From 38f7906ecb8d0e4867568d211b3a346efbe50a4b Mon Sep 17 00:00:00 2001
From: Raja Subramanian <raja.gobi@tutanota.com>
Date: Fri, 29 Aug 2025 18:18:32 +0530
Subject: [PATCH] Handle migration better in single peer connection case.
 (#3886)

* Handle migration better in single peer connection case.

Have to avoid creating a sender for publish tracks.
Otherwise, the sender messes up the answer SDP because
it forced to inactive and that gets used by type match when setting
remote description offer.

Create sender only for m-lines which was used to send.

* remove unnecessary RemoveTrack
---
 pkg/rtc/transport.go        | 63 +++++++++++++++++++++++++------------
 pkg/rtc/transportmanager.go |  2 +-
 2 files changed, 44 insertions(+), 21 deletions(-)

diff --git a/pkg/rtc/transport.go b/pkg/rtc/transport.go
index 1164e3b0e..c3f89e60c 100644
--- a/pkg/rtc/transport.go
+++ b/pkg/rtc/transport.go
@@ -1951,6 +1951,18 @@ func (t *PCTransport) initPCWithPreviousAnswer(previousAnswer webrtc.SessionDesc
 			continue
 		}
 
+		if !t.params.IsOfferer {
+			// `sendrecv` or `sendonly` means this transceiver is used for sending
+
+			// Note that a transceiver previously used to send could be `inactive`.
+			// Let those transceivers be created when remote description is set.
+			_, ok1 := m.Attribute(webrtc.RTPTransceiverDirectionSendrecv.String())
+			_, ok2 := m.Attribute(webrtc.RTPTransceiverDirectionSendonly.String())
+			if !ok1 && !ok2 {
+				continue
+			}
+		}
+
 		tr, err := t.pc.AddTransceiverFromKind(
 			codecType,
 			webrtc.RTPTransceiverInit{
@@ -1976,35 +1988,46 @@ func (t *PCTransport) initPCWithPreviousAnswer(previousAnswer webrtc.SessionDesc
 	return senders, nil
 }
 
-func (t *PCTransport) SetPreviousSdp(offer, answer *webrtc.SessionDescription) {
+func (t *PCTransport) SetPreviousSdp(localDescription, remoteDescription *webrtc.SessionDescription) {
 	// when there is no answer, cannot migrate, force a full reconnect
-	if answer == nil {
+	if (t.params.IsOfferer && remoteDescription == nil) || (!t.params.IsOfferer && localDescription == nil) {
 		t.onNegotiationFailed(true, "no previous answer")
 		return
 	}
 
 	t.lock.Lock()
-	if t.pc.RemoteDescription() == nil && t.previousAnswer == nil {
-		if t.params.IsOfferer {
-			t.previousAnswer = answer
+	var (
+		senders   map[string]*webrtc.RTPSender
+		err       error
+		parseMids bool
+	)
+	if t.params.IsOfferer {
+		if t.pc.RemoteDescription() == nil && t.previousAnswer == nil {
+			t.previousAnswer = remoteDescription
+			senders, err = t.initPCWithPreviousAnswer(*remoteDescription)
+			parseMids = true
 		}
-		senders, err := t.initPCWithPreviousAnswer(*answer)
-		if err != nil {
-			t.lock.Unlock()
-
-			t.onNegotiationFailed(true, fmt.Sprintf("initPCWithPreviousAnswer failed, error: %s", err))
-			return
+	} else {
+		if t.pc.LocalDescription() == nil {
+			senders, err = t.initPCWithPreviousAnswer(*localDescription)
+			parseMids = true
 		}
+	}
+	if err != nil {
+		t.lock.Unlock()
+		t.onNegotiationFailed(true, fmt.Sprintf("initPCWithPreviousAnswer failed, error: %s", err))
+		return
+	}
 
-		if offer != nil {
-			// in migration case, can't reuse transceiver before negotiated except track subscribed at previous node
-			t.canReuseTransceiver = false
-			if err := t.parseTrackMid(*offer, senders); err != nil {
-				t.params.Logger.Warnw(
-					"parse previous offer failed", err,
-					"offer", offer.SDP,
-				)
-			}
+	if localDescription != nil && parseMids {
+		// in migration case, can't reuse transceiver before negotiating excepted tracks
+		// that were subscribed at previous node
+		t.canReuseTransceiver = false
+		if err := t.parseTrackMid(*localDescription, senders); err != nil {
+			t.params.Logger.Warnw(
+				"parse previous local description failed", err,
+				"localDescription", localDescription.SDP,
+			)
 		}
 	}
 
diff --git a/pkg/rtc/transportmanager.go b/pkg/rtc/transportmanager.go
index 7302e178b..68bcfb63f 100644
--- a/pkg/rtc/transportmanager.go
+++ b/pkg/rtc/transportmanager.go
@@ -816,7 +816,7 @@ func (t *TransportManager) SetMigrateInfo(
 	}
 
 	if t.params.UseSinglePeerConnection {
-		t.publisher.SetPreviousSdp(previousOffer, previousAnswer)
+		t.publisher.SetPreviousSdp(previousAnswer, previousOffer)
 	} else {
 		t.subscriber.SetPreviousSdp(previousOffer, previousAnswer)
 	}