func (t *Throttle) Do() error {
	for {
		select {
		case t.ch <- struct{}{}:
			t.wg.Add(1)
			return nil
		case err := <-t.errCh:
			if err != nil {
				return err
			}
		}
	}
}
func (t *Throttle) Do() error {
	for {
		select {
		case t.ch <- struct{}{}:
			t.wg.Add(1)
			return nil
		case err := <-t.errCh:
			if err != nil {
				return err
			}
		}
	}
}
// Do should be called by workers before they start working. It blocks if there
// are already maximum number of workers working. If it detects an error from
// previously Done workers, it would return it.