SQLite

** This function is called while stepping or preparing a statement 
** associated with connection db. The operation will return SQLITE_LOCKED
** to the user because it requires a lock that will not be available
** until connection pBlocker concludes its current transaction.
*/
void sqlite3ConnectionBlocked(sqlite3 *db, sqlite3 *pBlocker){
  enterMutex();
  if( db->pBlockingConnection==0 ){
    /* 
    ** We can not register an unlock callback unless we think we are 
    ** blocked.
    */
    assert( db->pUnlockConnection==0 );
    addToBlockedList(db);
  }
  db->pBlockingConnection = pBlocker;
  leaveMutex();
}

/*

      xUnlockNotify = p->xUnlockNotify;
      p->pUnlockConnection = 0;
      p->xUnlockNotify = 0;
      p->pUnlockArg = 0;
    }

    /* Step 3. */
    if( p->pBlockingConnection==0 ){
      /* 
      ** If we were blocked on db, we would set
      ** pBlockingConnection to 0 above.  And we can 
      ** only wait on a connection we are blocked on.
      ** So if we were waiting on db (pUnlockConnection==db)
      ** then it would have been set to 0 above as well.
      */
      assert( p->pUnlockConnection==0 );
      /* Remove connection p from the blocked connections list. */
      *pp = p->pNextBlocked;
      p->pNextBlocked = 0;
    }else{
      pp = &p->pNextBlocked;
    }
  }