SQLite

** Enter the mutexes in accending order by BtShared pointer address
** to avoid the possibility of deadlock when two threads with
** two or more btrees in common both try to lock all their btrees
** at the same instant.
*/
void sqlite3BtreeEnterAll(sqlite3 *db){
  int i;
  Btree *p, *pLater;
  assert( sqlite3_mutex_held(db->mutex) );
  for(i=0; i<db->nDb; i++){
    p = db->aDb[i].pBt;
    assert( !p || (p->locked==0 && p->sharable) || p->pBt->db==p->db );
    if( p && p->sharable ){
      p->wantToLock++;
      if( !p->locked ){
        assert( p->wantToLock==1 );
        while( p->pPrev ) p = p->pPrev;
        /* Reason for ALWAYS:  There must be at least one unlocked Btree in
        ** the chain.  Otherwise the !p->locked test above would have failed */
        while( p->locked && ALWAYS(p->pNext) ) p = p->pNext;
        for(pLater = p->pNext; pLater; pLater=pLater->pNext){
          if( pLater->locked ){
            unlockBtreeMutex(pLater);
          }
        }
        while( p ){
          lockBtreeMutex(p);
          p = p->pNext;
        }
      }
    }
  }
}
void sqlite3BtreeLeaveAll(sqlite3 *db){
  int i;
  Btree *p;
  assert( sqlite3_mutex_held(db->mutex) );
  for(i=0; i<db->nDb; i++){
    p = db->aDb[i].pBt;
    if( p && p->sharable ){
      assert( p->wantToLock>0 );
      p->wantToLock--;
      if( p->wantToLock==0 ){
        unlockBtreeMutex(p);
      }
    }
  }
}

#ifndef NDEBUG
/*
** Return true if the current thread holds the database connection
** mutex and all required BtShared mutexes.