Allow batching in `JetStreamConsumer` (#2686)

This allows us to receive more than one message from NATS at a time if we want.

19 files changed, 88 insertions, 55 deletions

diff --git a/appservice/consumers/roomserver.go b/appservice/consumers/roomserver.go
index d567408b..21b52bc3 100644
--- a/appservice/consumers/roomserver.go
+++ b/appservice/consumers/roomserver.go
@@ -68,14 +68,15 @@ func NewOutputRoomEventConsumer(
 // Start consuming from room servers
 func (s *OutputRoomEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
 // onMessage is called when the appservice component receives a new event from
 // the room server output log.
-func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// Parse out the event JSON
 	var output api.OutputEvent
 	if err := json.Unmarshal(msg.Data, &output); err != nil {
diff --git a/federationapi/consumers/keychange.go b/federationapi/consumers/keychange.go
index 6d3cf0e4..f3314bc9 100644
--- a/federationapi/consumers/keychange.go
+++ b/federationapi/consumers/keychange.go
@@ -67,14 +67,15 @@ func NewKeyChangeConsumer(
 // Start consuming from key servers
 func (t *KeyChangeConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		t.ctx, t.jetstream, t.topic, t.durable, t.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		t.ctx, t.jetstream, t.topic, t.durable, 1,
+		t.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
 // onMessage is called in response to a message received on the
 // key change events topic from the key server.
-func (t *KeyChangeConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (t *KeyChangeConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	var m api.DeviceMessage
 	if err := json.Unmarshal(msg.Data, &m); err != nil {
 		logrus.WithError(err).Errorf("failed to read device message from key change topic")
diff --git a/federationapi/consumers/presence.go b/federationapi/consumers/presence.go
index a65d2aa0..e76103cd 100644
--- a/federationapi/consumers/presence.go
+++ b/federationapi/consumers/presence.go
@@ -69,14 +69,15 @@ func (t *OutputPresenceConsumer) Start() error {
 		return nil
 	}
 	return jetstream.JetStreamConsumer(
-		t.ctx, t.jetstream, t.topic, t.durable, t.onMessage,
+		t.ctx, t.jetstream, t.topic, t.durable, 1, t.onMessage,
 		nats.DeliverAll(), nats.ManualAck(), nats.HeadersOnly(),
 	)
 }
 
 // onMessage is called in response to a message received on the presence
 // events topic from the client api.
-func (t *OutputPresenceConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (t *OutputPresenceConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// only send presence events which originated from us
 	userID := msg.Header.Get(jetstream.UserID)
 	_, serverName, err := gomatrixserverlib.SplitID('@', userID)
diff --git a/federationapi/consumers/receipts.go b/federationapi/consumers/receipts.go
index 2c9d79bc..366cb264 100644
--- a/federationapi/consumers/receipts.go
+++ b/federationapi/consumers/receipts.go
@@ -65,14 +65,15 @@ func NewOutputReceiptConsumer(
 // Start consuming from the clientapi
 func (t *OutputReceiptConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		t.ctx, t.jetstream, t.topic, t.durable, t.onMessage,
+		t.ctx, t.jetstream, t.topic, t.durable, 1, t.onMessage,
 		nats.DeliverAll(), nats.ManualAck(), nats.HeadersOnly(),
 	)
 }
 
