code: func (d *requestDistributor) cancel(r *distReq) bool {
  	d.lock.Lock()
  	defer d.lock.Unlock()
  
  	if r.sentChn == nil {
  		return false
  	}
  
  	close(r.sentChn)
  	d.remove(r)
  	return true
  }
docs: // cancel removes a request from the queue if it has not been sent yet (returns
  // false if it has been sent already). It is guaranteed that the callback functions
  // will not be called after cancel returns.