Reduce chances of missing layer post migration. (#2612)

When migrating out, it is possible that a published layer is not
notified to the migrating in node. Reduce chances of that layer not
getting published to the new node by curbing RTCP during migration.
It could still happen if stars line up, but this should reduce the
window to a much smaller one.

pkg/rtc/participant.go

@@ -2341,6 +2341,25 @@ func (p *ParticipantImpl) DebugInfo() map[string]interface{} {
 }
 
 func (p *ParticipantImpl) postRtcp(pkts []rtcp.Packet) {
+	p.lock.RLock()
+	migrationTimer := p.migrationTimer
+	p.lock.RUnlock()
+
+	// Once migration out is active, layers getting added would not be communicated to
+	// where the publisher is migrating to. Without SSRC, `UnhandleSimulcastInterceptor`
+	// cannot be set up on the migrating in node. Without that interceptor, simulcast
+	// probing will fail.
+	//
+	// Clients usually send `rid` RTP header extension till they get an RTCP Receiver Report
+	// from the remote side. So, by curbing RTCP when migration is active, even if a new layer
+	// get published to this node, client should continue to send `rid` to the new node
+	// post migration and the new node can do regular simulcast probing (without the
+	// `UnhandleSimulcastInterceptor`) to fire `OnTrack` on that layer. And when the new node
+	// sends RTCP Receiver Report back to the client, client will stop `rid`.
+	if migrationTimer != nil {
+		return
+	}
+
 	p.pubRTCPQueue.Enqueue(func() {
 		if err := p.TransportManager.WritePublisherRTCP(pkts); err != nil && !IsEOF(err) {
 			p.pubLogger.Errorw("could not write RTCP to participant", err)