// 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
}