code: func (p *pullMediatorImpl) RegisterMsgHook(pullMsgType MsgType, hook MessageHook) {
  	p.Lock()
  	defer p.Unlock()
  	p.msgType2Hook[pullMsgType] = append(p.msgType2Hook[pullMsgType], hook)
  
  }
docs: // RegisterMsgHook registers a message hook to a specific type of pull message