SQLite

    }
    WALTRACE(("WAL%p: closed\n", pWal));
    sqlite3_free((void *)pWal->apWiData);
    sqlite3_free(pWal);
  }
  return rc;
}

/*
** Return true if the wal-index header seems valid based on the
** checksum.
*/
static int walIndexHdrValid(WalIndexHdr *p){
  u32 aCksum[2];
  if( p->isInit==0 ) return 0;
  walChecksumBytes(1, (u8*)p, sizeof(*p)-sizeof(p->aCksum), 0, aCksum);
  if( aCksum[0]!=p->aCksum[0] ) return 0;
  if( aCksum[1]!=p->aCksum[1] ) return 0;
  return 1;
}

/*
** Try to read the wal-index header.  Return 0 on success and 1 if
** there is a problem.
**
** The wal-index is in shared memory.  Another thread or process might
** be writing the header at the same time this procedure is trying to
** read it, which might result in inconsistency.  A dirty read is detected
** by verifying that both copies of the header are the same and also by
** a checksum on the header.
**
** If and only if the read is consistent and the header is different from
** pWal->hdr, then pWal->hdr is updated to the content of the new header
** and *pChanged is set to 1.
**
** If the checksum cannot be verified return non-zero. If the header
** is read successfully and the checksum verified, return zero.
*/
static SQLITE_NO_TSAN int walIndexTryHdr(Wal *pWal, int *pChanged){
static SQLITE_NO_TSAN int walIndexTryHdr(Wal *pWal, int *pChanged, int cnt){
  u32 aCksum[2];                  /* Checksum on the header content */
  WalIndexHdr h1, h2;             /* Two copies of the header content */
  WalIndexHdr volatile *aHdr;     /* Header in shared memory */

  /* The first page of the wal-index must be mapped at this point. */
  assert( pWal->nWiData>0 && pWal->apWiData[0] );

  ** give false-positive warnings about these accesses because the tools do not
  ** account for the double-read and the memory barrier. The use of mutexes
  ** here would be problematic as the memory being accessed is potentially
  ** shared among multiple processes and not all mutex implementions work
  ** reliably in that environment.
  */
  aHdr = walIndexHdr(pWal);
  memcpy(&h1, (void *)&aHdr[0], sizeof(h1)); /* Possible TSAN false-positive */
  memcpy(&h1, (void *)&aHdr[0], sizeof(h1));
  walShmBarrier(pWal);
  memcpy(&h2, (void *)&aHdr[1], sizeof(h2));

  if( memcmp(&h1, &h2, sizeof(h1))!=0 ){
    return 1;   /* Dirty read */
  }  
    if( cnt<15 ) return 1;
    if( !walIndexHdrValid(&h1) ){
      memcpy(&h1, &h2, sizeof(h1));
    }
  if( h1.isInit==0 ){
    return 1;   /* Malformed header - probably all zeros */
  }
  walChecksumBytes(1, (u8*)&h1, sizeof(h1)-sizeof(h1.aCksum), 0, aCksum);
  if( aCksum[0]!=h1.aCksum[0] || aCksum[1]!=h1.aCksum[1] ){
    return 1;   /* Checksum does not match */
  if( !walIndexHdrValid(&h1) ) return 1;
  }

  if( memcmp(&pWal->hdr, &h1, sizeof(WalIndexHdr)) ){
    *pChanged = 1;
    memcpy(&pWal->hdr, &h1, sizeof(WalIndexHdr));
    pWal->szPage = (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
    testcase( pWal->szPage<=32768 );
    testcase( pWal->szPage>=65536 );

** Set *pChanged to 1 if the wal-index header value in pWal->hdr is
** changed by this operation.  If pWal->hdr is unchanged, set *pChanged
** to 0.
**
** If the wal-index header is successfully read, return SQLITE_OK. 
** Otherwise an SQLite error code.
*/
static int walIndexReadHdr(Wal *pWal, int *pChanged){
static int walIndexReadHdr(Wal *pWal, int *pChanged, int cnt){
  int rc;                         /* Return code */
  int badHdr;                     /* True if a header read failed */
  volatile u32 *page0;            /* Chunk of wal-index containing header */

  /* Ensure that page 0 of the wal-index (the page that contains the 
  ** wal-index header) is mapped. Return early if an error occurs here.
  */

  assert( page0!=0 || pWal->writeLock==0 );

  /* If the first page of the wal-index has been mapped, try to read the
  ** wal-index header immediately, without holding any lock. This usually
  ** works, but may fail if the wal-index header is corrupt or currently 
  ** being modified by another thread or process.
  */
  badHdr = (page0 ? walIndexTryHdr(pWal, pChanged) : 1);
  badHdr = (page0 ? walIndexTryHdr(pWal, pChanged, 0) : 1);

  /* If the first attempt failed, it might have been due to a race
  ** with a writer.  So get a WRITE lock and try again.
  */
  if( badHdr ){
    if( pWal->bShmUnreliable==0 && (pWal->readOnly & WAL_SHM_RDONLY) ){
      if( SQLITE_OK==(rc = walLockShared(pWal, WAL_WRITE_LOCK)) ){
        walUnlockShared(pWal, WAL_WRITE_LOCK);
        rc = SQLITE_READONLY_RECOVERY;
      }
    }else{
      int bWriteLock = pWal->writeLock;
      if( bWriteLock || SQLITE_OK==(rc = walLockWriter(pWal)) ){
        pWal->writeLock = 1;
        if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){
          badHdr = walIndexTryHdr(pWal, pChanged);
          badHdr = walIndexTryHdr(pWal, pChanged, cnt);
          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.
            */
            rc = walIndexRecover(pWal);
            *pChanged = 1;

    if( cnt>=10 ) nDelay = (cnt-9)*(cnt-9)*39;
    sqlite3OsSleep(pWal->pVfs, nDelay);
  }

  if( !useWal ){
    assert( rc==SQLITE_OK );
    if( pWal->bShmUnreliable==0 ){
      rc = walIndexReadHdr(pWal, pChanged);
      rc = walIndexReadHdr(pWal, pChanged, cnt);
    }
    if( rc==SQLITE_BUSY ){
      /* If there is not a recovery running in another thread or process
      ** then convert BUSY errors to WAL_RETRY.  If recovery is known to
      ** be running, convert BUSY to BUSY_RECOVERY.  There is a race here
      ** which might cause WAL_RETRY to be returned even if BUSY_RECOVERY
      ** would be technically correct.  But the race is benign since with

    }
  }


  /* Read the wal-index header. */
  if( rc==SQLITE_OK ){
    walDisableBlocking(pWal);
    rc = walIndexReadHdr(pWal, &isChanged);
    rc = walIndexReadHdr(pWal, &isChanged, 0);
    (void)walEnableBlocking(pWal);
    if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
      sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
    }
  }

  /* Copy data from the log to the database file. */