// This method is called when we're doing the zero index handling
  // and gives a chance to the queues to do any pre-enqueueing
  // processing they might want to do on the buffer. It should return
  // true if the buffer should be enqueued, or false if enough
  // entries were cleared from it so that it can be re-used. It should
  // not return false if the buffer is still full (otherwise we can
  // get into an infinite loop).

  {- -------------------------------------------
  (1) true をリターンするだけ.
      ---------------------------------------- -}

      virtual bool should_enqueue_buffer() { return true; }