code: func (t *servingTask) start() bool {
  	select {
  	case t.token = <-t.sq.tokenCh:
  	default:
  		t.tokenCh = make(chan runToken, 1)
  		select {
  		case t.sq.queueAddCh <- t:
  		case <-t.sq.quit:
  			return false
  		}
  		select {
  		case t.token = <-t.tokenCh:
  		case <-t.sq.quit:
  			return false
  		}
  	}
  	if t.token == nil {
  		return false
  	}
  	t.servingTime -= uint64(mclock.Now())
  	return true
  }
docs: // start blocks until the task can start and returns true if it is allowed to run.
  // Returning false means that the task should be cancelled.