remove redundant video grant protocol

cmd/mxl-player/main.go

@@ -358,7 +358,7 @@ func main() {
 		video  string
 		audio  string
 	}
-	// One control channel: grant (empty params) or reconnect (with params).
+	// Reconnect requests from GUI or automatic retry
 	control := make(chan reconnectParams, 1)
 	videoBridge := playback.NewVideoBridge()
 
@@ -377,7 +377,7 @@ func main() {
 			audioSrc = nil
 		}
 		// Try once. Return error if fails — caller loops back to select
-		// and can pick up new reconnect params or a new grant.
+		// and can pick up newer reconnect request.
 		if params.video != "" && params.audio != "" {
 			s, e := source.OpenSameDomainSync(params.domain, params.video, params.audio)
 			if e == nil {
@@ -438,7 +438,7 @@ func main() {
 	}
 
 	go func() {
-		// Audio-only mode: independent loop, no grant/staged handshake.
+		// Audio-only mode: independent loop.
 		if audioSrc != nil && syncSrc == nil && videoSrc == nil {
 			for {
 				select {
@@ -503,31 +503,34 @@ func main() {
 			}
 		}
 
-		// Video (with or without sync) mode: grant/staged handshake.
+		// Video bridge provides backpressure: only one borrowed frame is in flight.
 		for {
-			params := <-control
-			if params.video != "" || params.audio != "" {
-				// Reconnect request from Connect button or auto-retry.
 			select {
-				case <-control: // drain any pending grant
-				default:
-				}
+			case <-ctx.Done():
+				return
+
+			case params := <-control:
 				if rerr := reopen(params); rerr != nil {
 					if errors.Is(rerr, context.Canceled) {
 						return
 					}
 					log.Printf("source: reopen failed: %v, retrying", rerr)
+
 					select {
 					case <-time.After(500 * time.Millisecond):
 					case <-ctx.Done():
 						return
 					}
+
 					select {
 					case control <- params:
 					default:
+						// Preserve an already queued, potentially newer request.
 					}
 				}
 				continue
+
+			default:
 			}
 
 			var videoFrame playback.VideoFrame
@@ -538,15 +541,16 @@ func main() {
 						return
 					}
 					log.Printf("source: %v", err)
-					// Drain any pending grant, then send reconnect.
-					select {
-					case <-control:
-					default:
+					// Request a reconnect after the read failure.
+					params := reconnectParams{
+						domain: domainStr,
+						video:  videoStr,
+						audio:  audioStr,
 					}
 					select {
-					case control <- reconnectParams{domain: domainStr, video: videoStr, audio: audioStr}:
-					case <-ctx.Done():
-						return
+					case control <- params:
+					default:
+						// Preserve an already queued, potentially newer request.
 					}
 					continue
 				}
@@ -570,14 +574,15 @@ func main() {
 						return
 					}
 					log.Printf("source: %v", err)
-					select {
-					case <-control:
-					default:
+					params := reconnectParams{
+						domain: domainStr,
+						video:  videoStr,
+						audio:  audioStr,
 					}
 					select {
-					case control <- reconnectParams{domain: domainStr, video: videoStr, audio: audioStr}:
-					case <-ctx.Done():
-						return
+					case control <- params:
+					default:
+						// Preserve an already queued, potentially newer request.
 					}
 					continue
 				}
@@ -604,7 +609,6 @@ func main() {
 
 	running := true
 	resized := false
-	granted := false
 	fullscreen := args.IsFullscreen
 	if fullscreen {
 		sdl.SetWindowFullscreen(windowHandler, true)
@@ -662,15 +666,6 @@ func main() {
 			}
 			resized = false
 		}
-		if !granted {
-			select {
-			case control <- reconnectParams{}:
-				granted = true
-			case <-ctx.Done():
-				running = false
-				continue
-			}
-		}
 		var shownIndex uint64
 		hasFrame := false
 
@@ -697,15 +692,11 @@ func main() {
 			}
 
 			shownIndex = pendingFrame.Frame.Index
-			granted = false
 			hasFrame = true
 		} else if frameErr != nil &&
 			!errors.Is(frameErr, context.DeadlineExceeded) &&
 			!errors.Is(frameErr, context.Canceled) {
 			panic(frameErr)
-		} else {
-			// No frame arrived before the deadline.
-			granted = false
 		}
 
 		// stats

imgui.ini

@@ -14,7 +14,7 @@ Size=200,200
 Collapsed=0
 
 [Window][Connection]
-Pos=1250,701
+Pos=500,330
 Size=523,153
 Collapsed=0
 

notes.md

@@ -0,0 +1,6 @@
+That makes sense. The GUI is likely more responsive because frame staging is now serialized with rendering:
+- The background goroutine no longer copies a large frame into Vulkan-mapped memory concurrently with GUI/render work.
+- StageFrame waits for the GPU fence before writing, removing CPU/GPU memory contention and undefined synchronization.
+- The bridge creates deterministic backpressure: the source cannot begin another frame until the current payload is staged.
+- The main thread now controls the complete render sequence instead of coordinating loosely through two channels.
+So we fixed both correctness and scheduling stability without adding another frame copy.