p = pDbFd->pShm;
  if( p==0 ) return SQLITE_OK;
  pShmNode = p->pShmNode;

  assert( pShmNode==pDbFd->pInode->pShmNode );
  assert( pShmNode->pInode==pDbFd->pInode );

  /* Verify that the connection being closed holds no locks */
  assert( p->exclMask==0 );
  assert( p->sharedMask==0 );

  /* Remove connection p from the set of connections associated
  ** with pShmNode */
  sqlite3_mutex_enter(pShmNode->mutex);
  for(pp=&pShmNode->pFirst; (*pp)!=p; pp = &(*pp)->pNext){}
  *pp = p->pNext;

  /* Free the connection p */

  p = pDbFd->pShm;
  if( p==0 ) return SQLITE_OK;
  pShmNode = p->pShmNode;

  assert( pShmNode==pDbFd->pInode->pShmNode );
  assert( pShmNode->pInode==pDbFd->pInode );





  /* Remove connection p from the set of connections associated
  ** with pShmNode */
  sqlite3_mutex_enter(pShmNode->mutex);
  for(pp=&pShmNode->pFirst; (*pp)!=p; pp = &(*pp)->pNext){}
  *pp = p->pNext;

  /* Free the connection p */

    pPager->state = PAGER_SHARED;

    /* If the connection was in locking_mode=exclusive mode but is no longer,
    ** drop the EXCLUSIVE lock held on the database file.
    */
    if( rc2==SQLITE_OK 
     && !pPager->exclusiveMode 
     && sqlite3WalExclusiveMode(pPager->pWal, -1) 
    ){
      sqlite3WalExclusiveMode(pPager->pWal, 0);
      rc2 = osUnlock(pPager->fd, SHARED_LOCK);
    }
  }else if( !pPager->exclusiveMode ){
    rc2 = osUnlock(pPager->fd, SHARED_LOCK);
    pPager->state = PAGER_SHARED;
    pPager->changeCountDone = 0;
  }else if( pPager->state==PAGER_SYNCED ){
................................................................................
** other writers or checkpointers.
*/
static int pagerBeginReadTransaction(Pager *pPager){
  int rc;                         /* Return code */
  int changed = 0;                /* True if cache must be reset */

  assert( pagerUseWal(pPager) );








  rc = sqlite3WalBeginReadTransaction(pPager->pWal, &changed);
  if( rc==SQLITE_OK ){
    int dummy;
    if( changed ){
      pager_reset(pPager);
      assert( pPager->errCode || pPager->dbSizeValid==0 );
................................................................................
    assert( pPager->pInJournal==0 );
    assert( !MEMDB && !pPager->tempFile );

    if( pagerUseWal(pPager) ){
      /* If the pager is configured to use locking_mode=exclusive, and an
      ** exclusive lock on the database is not already held, obtain it now.
      */
      if( pPager->exclusiveMode && !sqlite3WalExclusiveMode(pPager->pWal, -1) ){
        rc = sqlite3OsLock(pPager->fd, EXCLUSIVE_LOCK);
        pPager->state = PAGER_SHARED;
        if( rc!=SQLITE_OK ){
          return rc;
        }
        sqlite3WalExclusiveMode(pPager->pWal, 1);
      }

    pPager->state = PAGER_SHARED;

    /* If the connection was in locking_mode=exclusive mode but is no longer,
    ** drop the EXCLUSIVE lock held on the database file.
    */
    if( rc2==SQLITE_OK 
     && !pPager->exclusiveMode 
     && sqlite3WalExclusiveMode(pPager->pWal, 0) 
    ){

      rc2 = osUnlock(pPager->fd, SHARED_LOCK);
    }
  }else if( !pPager->exclusiveMode ){
    rc2 = osUnlock(pPager->fd, SHARED_LOCK);
    pPager->state = PAGER_SHARED;
    pPager->changeCountDone = 0;
  }else if( pPager->state==PAGER_SYNCED ){
................................................................................
** other writers or checkpointers.
*/
static int pagerBeginReadTransaction(Pager *pPager){
  int rc;                         /* Return code */
  int changed = 0;                /* True if cache must be reset */

  assert( pagerUseWal(pPager) );

