From a002337db11b8be9036f9f87e68a3cbba757cd80 Mon Sep 17 00:00:00 2001
From: Raja Subramanian <raja.gobi@tutanota.com>
Date: Wed, 29 Apr 2026 22:43:33 +0530
Subject: [PATCH] Legacy TrackInfo.Simulcast flag. (#4493)

* Legacy TrackInfo.Simulcast flag.

When AddTrack did not send SimulcastCodecs, the legacy `Simulcast` flag
was not set. Fix it by setting the flag when a second layer is
published.

* staticcheck

* use the existing PrimaryReceiver function
---
 pkg/rtc/mediatrack.go         |  1 +
 pkg/rtc/mediatrackreceiver.go | 28 ++++++++++++++++++++++++----
 pkg/rtc/participant.go        |  3 ++-
 pkg/sfu/receiver.go           | 14 ++++++++++++++
 4 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/pkg/rtc/mediatrack.go b/pkg/rtc/mediatrack.go
index 56f8b1b0c..bc4238aa9 100644
--- a/pkg/rtc/mediatrack.go
+++ b/pkg/rtc/mediatrack.go
@@ -564,6 +564,7 @@ func (t *MediaTrack) AddReceiver(receiver *webrtc.RTPReceiver, track sfu.TrackRe
 		return newCodec, false
 	}
 
+	t.MediaTrackReceiver.MaybeSetSimulcast()
 	t.MediaTrackReceiver.SetLayerSsrcsForRid(mimeType, track.RID(), uint32(track.SSRC()), 0)
 
 	if regressCodec {
diff --git a/pkg/rtc/mediatrackreceiver.go b/pkg/rtc/mediatrackreceiver.go
index 2fa2650bd..f4643af94 100644
--- a/pkg/rtc/mediatrackreceiver.go
+++ b/pkg/rtc/mediatrackreceiver.go
@@ -649,6 +649,29 @@ func (t *MediaTrackReceiver) updateTrackInfoOfReceivers() {
 	t.MediaTrackSubscriptions.SetMuted(ti.GetMuted())
 }
 
+func (t *MediaTrackReceiver) MaybeSetSimulcast() {
+	// only primary receiver (i.e. receiver at index 0) for legacy use case
+	primaryReceiver := t.PrimaryReceiver()
+	if primaryReceiver == nil {
+		return
+	}
+	if wr, ok := primaryReceiver.(*sfu.WebRTCReceiver); !ok || wr.NumUpTracks() < 2 {
+		return
+	}
+
+	t.lock.Lock()
+	trackInfo := t.TrackInfoClone()
+	if trackInfo.Simulcast {
+		t.lock.Unlock()
+		return
+	}
+	trackInfo.Simulcast = true
+	t.trackInfo.Store(trackInfo)
+	t.lock.Unlock()
+
+	t.updateTrackInfoOfReceivers()
+}
+
 func (t *MediaTrackReceiver) SetLayerSsrcsForRid(mimeType mime.MimeType, rid string, ssrc uint32, repairSSRC uint32) {
 	t.lock.Lock()
 	trackInfo := t.TrackInfoClone()
@@ -1058,10 +1081,7 @@ func (t *MediaTrackReceiver) GetQualityForDimension(mimeType mime.MimeType, widt
 	if origSize == 0 {
 		for i := len(mediaSizes) - 1; i >= 0; i-- {
 			if mediaSizes[i].Height > 0 {
-				origSize = mediaSizes[i].Height
-				if mediaSizes[i].Width < mediaSizes[i].Height {
-					origSize = mediaSizes[i].Width
-				}
+				origSize = min(mediaSizes[i].Width, mediaSizes[i].Height)
 				break
 			}
 		}
diff --git a/pkg/rtc/participant.go b/pkg/rtc/participant.go
index 433da1442..bffda51b6 100644
--- a/pkg/rtc/participant.go
+++ b/pkg/rtc/participant.go
@@ -2290,6 +2290,7 @@ func (p *ParticipantImpl) onMediaTrack(rtcTrack *webrtc.TrackRemote, rtpReceiver
 	)
 
 	if !isNewTrack && !publishedTrack.HasPendingCodec() && p.IsReady() {
+		p.dirty.Store(true)
 		p.listener().OnTrackUpdated(p, publishedTrack)
 	}
 }
@@ -2865,7 +2866,7 @@ func (p *ParticipantImpl) addPendingTrackLocked(req *livekit.AddTrackRequest) *l
 
 	if len(req.SimulcastCodecs) == 0 {
 		// clients not supporting simulcast codecs, synthesise a codec
-		videoLayerMode := livekit.VideoLayer_MODE_UNUSED
+		videoLayerMode := livekit.VideoLayer_ONE_SPATIAL_LAYER_PER_STREAM
 		if p.params.ClientInfo.isOBS() {
 			videoLayerMode = livekit.VideoLayer_ONE_SPATIAL_LAYER_PER_STREAM_INCOMPLETE_RTCP_SR
 		}
diff --git a/pkg/sfu/receiver.go b/pkg/sfu/receiver.go
index a134e16e7..0bc566332 100644
--- a/pkg/sfu/receiver.go
+++ b/pkg/sfu/receiver.go
@@ -200,6 +200,20 @@ func (w *WebRTCReceiver) AddUpTrack(track TrackRemote, buff *buffer.Buffer) erro
 	return nil
 }
 
+func (w *WebRTCReceiver) NumUpTracks() int {
+	numUpTracks := 0
+
+	w.upTracksMu.Lock()
+	for _, track := range w.upTracks {
+		if track != nil {
+			numUpTracks++
+		}
+	}
+	w.upTracksMu.Unlock()
+
+	return numUpTracks
+}
+
 func (w *WebRTCReceiver) UpdateTrackInfo(ti *livekit.TrackInfo) {
 	w.ReceiverBase.UpdateTrackInfo(ti)
 	w.connectionStats.UpdateMute(ti.GetMuted())