code: func (r *sentReq) stop(err error) {
  	r.lock.Lock()
  	if !r.stopped {
  		r.stopped = true
  		r.err = err
  		close(r.stopCh)
  	}
  	r.lock.Unlock()
  }
docs: // stop stops the retrieval process and sets an error code that will be returned
  // by getError