From c7f625d626f12eb485afef22322e626896559383 Mon Sep 17 00:00:00 2001
From: Raja Subramanian <raja.gobi@tutanota.com>
Date: Sat, 4 Oct 2025 00:17:17 +0530
Subject: [PATCH] Do not force codec regression between opus and red. (#3980)

The following scenario produced silence audio in egress
1. JS SDK publishing audio/red
2. Egress joins
3. Egress picks the RED primary receiver -> this converts RED to Opus in SFU
4. At the same time, codec regression is triggered back to the publisher. Publisher switches to publishing audio/opus and stops audio/red.
5. But, egress is still attached to audio/red and pulls down only silence.

Fix by checking if the negotiated codec needs publish before sending
subscribed codec udpate.
---
 pkg/rtc/subscribedtrack.go |  2 +-
 pkg/rtc/wrappedreceiver.go | 17 +++++++++++++----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/pkg/rtc/subscribedtrack.go b/pkg/rtc/subscribedtrack.go
index 0aeda7199..2c39bda3a 100644
--- a/pkg/rtc/subscribedtrack.go
+++ b/pkg/rtc/subscribedtrack.go
@@ -433,7 +433,7 @@ func (t *SubscribedTrack) OnRttUpdate(rtt uint32) {
 }
 
 func (t *SubscribedTrack) OnCodecNegotiated(codec webrtc.RTPCodecCapability) {
-	if !t.params.WrappedReceiver.DetermineReceiver(codec) {
+	if isAvailable, needsPublish := t.params.WrappedReceiver.DetermineReceiver(codec); !isAvailable || !needsPublish {
 		return
 	}
 
diff --git a/pkg/rtc/wrappedreceiver.go b/pkg/rtc/wrappedreceiver.go
index fc8edbb45..e29dfe799 100644
--- a/pkg/rtc/wrappedreceiver.go
+++ b/pkg/rtc/wrappedreceiver.go
@@ -88,8 +88,13 @@ func (r *WrappedReceiver) StreamID() string {
 	return r.params.StreamId
 }
 
-// DetermineReceiver determines the receiver of negotiated codec and return if there is a match
-func (r *WrappedReceiver) DetermineReceiver(codec webrtc.RTPCodecCapability) bool {
+// DetermineReceiver determines the receiver of negotiated codec and returns
+//
+// isAvailable: returns true if given codec is a potential codec from publisher or if an existing published codec can be translated
+// needsPublish: indicates if the codec is needed from publisher, some combinations can be achieved via codec translation internally,
+//
+//	example: unecrypted opus -> RED translation and vice-versa can be done without the need for publisher to send the other codec.
+func (r *WrappedReceiver) DetermineReceiver(codec webrtc.RTPCodecCapability) (isAvailable bool, needsPublish bool) {
 	r.lock.Lock()
 
 	codecMimeType := mime.NormalizeMimeType(codec.MimeType)
@@ -98,6 +103,8 @@ func (r *WrappedReceiver) DetermineReceiver(codec webrtc.RTPCodecCapability) boo
 		receiverMimeType := receiver.Mime()
 		if receiverMimeType == codecMimeType {
 			trackReceiver = receiver
+			isAvailable = true
+			needsPublish = true
 			break
 		}
 
@@ -105,9 +112,11 @@ func (r *WrappedReceiver) DetermineReceiver(codec webrtc.RTPCodecCapability) boo
 			if receiverMimeType == mime.MimeTypeRED && codecMimeType == mime.MimeTypeOpus {
 				// audio opus/red can match opus only
 				trackReceiver = receiver.GetPrimaryReceiverForRed()
+				isAvailable = true
 				break
 			} else if receiverMimeType == mime.MimeTypeOpus && codecMimeType == mime.MimeTypeRED {
 				trackReceiver = receiver.GetRedReceiver()
+				isAvailable = true
 				break
 			}
 		}
@@ -115,7 +124,7 @@ func (r *WrappedReceiver) DetermineReceiver(codec webrtc.RTPCodecCapability) boo
 	if trackReceiver == nil {
 		r.lock.Unlock()
 		r.params.Logger.Warnw("can't determine receiver for codec", nil, "codec", codec.MimeType)
-		return false
+		return
 	}
 	r.TrackReceiver = trackReceiver
 
@@ -127,7 +136,7 @@ func (r *WrappedReceiver) DetermineReceiver(codec webrtc.RTPCodecCapability) boo
 		trackReceiver.AddOnReady(f)
 	}
 
-	return true
+	return
 }
 
 func (r *WrappedReceiver) Codecs() []webrtc.RTPCodecParameters {