 // onMessage is called in response to a message received on the receipt
 // events topic from the client api.
-func (t *OutputReceiptConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (t *OutputReceiptConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	receipt := syncTypes.OutputReceiptEvent{
 		UserID:  msg.Header.Get(jetstream.UserID),
 		RoomID:  msg.Header.Get(jetstream.RoomID),
diff --git a/federationapi/consumers/roomserver.go b/federationapi/consumers/roomserver.go
index 2622ecb3..349b50b0 100644
--- a/federationapi/consumers/roomserver.go
+++ b/federationapi/consumers/roomserver.go
@@ -68,8 +68,8 @@ func NewOutputRoomEventConsumer(
 // Start consuming from room servers
 func (s *OutputRoomEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
@@ -77,7 +77,8 @@ func (s *OutputRoomEventConsumer) Start() error {
 // It is unsafe to call this with messages for the same room in multiple gorountines
 // because updates it will likely fail with a types.EventIDMismatchError when it
 // realises that it cannot update the room state using the deltas.
-func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// Parse out the event JSON
 	var output api.OutputEvent
 	if err := json.Unmarshal(msg.Data, &output); err != nil {
diff --git a/federationapi/consumers/sendtodevice.go b/federationapi/consumers/sendtodevice.go
index f99a895e..e44bad72 100644
--- a/federationapi/consumers/sendtodevice.go
+++ b/federationapi/consumers/sendtodevice.go
@@ -63,14 +63,15 @@ func NewOutputSendToDeviceConsumer(
 // Start consuming from the client api
 func (t *OutputSendToDeviceConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		t.ctx, t.jetstream, t.topic, t.durable, t.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		t.ctx, t.jetstream, t.topic, t.durable, 1,
+		t.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
 // onMessage is called in response to a message received on the
 // send-to-device events topic from the client api.
-func (t *OutputSendToDeviceConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (t *OutputSendToDeviceConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// only send send-to-device events which originated from us
 	sender := msg.Header.Get("sender")
 	_, originServerName, err := gomatrixserverlib.SplitID('@', sender)
diff --git a/federationapi/consumers/typing.go b/federationapi/consumers/typing.go
index 428e1a86..9c737913 100644
--- a/federationapi/consumers/typing.go
+++ b/federationapi/consumers/typing.go
@@ -62,14 +62,15 @@ func NewOutputTypingConsumer(
 // Start consuming from the clientapi
 func (t *OutputTypingConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		t.ctx, t.jetstream, t.topic, t.durable, t.onMessage,
+		t.ctx, t.jetstream, t.topic, t.durable, 1, t.onMessage,
 		nats.DeliverAll(), nats.ManualAck(), nats.HeadersOnly(),
 	)
 }
 
 // onMessage is called in response to a message received on the typing
 // events topic from the client api.
-func (t *OutputTypingConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (t *OutputTypingConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// Extract the typing event from msg.
 	roomID := msg.Header.Get(jetstream.RoomID)
 	userID := msg.Header.Get(jetstream.UserID)
diff --git a/keyserver/consumers/devicelistupdate.go b/keyserver/consumers/devicelistupdate.go
index f4f24628..d15f9426 100644
--- a/keyserver/consumers/devicelistupdate.go
+++ b/keyserver/consumers/devicelistupdate.go
@@ -55,14 +55,15 @@ func NewDeviceListUpdateConsumer(
 // Start consuming from key servers
 func (t *DeviceListUpdateConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		t.ctx, t.jetstream, t.topic, t.durable, t.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		t.ctx, t.jetstream, t.topic, t.durable, 1,
+		t.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
 // onMessage is called in response to a message received on the
 // key change events topic from the key server.
-func (t *DeviceListUpdateConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (t *DeviceListUpdateConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	var m gomatrixserverlib.DeviceListUpdateEvent
 	if err := json.Unmarshal(msg.Data, &m); err != nil {
 		logrus.WithError(err).Errorf("Failed to read from device list update input topic")
diff --git a/setup/jetstream/helpers.go b/setup/jetstream/helpers.go
index 1c07583e..f47637c6 100644
--- a/setup/jetstream/helpers.go
+++ b/setup/jetstream/helpers.go
@@ -9,9 +9,16 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+// JetStreamConsumer starts a durable consumer on the given subject with the
+// given durable name. The function will be called when one or more messages
+// is available, up to the maximum batch size specified. If the batch is set to
+// 1 then messages will be delivered one at a time. If the function is called,
+// the messages array is guaranteed to be at least 1 in size. Any provided NATS
+// options will be passed through to the pull subscriber creation. The consumer
+// will continue to run until the context expires, at which point it will stop.
 func JetStreamConsumer(
-	ctx context.Context, js nats.JetStreamContext, subj, durable string,
-	f func(ctx context.Context, msg *nats.Msg) bool,
+	ctx context.Context, js nats.JetStreamContext, subj, durable string, batch int,
+	f func(ctx context.Context, msgs []*nats.Msg) bool,
 	opts ...nats.SubOpt,
 ) error {
 	defer func() {
@@ -27,6 +34,14 @@ func JetStreamConsumer(
 		}
 	}()
 
