track: detect stalled publishers and switch layer

Relates to #131.

Dockerfile

@@ -1,7 +1,7 @@
 ##
 ## Build
 ##
-FROM golang:1.19 AS build
+FROM golang:1.20 AS build
 
 WORKDIR /app
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/matrix-org/waterfall
 
-go 1.19
+go 1.20
 
 require github.com/pion/webrtc/v3 v3.1.31
 

pkg/conference/publisher/publisher.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/pion/rtp"
 	"github.com/sirupsen/logrus"
+	"golang.org/x/exp/maps"
 )
 
 var ErrSubscriptionExists = errors.New("subscription already exists")
@@ -77,15 +78,17 @@ func NewPublisher(
 	return publisher, observer.statusCh
 }
 
-func (p *Publisher) AddSubscription(subscription Subscription) {
+func (p *Publisher) AddSubscription(subscriptions ...Subscription) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
-	if _, ok := p.subscriptions[subscription]; ok {
-		return
-	}
+	for _, s := range subscriptions {
+		if _, ok := p.subscriptions[s]; ok {
+			continue
+		}
 
-	p.subscriptions[subscription] = struct{}{}
+		p.subscriptions[s] = struct{}{}
+	}
 }
 
 func (p *Publisher) RemoveSubscription(subscription Subscription) {
@@ -94,6 +97,16 @@ func (p *Publisher) RemoveSubscription(subscription Subscription) {
 	delete(p.subscriptions, subscription)
 }
 
+func (p *Publisher) DrainSubscriptions() []Subscription {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	subscriptions := maps.Keys(p.subscriptions)
+	maps.Clear(p.subscriptions)
+
+	return subscriptions
+}
+
 func (p *Publisher) GetTrack() Track {
 	p.mu.Lock()
 	defer p.mu.Unlock()

pkg/conference/track/internal.go

@@ -1,6 +1,8 @@
 package track
 
 import (
+	"fmt"
+
 	"github.com/matrix-org/waterfall/pkg/conference/publisher"
 	"github.com/matrix-org/waterfall/pkg/webrtc_ext"
 	"github.com/matrix-org/waterfall/pkg/worker"
@@ -49,28 +51,59 @@ func forward(sender *webrtc.TrackRemote, receiver *webrtc.TrackLocalStaticRTP, s
 }
 
 func (p *PublishedTrack[SubscriberID]) addVideoPublisher(track *webrtc.TrackRemote) {
+	// Detect simulcast layer of a publisher and create loggers and scoped telemetry.
 	simulcast := webrtc_ext.RIDToSimulcastLayer(track.RID())
-	pub, statusCh := publisher.NewPublisher(
-		&publisher.RemoteTrack{track},
-		p.stopPublishers,
-		p.logger.WithField("layer", simulcast),
-	)
+	layerTelemetry := p.telemetry.CreateChild("layer", attribute.String("layer", simulcast.String()))
+	layerLogger := p.logger.WithField("layer", simulcast.String())
 
+	// Create a new publisher for the track.
+	pub, statusCh := publisher.NewPublisher(&publisher.RemoteTrack{track}, p.stopPublishers, layerLogger)
 	p.video.publishers[simulcast] = pub
 
-	defer p.telemetry.AddEvent("video publisher started", attribute.String("simulcast", simulcast.String()))
-
-	// Listen on `done` and remove the track once it's done.
+	// Observe the status of the publisher.
 	p.activePublishers.Add(1)
 	go func() {
+		// Once this go-routine is done, inform that this publisher is stopped.
 		defer p.activePublishers.Done()
-		defer p.telemetry.AddEvent("video publisher stopped", attribute.String("simulcast", simulcast.String()))
+		defer layerTelemetry.End()
+
+		// Observe publisher's status events.
+		for status := range statusCh {
+			switch status {
+			// Publisher is not active (no packets received for a while).
+			case publisher.StatusStalled:
+				p.mutex.Lock()
+				defer p.mutex.Unlock()
+
+				// Let's check if we're muted. If we are, it's ok to not receive packets.
+				if p.metadata.Muted {
+					layerLogger.Info("Publisher is stalled but we're muted, ignoring")
+					layerTelemetry.AddEvent("muted")
+					continue
+				}
+
+				// Otherwise, remove all subscriptions and switch them to the lowest layer if available.
+				// We assume that the lowest layer is the latest to fail (normally, lowest layer always
+				// receive packets even if other layers are stalled).
+				subscriptions := pub.DrainSubscriptions()
+				lowLayer := p.video.publishers[webrtc_ext.SimulcastLayerLow]
+				if lowLayer != nil {
+					layerLogger.Info("Publisher is stalled, switching to the lowest layer")
+					layerTelemetry.AddEvent("stalled, switched to the low layer")
+					lowLayer.AddSubscription(subscriptions...)
+					continue
+				}
+
+				// Otherwise, we have no other layer to switch to. Bummer.
+				layerLogger.Warn("Publisher is stalled and we have no other layer to switch to")
+				layerTelemetry.Fail(fmt.Errorf("stalled"))
+				continue
 
-		// Wait for the channel to be closed. Ignore statuses for now.
-		for {
-			_, closed := <-statusCh
-			if closed {
-				break
+			// Publisher is active again (new packets received).
+			case publisher.StatusRecovered:
+				// Currently, we don't have any actions when the publisher is recovered, i.e. we
+				// do not switch subscriptions that **used to be subscribed to this layer** back.
+				// But we may want to do it once we have congestion control and bandwidth allocation.
 			}
 		}