Use AckExplicitPolicy instead of AckAllPolicy, fix back pressure metric

roomserver/internal/input/input.go

@@ -119,9 +119,14 @@ func (r *Inputer) startWorkerForRoom(roomID string) {
 	w.Lock()
 	defer w.Unlock()
 	if !loaded || w.subscription == nil {
+		streamName := r.Cfg.Matrix.JetStream.Prefixed(jetstream.InputRoomEvent)
 		consumer := r.Cfg.Matrix.JetStream.Prefixed("RoomInput" + jetstream.Tokenise(w.roomID))
 		subject := r.Cfg.Matrix.JetStream.Prefixed(jetstream.InputRoomEventSubj(w.roomID))
 
+		logger := logrus.WithFields(logrus.Fields{
+			"stream_name": streamName,
+			"consumer":    consumer,
+		})
 		// Create the consumer. We do this as a specific step rather than
 		// letting PullSubscribe create it for us because we need the consumer
 		// to outlive the subscription. If we do it this way, we can Bind in the
@@ -135,20 +140,51 @@ func (r *Inputer) startWorkerForRoom(roomID string) {
 		// before it. This is necessary because otherwise our consumer will never
 		// acknowledge things we filtered out for other subjects and therefore they
 		// will linger around forever.
-		if _, err := w.r.JetStream.AddConsumer(
-			r.Cfg.Matrix.JetStream.Prefixed(jetstream.InputRoomEvent),
-			&nats.ConsumerConfig{
+
+		info, err := w.r.JetStream.ConsumerInfo(streamName, consumer)
+		if err != nil {
+			// log and return, we will retry anyway
+			logger.WithError(err).Errorf("failed to get consumer info")
+			return
+		}
+
+		consumerConfig := &nats.ConsumerConfig{
 			Durable:           consumer,
-				AckPolicy:         nats.AckAllPolicy,
+			AckPolicy:         nats.AckExplicitPolicy,
 			DeliverPolicy:     nats.DeliverAllPolicy,
 			FilterSubject:     subject,
 			AckWait:           MaximumMissingProcessingTime + (time.Second * 10),
 			InactiveThreshold: inactiveThreshold,
-			},
-		); err != nil {
-			logrus.WithError(err).Errorf("Failed to create consumer for room %q", w.roomID)
+		}
+
+		// The consumer already exists, try to update if necessary.
+		if info != nil {
+			switch {
+			case info.Config.AckPolicy != consumerConfig.AckPolicy:
+				logger.Warn("Consumer already exists, trying to update it.")
+				// Try updating the consumer first
+				if _, err = w.r.JetStream.UpdateConsumer(streamName, consumerConfig); err != nil {
+					// We failed to update the consumer, recreate it
+					logger.WithError(err).Warnf("Unable to update consumer %q, recreating...", consumer)
+					if err = w.r.JetStream.DeleteConsumer(streamName, consumer); err != nil {
+						logger.WithError(err).Fatalf("Unable to delete consumer %q", consumer)
 						return
 					}
+					info = nil
+				}
+			}
+		}
+
+		if info == nil {
+			// Create the consumer with the correct config
+			if _, err = w.r.JetStream.AddConsumer(
+				r.Cfg.Matrix.JetStream.Prefixed(jetstream.InputRoomEvent),
+				consumerConfig,
+			); err != nil {
+				logger.WithError(err).Errorf("Failed to create consumer for room %q", w.roomID)
+				return
+			}
+		}
 
 		// Bind to our durable consumer. We want to receive all messages waiting
 		// for this subject and we want to manually acknowledge them, so that we
@@ -162,7 +198,7 @@ func (r *Inputer) startWorkerForRoom(roomID string) {
 			nats.InactiveThreshold(inactiveThreshold),
 		)
 		if err != nil {
-			logrus.WithError(err).Errorf("Failed to subscribe to stream for room %q", w.roomID)
+			logger.WithError(err).Errorf("Failed to subscribe to stream for room %q", w.roomID)
 			return
 		}
 
@@ -276,7 +312,7 @@ func (w *worker) _next() {
 	if scope := sentry.CurrentHub().Scope(); scope != nil {
 		scope.SetTag("event_id", inputRoomEvent.Event.EventID())
 	}
-	roomserverInputBackpressure.With(prometheus.Labels{"room_id": w.roomID}).Inc()
+
 	defer roomserverInputBackpressure.With(prometheus.Labels{"room_id": w.roomID}).Dec()
 
 	// Process the room event. If something goes wrong then we'll tell
@@ -381,6 +417,7 @@ func (r *Inputer) queueInputRoomEvents(
 			}).Error("Roomserver failed to queue async event")
 			return nil, fmt.Errorf("r.JetStream.PublishMsg: %w", err)
 		}
+		roomserverInputBackpressure.With(prometheus.Labels{"room_id": roomID}).Inc()
 	}
 	return
 }