Introduce a new stream for the appservice consumer (#3277)

This introduces a new stream the syncAPI produces to once it processed a
`OutputRoomEvent` and the appservices consumes.
This is to work around a race condition where appservices receive an
event before the syncAPI has handled it, this can result in e.g. calls
to `/joined_members` returning a wrong membership list.

syncapi/consumers/roomserver.go

@@ -31,6 +31,7 @@ import (
 	"github.com/matrix-org/dendrite/setup/jetstream"
 	"github.com/matrix-org/dendrite/setup/process"
 	"github.com/matrix-org/dendrite/syncapi/notifier"
+	"github.com/matrix-org/dendrite/syncapi/producers"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/streams"
 	"github.com/matrix-org/dendrite/syncapi/synctypes"
@@ -55,6 +56,7 @@ type OutputRoomEventConsumer struct {
 	inviteStream streams.StreamProvider
 	notifier     *notifier.Notifier
 	fts          fulltext.Indexer
+	asProducer   *producers.AppserviceEventProducer
 }
 
 // NewOutputRoomEventConsumer creates a new OutputRoomEventConsumer. Call Start() to begin consuming from room servers.
@@ -68,6 +70,7 @@ func NewOutputRoomEventConsumer(
 	inviteStream streams.StreamProvider,
 	rsAPI api.SyncRoomserverAPI,
 	fts *fulltext.Search,
+	asProducer *producers.AppserviceEventProducer,
 ) *OutputRoomEventConsumer {
 	return &OutputRoomEventConsumer{
 		ctx:          process.Context(),
@@ -81,6 +84,7 @@ func NewOutputRoomEventConsumer(
 		inviteStream: inviteStream,
 		rsAPI:        rsAPI,
 		fts:          fts,
+		asProducer:   asProducer,
 	}
 }
 
@@ -119,6 +123,11 @@ func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msgs []*nats.Ms
 			}
 		}
 		err = s.onNewRoomEvent(s.ctx, *output.NewRoomEvent)
+		if err == nil && s.asProducer != nil {
+			if err = s.asProducer.ProduceRoomEvents(msg); err != nil {
+				log.WithError(err).Warn("failed to produce OutputAppserviceEvent")
+			}
+		}
 	case api.OutputTypeOldRoomEvent:
 		err = s.onOldRoomEvent(s.ctx, *output.OldRoomEvent)
 	case api.OutputTypeNewInviteEvent:

syncapi/producers/appservices.go

@@ -0,0 +1,33 @@
+// Copyright 2023 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package producers
+
+import (
+	"github.com/nats-io/nats.go"
+)
+
+// AppserviceEventProducer produces events for the appservice API to consume
+type AppserviceEventProducer struct {
+	Topic     string
+	JetStream nats.JetStreamContext
+}
+
+func (a *AppserviceEventProducer) ProduceRoomEvents(
+	msg *nats.Msg,
+) error {
+	msg.Subject = a.Topic
+	_, err := a.JetStream.PublishMsg(msg)
+	return err
+}

syncapi/syncapi.go

@@ -100,9 +100,16 @@ func AddPublicRoutes(
 		logrus.WithError(err).Panicf("failed to start key change consumer")
 	}
 
+	var asProducer *producers.AppserviceEventProducer
+	if len(dendriteCfg.AppServiceAPI.Derived.ApplicationServices) > 0 {
+		asProducer = &producers.AppserviceEventProducer{
+			JetStream: js, Topic: dendriteCfg.Global.JetStream.Prefixed(jetstream.OutputAppserviceEvent),
+		}
+	}
+
 	roomConsumer := consumers.NewOutputRoomEventConsumer(
 		processContext, &dendriteCfg.SyncAPI, js, syncDB, notifier, streams.PDUStreamProvider,
-		streams.InviteStreamProvider, rsAPI, fts,
+		streams.InviteStreamProvider, rsAPI, fts, asProducer,
 	)
 	if err = roomConsumer.Start(); err != nil {
 		logrus.WithError(err).Panicf("failed to start room server consumer")