// Dequeue returns Enqueue()'d items in FIFO order. If the // queue is empty, Dequeue blocks until items are available. func (q *PriorityQueue) Dequeue() (string, error) { // TODO: fewer round trips by fetching more than one key resp, err := q.client.Get(q.ctx, q.key, v3.WithFirstKey()...) if err != nil { return "", err } kv, err := claimFirstKey(q.client, resp.Kvs) if err != nil { return "", err } else if kv != nil { return string(kv.Value), nil } else if resp.More { // missed some items, retry to read in more return q.Dequeue() } // nothing to dequeue; wait on items ev, err := WaitPrefixEvents( q.client, q.key, resp.Header.Revision, []mvccpb.Event_EventType{mvccpb.PUT}) if err != nil { return "", err } ok, err := deleteRevKey(q.client, string(ev.Kv.Key), ev.Kv.ModRevision) if err != nil { return "", err } else if !ok { return q.Dequeue() } return string(ev.Kv.Value), err }
// Wait blocks on the barrier key until it is deleted. If there is no key, Wait // assumes Release has already been called and returns immediately. func (b *Barrier) Wait() error { resp, err := b.client.Get(b.ctx, b.key, v3.WithFirstKey()...) if err != nil { return err } if len(resp.Kvs) == 0 { // key already removed return nil } _, err = WaitEvents( b.client, b.key, resp.Header.Revision, []storagepb.Event_EventType{storagepb.PUT, storagepb.DELETE}) return err }