docs: def _peek_async_request(self, msg_id, msg_name):
          """Peek at the set of callbacks for a request.
  
          Return tuple of Nones if callbacks don't exist.
  
          """
          assert get_thread_ident() == self.ioloop_thread_id
          if msg_id is None:
              msg_id = self._msg_id_for_name(msg_name)
          if msg_id in self._async_queue:
              return self._async_queue[msg_id]
          else:
              return None, None, None, None, None