *)sqlite3MallocZero(WALINDEX_PGSZ);
      if( !pWal->apWiData[iPage] ) rc = SQLITE_NOMEM;
    }else{
      rc = sqlite3OsShmMap(pWal->pDbFd, iPage, WALINDEX_PGSZ, 
          pWal->writeLock, (void volatile **)&pWal->apWiData[iPage]
      );
    }
    if( rc ) walTrace("xShmMap():%d in walIndexPage(iPage=%d)",rc,iPage);
  }

  *ppPage = pWal->apWiData[iPage];
  assert( iPage==0 || *ppPage || rc!=SQLITE_OK );
  return rc;
}

  /* If the first attempt failed, it might have been due to a race
  ** with a writer.  So get a WRITE lock and try again.
  */
  assert( badHdr==0 || pWal->writeLock==0 );
  if( badHdr && SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) ){
    pWal->writeLock = 1;
    walTrace("trying walIndexTryHdr w/lock");
    if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
      badHdr = walIndexTryHdr(pWal, pChanged);
      if( badHdr ){
        /* If the wal-index header is still malformed even while holding
        ** a WRITE lock, it can only mean that the header is corrupted and
        ** needs to be reconstructed.  So run recovery to do exactly that.
        */
        walTrace("walIndexTryHdr() failed w/lock");
        rc = walIndexRecover(pWal);
        if( rc ) walTrace("walIndexRecover():%d", rc);
        *pChanged = 1;
      }
    }
    pWal->writeLock = 0;
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
  }
  else if(badHdr) walTrace("walLockExcl():%d in walIndexReadHdr()", rc);

  /* If the header is read successfully, check the version number to make
  ** sure the wal-index was not constructed with some future format that
  ** this version of SQLite cannot understand.
  */
  if( badHdr==0 && pWal->hdr.iVersion!=WALINDEX_MAX_VERSION ){
    rc = SQLITE_CANTOPEN_BKPT;

  int i;                          /* Loop counter */
  int rc = SQLITE_OK;             /* Return code  */

  assert( pWal->readLock<0 );     /* Not currently locked */

  /* Take steps to avoid spinning forever if there is a protocol error. */
  if( cnt>5 ){
    walTrace("cnt=%d",cnt);
    if( cnt>100 ) return SQLITE_PROTOCOL;
    sqlite3OsSleep(pWal->pVfs, 1);
  }

  if( !useWal ){
    rc = walIndexReadHdr(pWal, pChanged);
    if( rc==SQLITE_BUSY ){

        **
        ** However, if frames have been appended to the log (or if the log 
        ** is wrapped and written for that matter) before the READ_LOCK(0)
        ** is obtained, that is not necessarily true. A checkpointer may
        ** have started to backfill the appended frames but crashed before
        ** it finished. Leaving a corrupt image in the database file.
        */
        walTrace("wal read/write race - writer won"); 
        walUnlockShared(pWal, WAL_READ_LOCK(0));
        return WAL_RETRY;
      }
      pWal->readLock = 0;
      return SQLITE_OK;
    }else if( rc!=SQLITE_BUSY ){
      walTrace("walLockShared(0):%d in walTryBeginRead", rc);
      return rc;
    }
  }

  /* If we get this far, it means that the reader will want to use
  ** the WAL to get at content from recent commits.  The job now is
  ** to select one of the aReadMark[] entries that is closest to

    ** be mxFrame, then retry.
    */
    rc = walLockExclusive(pWal, WAL_READ_LOCK(1), 1);
    if( rc==SQLITE_OK ){
      pInfo->aReadMark[1] = pWal->hdr.mxFrame;
      walUnlockExclusive(pWal, WAL_READ_LOCK(1), 1);
      rc = WAL_RETRY;
      walTrace("aReadMark[1] <- %d", pWal->hdr.mxFrame);
    }else if( rc==SQLITE_BUSY ){
      rc = WAL_RETRY;
      walTrace("aReadMark[1] is busy");
    }
    return rc;
  }else{
    if( mxReadMark < pWal->hdr.mxFrame ){
      for(i=1; i<WAL_NREADER; i++){
        rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
        if( rc==SQLITE_OK ){
          mxReadMark = pInfo->aReadMark[i] = pWal->hdr.mxFrame;
          mxI = i;
          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          break;
        }else if( rc!=SQLITE_BUSY ){
          walTrace("walLockExclusive(%d):%d", i, rc);
          return rc;
        }
      }
    }

    rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
    if( rc ){
      walTrace("walLockShared(mxI=%d):%d", mxI, rc);
      return rc==SQLITE_BUSY ? WAL_RETRY : rc;
    }
    /* Now that the read-lock has been obtained, check that neither the
    ** value in the aReadMark[] array or the contents of the wal-index
    ** header have changed.
    **
    ** It is necessary to check that the wal-index header did not change