From cd8f7e125172629984f60546446e7c9d261b26c0 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Fri, 14 Oct 2022 15:14:29 +0100
Subject: [PATCH] Set inactivity threshold on durable consumers in the
 roomserver input API (#2795)

This prevents us from holding onto durable consumers indefinitely for
rooms that have long since turned inactive, since they do have a bit of
a processing overhead in the NATS Server. If we clear up a consumer and
then a room becomes active again, the consumer gets recreated as needed.

The threshold is set to 24 hours for now, we can tweak it later if needs
be.
---
 roomserver/internal/input/input.go | 34 +++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/roomserver/internal/input/input.go b/roomserver/internal/input/input.go
index c47793f0..f5099ca1 100644
--- a/roomserver/internal/input/input.go
+++ b/roomserver/internal/input/input.go
@@ -89,6 +89,13 @@ type Inputer struct {
 	Queryer *query.Queryer
 }
 
+// If a room consumer is inactive for a while then we will allow NATS
+// to clean it up. This stops us from holding onto durable consumers
+// indefinitely for rooms that might no longer be active, since they do
+// have an interest overhead in the NATS Server. If the room becomes
+// active again then we'll recreate the consumer anyway.
+const inactiveThreshold = time.Hour * 24
+
 type worker struct {
 	phony.Inbox
 	sync.Mutex
@@ -125,11 +132,12 @@ func (r *Inputer) startWorkerForRoom(roomID string) {
 		if _, err := w.r.JetStream.AddConsumer(
 			r.Cfg.Matrix.JetStream.Prefixed(jetstream.InputRoomEvent),
 			&nats.ConsumerConfig{
-				Durable:       consumer,
-				AckPolicy:     nats.AckAllPolicy,
-				DeliverPolicy: nats.DeliverAllPolicy,
-				FilterSubject: subject,
-				AckWait:       MaximumMissingProcessingTime + (time.Second * 10),
+				Durable:           consumer,
+				AckPolicy:         nats.AckAllPolicy,
+				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)
@@ -145,6 +153,7 @@ func (r *Inputer) startWorkerForRoom(roomID string) {
 			nats.DeliverAll(),
 			nats.AckWait(MaximumMissingProcessingTime+(time.Second*10)),
 			nats.Bind(r.InputRoomEventTopic, consumer),
+			nats.InactiveThreshold(inactiveThreshold),
 		)
 		if err != nil {
 			logrus.WithError(err).Errorf("Failed to subscribe to stream for room %q", w.roomID)
@@ -180,6 +189,21 @@ func (r *Inputer) Start() error {
 		nats.ReplayInstant(),
 		nats.BindStream(r.InputRoomEventTopic),
 	)
+
+	// Make sure that the room consumers have the right config.
+	stream := r.Cfg.Matrix.JetStream.Prefixed(jetstream.InputRoomEvent)
+	for consumer := range r.JetStream.Consumers(stream) {
+		switch {
+		case consumer.Config.Durable == "":
+			continue // Ignore ephemeral consumers
+		case consumer.Config.InactiveThreshold != inactiveThreshold:
+			consumer.Config.InactiveThreshold = inactiveThreshold
+			if _, cerr := r.JetStream.UpdateConsumer(stream, &consumer.Config); cerr != nil {
+				logrus.WithError(cerr).Warnf("Failed to update inactive threshold on consumer %q", consumer.Name)
+			}
+		}
+	}
+
 	return err
 }