Event-driven encode: per-track goroutine

sender/frame_buffer.go

@@ -128,24 +128,34 @@ func (f *FrameBuffer) ResetInitialized() {
 }
 
 // Read returns the next available frame from the buffer.
-// When initialized (normal operation), returns immediately with ErrNoFrameAvailable
-// if no frame is ready. When not initialized (encoder init), blocks up to 100ms
-// and returns a black frame for codec property detection.
+// When initialized (normal operation), it returns immediately with
+// ErrNoFrameAvailable if no frame is ready. The vpx encoder's Read holds
+// its internal mutex across the inner Read call, so blocking here would
+// also block concurrent DynamicQPControl bitrate updates that try to take
+// the same mutex. runEncodeLoop handles ErrNoFrameAvailable by sleeping
+// briefly before retrying. When not initialized (encoder init), it blocks
+// up to 100ms and returns a black frame for codec property detection.
 //
 // Side effect: stores the popped frame's captureTSUs for LastCaptureTSUs.
 // Black-frame timeouts preserve the previous value so encoders with
 // lookahead can still correlate the previously-read real frame.
 func (f *FrameBuffer) Read() (image.Image, func(), error) {
 	if f.initialized {
+		// Check closeChan first so a closed buffer always reports
+		// ErrBufferClosed instead of racing with the default branch below
+		// (Go's select picks ready cases pseudo-randomly).
+		select {
+		case <-f.closeChan:
+			return nil, func() {}, ErrBufferClosed
+		default:
+		}
 		// Non-blocking fast path for normal operation.
 		select {
 		case fm := <-f.frameChan:
 			f.lastCaptureTSUs.Store(fm.captureTSUs)
 			f.lastDequeueWallUs.Store(time.Now().UnixMicro())
 
 			return fm.img, func() {}, nil
-		case <-f.closeChan:
-			return nil, func() {}, ErrBufferClosed
 		default:
 			return nil, func() {}, ErrNoFrameAvailable
 		}

sender/frame_buffer_test.go

@@ -113,11 +113,9 @@ func TestFrameBuffer_ReadWithoutFrames(t *testing.T) {
 	fb := NewFrameBuffer(640, 480)
 	defer func() { _ = fb.Close() }()
 
-	// Mark as initialized so we don't get black frames
+	// Mark as initialized so we take the non-blocking fast path.
 	fb.SetInitialized()
 
-	// Try to read without sending any frames
-	// This should return ErrNoFrameAvailable immediately (non-blocking)
 	img, release, err := fb.Read()
 	assert.Nil(t, img)
 	assert.NotNil(t, release)
@@ -165,7 +163,8 @@ func TestFrameBuffer_ReadInitializedAfterClose(t *testing.T) {
 	fb := NewFrameBuffer(640, 480)
 	fb.SetInitialized()
 
-	// Close, then read — fast path should detect closeChan.
+	// Close, then read — the closeChan precedence check should win over
+	// the default branch in the non-blocking select.
 	err := fb.Close()
 	require.NoError(t, err)