Send response back to the server.

def SendReply(self,
                rdf_value=None,
                session_id=None,
                message_type=rdf_flows.GrrMessage.Type.MESSAGE):
    """Send response back to the server."""
    # TODO(hanuszczak): This is pretty bad. Here we assume that if the session
    # id is not none we are "replying" to a well-known flow. If we are replying
    # to a well-known flow we cannot increment the response id (since these are
    # not really responses) because otherwise the parent flow will think it is
    # missing some responses.
    #
    # Moreover, the message queue (at least in the flow test helper workflow)
    # expects flow messages to have request id equal to 0. This is rather
    # convoluted and should be untangled in the future.
    #
    # Once we have a separate method for communicating with well-known flows
    # this if-statement should no longer be relevant (since setting custom
    # session ids would become illegal).
    if session_id is None:
      response_id = self.response_id
      request_id = self.message.request_id
      session_id = self.message.session_id
      self.response_id += 1
    else:
      response_id = 0
      request_id = 0

    self.grr_worker.SendReply(
        rdf_value,
        # This is not strictly necessary but adds context
        # to this response.
        name=self.__class__.__name__,
        session_id=session_id,
        response_id=response_id,
        request_id=request_id,
        message_type=message_type,
        task_id=self.message.Get("task_id") or None,
        require_fastpoll=self.require_fastpoll)