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storage: fix bug where WatchList would (rarely) return duplicate events (#17067)

This commit is contained in:
Dan Upton 2023-04-25 11:48:13 +01:00 committed by GitHub
parent e66b18d048
commit ba4a314772
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1 changed files with 21 additions and 0 deletions
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21
internal/storage/inmem/watch.go
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@ -26,6 +26,7 @@ type Watch struct {
// Next returns the next WatchEvent, blocking until one is available.
func (w *Watch) Next(ctx context.Context) (*pbresource.WatchEvent, error) {
var idx uint64
for {
e, err := w.nextEvent(ctx)
if err == stream.ErrSubForceClosed {
@ -35,6 +36,26 @@ func (w *Watch) Next(ctx context.Context) (*pbresource.WatchEvent, error) {
return nil, err
}
// This works around a *very* rare race-condition in the EventPublisher where
// it's possible to see duplicate events when events are published at the same
// time as the first subscription is created on a {topic, subject} pair.
//
// We see this problem when a call to WriteCAS is happening in parallel with
// a call to WatchList. It happens because our snapshot handler returns events
// that have not yet been published (in the gap between us committing changes
// to MemDB and the EventPublisher dispatching events onto its event buffers).
//
// An intuitive solution to this problem would be to take eventLock in the
// snapshot handler to avoid it racing with publishing, but this does not
// work because publishing is asynchronous.
//
// We should fix this problem at the root, but it's complicated, so for now
// we'll work around it.
if e.Index <= idx {
continue
}
idx = e.Index
event := e.Payload.(eventPayload).event
if w.query.matches(event.Resource) {
return event, nil