Merge pull request #10355 from hashicorp/dnephin/stream-fix-topic-head-bug

stream: fix a bug with subscriptions and the latest item in the topic buffers.

agent/consul/stream/event_buffer.go

@@ -81,7 +81,7 @@ func (b *eventBuffer) Append(events []Event) {
 	b.AppendItem(newBufferItem(events))
 }
 
-// AppendBuffer joins another buffer which may be the tail of a separate buffer
+// AppendItem joins another buffer which may be the tail of a separate buffer
 // for example a buffer that's had the events from a snapshot appended may
 // finally by linked to the topic buffer for the subsequent events so
 // subscribers can seamlessly consume the updates. Note that Events in item must
@@ -194,27 +194,19 @@ func (i *bufferItem) Next(ctx context.Context, closed <-chan struct{}) (*bufferI
 	return next, nil
 }
 
-// NextNoBlock returns the next item in the buffer without blocking. If it
+// NextNoBlock returns true and the next item in the buffer without blocking.
-// reaches the most recent item it will return nil.
+// If there are no subsequent items it returns false and an empty item.
-func (i *bufferItem) NextNoBlock() *bufferItem {
+// The empty item will be ignored by subscribers, but has the same link as this
+// bufferItem without the Events.
+// When the link.ch of the empty item is closed, subscriptions are notified of
+// the next item.
+func (i *bufferItem) NextNoBlock() (*bufferItem, bool) {
 	nextRaw := i.link.next.Load()
 	if nextRaw == nil {
-		return nil
-	}
-	return nextRaw.(*bufferItem)
-}
-
-// NextLink returns either the next item in the buffer if there is one, or
-// an empty item (that will be ignored by subscribers) that has a pointer to
-// the same link as this bufferItem (but none of the bufferItem content).
-// When the link.ch is closed, subscriptions will be notified of the next item.
-func (i *bufferItem) NextLink() *bufferItem {
-	next := i.NextNoBlock()
-	if next == nil {
 		// Return an empty item that can be followed to the next item published.
-		return &bufferItem{link: i.link}
+		return &bufferItem{link: i.link}, false
 	}
-	return next
+	return nextRaw.(*bufferItem), true
 }
 
 // HasEventIndex returns true if index matches the Event.Index of this item. Returns

agent/consul/stream/event_buffer_test.go

@@ -17,10 +17,6 @@ func TestEventBufferFuzz(t *testing.T) {
 		t.Skip("too slow for testing.Short")
 	}
 
-	if testing.Short() {
-		t.Skip("too slow for short run")
-	}
-
 	nReaders := 1000
 	nMessages := 1000
 

agent/consul/stream/event_publisher.go

@@ -171,7 +171,8 @@ func (e *EventPublisher) Subscribe(req *SubscribeRequest) (*Subscription, error)
 		subscriptionHead := buf.Head()
 		// splice the rest of the topic buffer onto the subscription buffer so
 		// the subscription will receive new events.
-		buf.AppendItem(topicHead.NextLink())
+		next, _ := topicHead.NextNoBlock()
+		buf.AppendItem(next)
 		return e.subscriptions.add(req, subscriptionHead), nil
 	}
 

agent/consul/stream/event_snapshot.go

@@ -49,34 +49,33 @@ func (s *eventSnapshot) appendAndSplice(req SubscribeRequest, fn SnapshotFunc, t
 func (s *eventSnapshot) spliceFromTopicBuffer(topicBufferHead *bufferItem, idx uint64) {
 	item := topicBufferHead
 	for {
-		next := item.NextNoBlock()
 		switch {
-		case next == nil:
+		case item.Err != nil:
-			// This is the head of the topic buffer (or was just now which is after
-			// the snapshot completed). We don't want any of the events (if any) in
-			// the snapshot buffer as they came before the snapshot but we do need to
-			// wait for the next update.
-			s.buffer.AppendItem(item.NextLink())
-			return
-
-		case next.Err != nil:
 			// This case is not currently possible because errors can only come
 			// from a snapshot func, and this is consuming events from a topic
 			// buffer which does not contain a snapshot.
 			// Handle this case anyway in case errors can come from other places
 			// in the future.
-			s.buffer.AppendItem(next)
+			s.buffer.AppendItem(item)
 			return
 
-		case len(next.Events) > 0 && next.Events[0].Index > idx:
+		case len(item.Events) > 0 && item.Events[0].Index > idx:
 			// We've found an update in the topic buffer that happened after our
 			// snapshot was taken, splice it into the snapshot buffer so subscribers
 			// can continue to read this and others after it.
-			s.buffer.AppendItem(next)
+			s.buffer.AppendItem(item)
 			return
 		}
 
-		// We don't need this item, continue to next
+		next, ok := item.NextNoBlock()
+		if !ok {
+			// We reached the head of the topic buffer. We don't want any of the
+			// events in the topic buffer as they came before the snapshot.
+			// Append a link to any future items.
+			s.buffer.AppendItem(next)
+			return
+		}
+		// Proceed to the next item in the topic buffer
 		item = next
 	}
 }