Fix deadlock (#2180)

* Fix deadlock

My previous PR to wrap layer notifier post in bind lock was
problematic as `onBinding` callback happens within that lock
and that onBinding callback can call set max layer which will
post to channel. Use a separate mutex.

* RUnlock

pkg/sfu/downtrack.go

@@ -269,7 +269,9 @@ type DownTrack struct {
 
 	pacer pacer.Pacer
 
-	maxLayerNotifierCh chan struct{}
+	maxLayerNotifierChMu     sync.RWMutex
+	maxLayerNotifierCh       chan struct{}
+	maxLayerNotifierChClosed bool
 
 	cbMu                        sync.RWMutex
 	onStatsUpdate               func(dt *DownTrack, stat *livekit.AnalyticsStat)
@@ -641,14 +643,14 @@ func (d *DownTrack) postMaxLayerNotifierEvent() {
 		return
 	}
 
-	d.bindLock.Lock()
-	if !d.IsClosed() {
+	d.maxLayerNotifierChMu.RLock()
+	if !d.maxLayerNotifierChClosed {
 		select {
 		case d.maxLayerNotifierCh <- struct{}{}:
 		default:
 		}
 	}
-	d.bindLock.Unlock()
+	d.maxLayerNotifierChMu.RUnlock()
 }
 
 func (d *DownTrack) maxLayerNotifierWorker() {
@@ -934,13 +936,12 @@ func (d *DownTrack) Close() {
 //  2. in case of session migration, participant migrate from other node, video track should
 //     be resumed with same participant, set flush=false since we don't need to flush decoder.
 func (d *DownTrack) CloseWithFlush(flush bool) {
-	d.bindLock.Lock()
 	if d.isClosed.Swap(true) {
-		d.bindLock.Unlock()
 		// already closed
 		return
 	}
 
+	d.bindLock.Lock()
 	d.params.Logger.Debugw("close down track", "flushBlankFrame", flush)
 	if d.bound.Load() {
 		d.forwarder.Mute(true, true)
@@ -975,12 +976,16 @@ func (d *DownTrack) CloseWithFlush(flush bool) {
 		d.rtcpReader.OnPacket(nil)
 	}
 
-	close(d.maxLayerNotifierCh)
 	d.bindLock.Unlock()
 	d.connectionStats.Close()
 	d.rtpStats.Stop()
 	d.params.Logger.Infow("rtp stats", "direction", "downstream", "mime", d.mime, "ssrc", d.ssrc, "stats", d.rtpStats.ToString())
 
+	d.maxLayerNotifierChMu.Lock()
+	d.maxLayerNotifierChClosed = true
+	close(d.maxLayerNotifierCh)
+	d.maxLayerNotifierChMu.Unlock()
+
 	if onCloseHandler := d.getOnCloseHandler(); onCloseHandler != nil {
 		onCloseHandler(!flush)
 	}