+	// If the batch size is greater than 1, we will want to acknowledge all
+	// received messages in the batch. Below we will send an acknowledgement
+	// for the most recent message in the batch and AckAll will ensure that
+	// all messages that came before it are also acknowledged implicitly.
+	if batch > 1 {
+		opts = append(opts, nats.AckAll())
+	}
+
 	name := durable + "Pull"
 	sub, err := js.PullSubscribe(subj, name, opts...)
 	if err != nil {
@@ -50,7 +65,7 @@ func JetStreamConsumer(
 			// enforce its own deadline (roughly 5 seconds by default). Therefore
 			// it is our responsibility to check whether our context expired or
 			// not when a context error is returned. Footguns. Footguns everywhere.
-			msgs, err := sub.Fetch(1, nats.Context(ctx))
+			msgs, err := sub.Fetch(batch, nats.Context(ctx))
 			if err != nil {
 				if err == context.Canceled || err == context.DeadlineExceeded {
 					// Work out whether it was the JetStream context that expired
@@ -74,13 +89,13 @@ func JetStreamConsumer(
 			if len(msgs) < 1 {
 				continue
 			}
-			msg := msgs[0]
+			msg := msgs[len(msgs)-1] // most recent message, in case of AckAll
 			if err = msg.InProgress(nats.Context(ctx)); err != nil {
 				logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.InProgress: %w", err))
 				sentry.CaptureException(err)
 				continue
 			}
-			if f(ctx, msg) {
+			if f(ctx, msgs) {
 				if err = msg.AckSync(nats.Context(ctx)); err != nil {
 					logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.AckSync: %w", err))
 					sentry.CaptureException(err)
diff --git a/syncapi/consumers/clientapi.go b/syncapi/consumers/clientapi.go
index 02633b56..f0588cab 100644
--- a/syncapi/consumers/clientapi.go
+++ b/syncapi/consumers/clientapi.go
@@ -75,15 +75,16 @@ func NewOutputClientDataConsumer(
 // Start consuming from room servers
 func (s *OutputClientDataConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
 // onMessage is called when the sync server receives a new event from the client API server output log.
 // It is not safe for this function to be called from multiple goroutines, or else the
 // sync stream position may race and be incorrectly calculated.
-func (s *OutputClientDataConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputClientDataConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// Parse out the event JSON
 	userID := msg.Header.Get(jetstream.UserID)
 	var output eventutil.AccountData
diff --git a/syncapi/consumers/keychange.go b/syncapi/consumers/keychange.go
index c8d88dda..c42e7197 100644
--- a/syncapi/consumers/keychange.go
+++ b/syncapi/consumers/keychange.go
@@ -75,12 +75,13 @@ func NewOutputKeyChangeEventConsumer(
 // Start consuming from the key server
 func (s *OutputKeyChangeEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
-func (s *OutputKeyChangeEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputKeyChangeEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	var m api.DeviceMessage
 	if err := json.Unmarshal(msg.Data, &m); err != nil {
 		logrus.WithError(err).Errorf("failed to read device message from key change topic")
diff --git a/syncapi/consumers/presence.go b/syncapi/consumers/presence.go
index db7d67fa..61bdc13d 100644
--- a/syncapi/consumers/presence.go
+++ b/syncapi/consumers/presence.go
@@ -128,12 +128,13 @@ func (s *PresenceConsumer) Start() error {
 		return nil
 	}
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.presenceTopic, s.durable, s.onMessage,
+		s.ctx, s.jetstream, s.presenceTopic, s.durable, 1, s.onMessage,
 		nats.DeliverAll(), nats.ManualAck(), nats.HeadersOnly(),
 	)
 }
 
