sfu: don't add a closed down track in SetReceiver (#5179)

Co-authored-by: Mansur Akhmadi <10739460+mansur-akhmadi@users.noreply.github.com>

pkg/sfu/downtrack.go

@@ -809,6 +809,12 @@ func (d *DownTrack) SetReceiver(r TrackReceiver) {
 	d.handleUpstreamCodecChange(r.Codec().MimeType)
 
 	d.bindLock.Lock()
+	// a close that ran while bindLock was released may already have removed the down
+	// track from r, adding it now would leave a closed down track registered there
+	if d.IsClosed() {
+		d.bindLock.Unlock()
+		return
+	}
 	if err := r.AddDownTrack(d); err != nil {
 		d.params.Logger.Warnw("failed to add downtrack to receiver", err)
 	}

pkg/sfu/downtrack_test.go

@@ -0,0 +1,93 @@
+// Copyright 2026 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sfu_test
+
+import (
+	"testing"
+
+	"github.com/pion/webrtc/v4"
+	"github.com/stretchr/testify/require"
+
+	"github.com/livekit/protocol/logger"
+
+	"github.com/livekit/livekit-server/pkg/sfu"
+	"github.com/livekit/livekit-server/pkg/sfu/buffer"
+	"github.com/livekit/livekit-server/pkg/sfu/pacer"
+	"github.com/livekit/livekit-server/pkg/sfu/sfufakes"
+)
+
+func TestDownTrackSetReceiver(t *testing.T) {
+	newDownTrack := func(t *testing.T, r sfu.TrackReceiver) *sfu.DownTrack {
+		dt, err := sfu.NewDownTrack(sfu.DownTrackParams{
+			Codecs:        []webrtc.RTPCodecParameters{vp8CodecParams},
+			Receiver:      r,
+			BufferFactory: buffer.NewFactoryOfBufferFactory(500, 500).CreateBufferFactory(),
+			Pacer:         pacer.NewPassThrough(logger.GetLogger(), newNullBWE()),
+			Logger:        logger.GetLogger(),
+			Listener:      &sfufakes.FakeDownTrackListener{},
+			StreamID:      "fake-stream",
+			SubID:         "fake-sub",
+			MaxTrack:      500,
+		})
+		require.NoError(t, err)
+		t.Cleanup(dt.Close)
+		return dt
+	}
+
+	t.Run("moves to the new receiver", func(t *testing.T) {
+		r1 := newFakeTrackReceiver(vp8CodecParams)
+		dt := newDownTrack(t, r1)
+
+		r2 := newFakeTrackReceiver(vp8CodecParams)
+		dt.SetReceiver(r2)
+
+		require.Equal(t, sfu.TrackReceiver(r2), dt.Receiver())
+		require.Equal(t, 1, r1.DeleteDownTrackCallCount())
+		require.Equal(t, 1, r2.AddDownTrackCallCount())
+		require.Same(t, dt, r2.AddDownTrackArgsForCall(0))
+	})
+
+	t.Run("closed before the switch", func(t *testing.T) {
+		r1 := newFakeTrackReceiver(vp8CodecParams)
+		dt := newDownTrack(t, r1)
+		dt.Close()
+
+		r2 := newFakeTrackReceiver(vp8CodecParams)
+		dt.SetReceiver(r2)
+
+		require.Equal(t, sfu.TrackReceiver(r1), dt.Receiver())
+		require.Zero(t, r2.AddOnReadyCallCount())
+		require.Zero(t, r2.AddDownTrackCallCount())
+	})
+
+	// SetReceiver releases bindLock while it registers for readiness and handles a codec
+	// change; a close in that window deletes the down track from the new receiver before
+	// it was added, so it must not be added afterwards
+	t.Run("closed during the switch", func(t *testing.T) {
+		r1 := newFakeTrackReceiver(vp8CodecParams)
+		dt := newDownTrack(t, r1)
+
+		r2 := newFakeTrackReceiver(vp8CodecParams)
+		r2.AddOnReadyCalls(func(f func()) {
+			dt.Close()
+			f()
+		})
+		dt.SetReceiver(r2)
+
+		require.True(t, dt.IsClosed())
+		require.Equal(t, 1, r2.DeleteDownTrackCallCount())
+		require.Zero(t, r2.AddDownTrackCallCount(), "closed down track was added to the new receiver")
+	})
+}