frame pacing fix

internal/playback/video_bridge.go

@@ -57,6 +57,19 @@ func (b *VideoBridge) Next(
 	}
 }
 
+// TryNext returns a frame only when a producer is already waiting. It never
+// blocks the caller, allowing UI/render loops to run independently of video
+// frame cadence. A returned frame has the same completion requirements as
+// one returned by Next.
+func (b *VideoBridge) TryNext() (*PendingVideoFrame, bool) {
+	select {
+	case pending := <-b.requests:
+		return pending, true
+	default:
+		return nil, false
+	}
+}
+
 func (f *PendingVideoFrame) Complete(err error) {
 	f.completeOnce.Do(func() {
 		f.result <- err

internal/playback/video_bridge_test.go

@@ -144,3 +144,52 @@ func TestVideoBridgeNextHonorsCancellation(t *testing.T) {
 		t.Fatalf("Next() error = %v, want %v", err, context.Canceled)
 	}
 }
+
+func TestVideoBridgeTryNextReturnsImmediatelyWhenEmpty(t *testing.T) {
+	bridge := NewVideoBridge()
+	if pending, ok := bridge.TryNext(); ok || pending != nil {
+		t.Fatalf("TryNext() = %#v, %t; want nil, false", pending, ok)
+	}
+}
+
+func TestVideoBridgeTryNextDeliversWithoutCopyAndRequiresCompletion(t *testing.T) {
+	bridge := NewVideoBridge()
+	frame := VideoFrame{Index: 9, Payload: []byte{1, 2, 3}}
+	consumeResult := make(chan error, 1)
+	started := make(chan struct{})
+	go func() {
+		close(started)
+		consumeResult <- bridge.ConsumeVideo(context.Background(), frame)
+	}()
+	<-started
+
+	deadline := time.Now().Add(videoBridgeTestTimeout)
+	var pending *PendingVideoFrame
+	for pending == nil && time.Now().Before(deadline) {
+		pending, _ = bridge.TryNext()
+		if pending == nil {
+			time.Sleep(time.Millisecond)
+		}
+	}
+	if pending == nil {
+		t.Fatal("TryNext() did not receive waiting producer")
+	}
+	if &pending.Frame.Payload[0] != &frame.Payload[0] {
+		t.Fatal("TryNext() copied borrowed payload")
+	}
+	select {
+	case err := <-consumeResult:
+		t.Fatalf("ConsumeVideo() returned before completion: %v", err)
+	default:
+	}
+
+	pending.Complete(nil)
+	select {
+	case err := <-consumeResult:
+		if err != nil {
+			t.Fatalf("ConsumeVideo() error = %v", err)
+		}
+	case <-time.After(videoBridgeTestTimeout):
+		t.Fatal("ConsumeVideo() did not return after completion")
+	}
+}