-func (s *PresenceConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *PresenceConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	userID := msg.Header.Get(jetstream.UserID)
 	presence := msg.Header.Get("presence")
 	timestamp := msg.Header.Get("last_active_ts")
diff --git a/syncapi/consumers/receipts.go b/syncapi/consumers/receipts.go
index 83156cf9..a18244c4 100644
--- a/syncapi/consumers/receipts.go
+++ b/syncapi/consumers/receipts.go
@@ -74,12 +74,13 @@ func NewOutputReceiptEventConsumer(
 // Start consuming receipts events.
 func (s *OutputReceiptEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
-func (s *OutputReceiptEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputReceiptEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	output := types.OutputReceiptEvent{
 		UserID:  msg.Header.Get(jetstream.UserID),
 		RoomID:  msg.Header.Get(jetstream.RoomID),
diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index f77b1673..6979eb48 100644
--- a/syncapi/consumers/roomserver.go
+++ b/syncapi/consumers/roomserver.go
@@ -79,15 +79,16 @@ func NewOutputRoomEventConsumer(
 // Start consuming from room servers
 func (s *OutputRoomEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
 // onMessage is called when the sync server receives a new event from the room server output log.
 // It is not safe for this function to be called from multiple goroutines, or else the
 // sync stream position may race and be incorrectly calculated.
-func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	// Parse out the event JSON
 	var err error
 	var output api.OutputEvent
diff --git a/syncapi/consumers/sendtodevice.go b/syncapi/consumers/sendtodevice.go
index 0b9153fc..89b01d7e 100644
--- a/syncapi/consumers/sendtodevice.go
+++ b/syncapi/consumers/sendtodevice.go
@@ -68,12 +68,13 @@ func NewOutputSendToDeviceEventConsumer(
 // Start consuming send-to-device events.
 func (s *OutputSendToDeviceEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
-func (s *OutputSendToDeviceEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputSendToDeviceEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	userID := msg.Header.Get(jetstream.UserID)
 	_, domain, err := gomatrixserverlib.SplitID('@', userID)
 	if err != nil {
diff --git a/syncapi/consumers/typing.go b/syncapi/consumers/typing.go
index 48e484ec..88db80f8 100644
--- a/syncapi/consumers/typing.go
+++ b/syncapi/consumers/typing.go
@@ -64,12 +64,13 @@ func NewOutputTypingEventConsumer(
 // Start consuming typing events.
 func (s *OutputTypingEventConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
-func (s *OutputTypingEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputTypingEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	roomID := msg.Header.Get(jetstream.RoomID)
 	userID := msg.Header.Get(jetstream.UserID)
 	typing, err := strconv.ParseBool(msg.Header.Get("typing"))
diff --git a/syncapi/consumers/userapi.go b/syncapi/consumers/userapi.go
index 010fa7c8..22782352 100644
--- a/syncapi/consumers/userapi.go
+++ b/syncapi/consumers/userapi.go
@@ -67,8 +67,8 @@ func NewOutputNotificationDataConsumer(
 // Start starts consumption.
 func (s *OutputNotificationDataConsumer) Start() error {
 	return jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	)
 }
 
@@ -76,7 +76,8 @@ func (s *OutputNotificationDataConsumer) Start() error {
 // the push server. It is not safe for this function to be called from
 // multiple goroutines, or else the sync stream position may race and
 // be incorrectly calculated.
-func (s *OutputNotificationDataConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputNotificationDataConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	userID := string(msg.Header.Get(jetstream.UserID))
 
 	// Parse out the event JSON
diff --git a/userapi/consumers/syncapi_readupdate.go b/userapi/consumers/syncapi_readupdate.go
index 067f9333..54654f75 100644
--- a/userapi/consumers/syncapi_readupdate.go
+++ b/userapi/consumers/syncapi_readupdate.go
@@ -56,15 +56,16 @@ func NewOutputReadUpdateConsumer(
 
 func (s *OutputReadUpdateConsumer) Start() error {
 	if err := jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	); err != nil {
 		return err
 	}
 	return nil
 }
 
-func (s *OutputReadUpdateConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputReadUpdateConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	var read types.ReadUpdate
 	if err := json.Unmarshal(msg.Data, &read); err != nil {
 		log.WithError(err).Error("userapi clientapi consumer: message parse failure")
diff --git a/userapi/consumers/syncapi_streamevent.go b/userapi/consumers/syncapi_streamevent.go
index ec351ef7..3ac6f58d 100644
--- a/userapi/consumers/syncapi_streamevent.go
+++ b/userapi/consumers/syncapi_streamevent.go
@@ -65,15 +65,16 @@ func NewOutputStreamEventConsumer(
 
 func (s *OutputStreamEventConsumer) Start() error {
 	if err := jetstream.JetStreamConsumer(
-		s.ctx, s.jetstream, s.topic, s.durable, s.onMessage,
-		nats.DeliverAll(), nats.ManualAck(),
+		s.ctx, s.jetstream, s.topic, s.durable, 1,
+		s.onMessage, nats.DeliverAll(), nats.ManualAck(),
 	); err != nil {
 		return err
 	}
 	return nil
 }
 
-func (s *OutputStreamEventConsumer) onMessage(ctx context.Context, msg *nats.Msg) bool {
+func (s *OutputStreamEventConsumer) onMessage(ctx context.Context, msgs []*nats.Msg) bool {
+	msg := msgs[0] // Guaranteed to exist if onMessage is called
 	var output types.StreamedEvent
 	output.Event = &gomatrixserverlib.HeaderedEvent{}
 	if err := json.Unmarshal(msg.Data, &output); err != nil {