  /* sqlite3WalEndReadTransaction() was not called for the previous
  ** transaction in locking_mode=EXCLUSIVE.  So call it now.  If we
  ** are in locking_mode=NORMAL and EndRead() was previously called,
  ** the duplicate call is harmless.
  */
  sqlite3WalEndReadTransaction(pPager->pWal);

  rc = sqlite3WalBeginReadTransaction(pPager->pWal, &changed);
  if( rc==SQLITE_OK ){
    int dummy;
    if( changed ){
      pager_reset(pPager);
      assert( pPager->errCode || pPager->dbSizeValid==0 );
................................................................................
    assert( pPager->pInJournal==0 );
    assert( !MEMDB && !pPager->tempFile );

    if( pagerUseWal(pPager) ){
      /* If the pager is configured to use locking_mode=exclusive, and an
      ** exclusive lock on the database is not already held, obtain it now.
      */
      if( pPager->exclusiveMode && sqlite3WalExclusiveMode(pPager->pWal, -1) ){
        rc = sqlite3OsLock(pPager->fd, EXCLUSIVE_LOCK);
        pPager->state = PAGER_SHARED;
        if( rc!=SQLITE_OK ){
          return rc;
        }
        sqlite3WalExclusiveMode(pPager->pWal, 1);
      }

  volatile WalIndexHdr *pHdr;     /* Header of the wal-index */
  volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */
  u32 mxReadMark;                 /* Largest aReadMark[] value */
  int mxI;                        /* Index of largest aReadMark[] value */
  int i;                          /* Loop counter */
  int rc;                         /* Return code  */

  assert( pWal->readLock<0 );  /* No read lock held on entry */

  if( !useWal ){
    rc = walIndexReadHdr(pWal, pChanged);
    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
................................................................................
    ret = pWal->iCallback;
    pWal->iCallback = 0;
  }
  return (int)ret;
}

/*
** This function is called to set or query the exclusive-mode flag 
** associated with the WAL connection passed as the first argument. The
** exclusive-mode flag should be set to indicate that the caller is
** holding an EXCLUSIVE lock on the database file (it does this in
** locking_mode=exclusive mode). If the EXCLUSIVE lock is to be dropped,
** the flag set by this function should be cleared before doing so.




**
** When the flag is set, this module does not call the VFS xShmLock()
** method to obtain any locks on the wal-index (as it assumes it
** has exclusive access to the wal and wal-index files anyhow). It
** continues to hold (and does not drop) the existing READ lock on
** the wal-index.





**
** To set or clear the flag, the "op" parameter is passed 1 or 0,
** respectively. To query the flag, pass -1. In all cases, the value
** returned is the value of the exclusive-mode flag (after its value
** has been modified, if applicable).




*/
int sqlite3WalExclusiveMode(Wal *pWal, int op){


  if( op>=0 ){

    pWal->exclusiveMode = (u8)op;


  }










  return pWal->exclusiveMode;





}

#endif /* #ifndef SQLITE_OMIT_WAL */

  volatile WalIndexHdr *pHdr;     /* Header of the wal-index */
  volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */
  u32 mxReadMark;                 /* Largest aReadMark[] value */
  int mxI;                        /* Index of largest aReadMark[] value */
  int i;                          /* Loop counter */
  int rc;                         /* Return code  */

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

  if( !useWal ){
    rc = walIndexReadHdr(pWal, pChanged);
    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
................................................................................
    ret = pWal->iCallback;
    pWal->iCallback = 0;
  }
  return (int)ret;
}

/*
** This function is called to change the WAL subsystem into or out
** of locking_mode=EXCLUSIVE.
**
** If op is zero, then attempt to change from locking_mode=EXCLUSIVE
** into locking_mode=NORMAL.  This means that we must acquire a lock
** on the pWal->readLock byte.  If the WAL is already in locking_mode=NORMAL
** or if the acquisition of the lock fails, then return 0.  If the
** transition out of exclusive-mode is successful, return 1.  This
** operation must occur while the pager is still holding the exclusive
** lock on the main database file.
**




** If op is one, then change from locking_mode=NORMAL into 
** locking_mode=EXCLUSIVE.  This means that the pWal->readLock must
** be released.  Return 1 if the transition is made and 0 if the
** WAL is already in exclusive-locking mode - meaning that this
** routine is a no-op.  The pager must already hold the exclusive lock
** on the main database file before invoking this operation.
**




** If op is negative, then do a dry-run of the op==1 case but do
** not actually change anything.  The pager uses this to see if it
** should acquire the database exclusive lock prior to invoking
** the op==1 case.
*/
int sqlite3WalExclusiveMode(Wal *pWal, int op){
  int rc;
  assert( pWal->writeLock==0 && pWal->readLock>=0 );
  if( op==0 ){
    if( pWal->exclusiveMode ){
      pWal->exclusiveMode = 0;
      if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){
        pWal->exclusiveMode = 1;
      }
      rc = pWal->exclusiveMode==0;
    }else{
      /* No changes.  Either already in locking_mode=NORMAL or else the 
      ** acquisition of the read-lock failed.  The pager must continue to
      ** hold the database exclusive lock. */
      rc = 0;
    }
  }else if( op>0 ){
    assert( pWal->exclusiveMode==0 );
    walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
    pWal->exclusiveMode = 1;
    rc = 1;
  }else{
    rc = pWal->exclusiveMode==0;
  }
  return rc;
}

#endif /* #ifndef SQLITE_OMIT_WAL */