From d980c4652a8c72ff64d8fede5045cd80001f6c0e Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Thu, 7 Nov 2019 17:34:32 +0100
Subject: [PATCH] app: don't touch the frame after returning from
 FrameEvent.Frame

It was already true that the GPU wouldn't touch the frame after
calling GPU.Draw. This change makes FrameEvent.Frame block until
GPU.Draw is complete.

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 app/window.go | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/app/window.go b/app/window.go
index eb3ab233..f9a4b12c 100644
--- a/app/window.go
+++ b/app/window.go
@@ -36,6 +36,7 @@ type Window struct {
 	ack         chan struct{}
 	invalidates chan struct{}
 	frames      chan *op.Ops
+	frameAck    chan struct{}
 
 	stage        system.Stage
 	animating    bool
@@ -95,6 +96,7 @@ func NewWindow(options ...Option) *Window {
 		ack:         make(chan struct{}),
 		invalidates: make(chan struct{}, 1),
 		frames:      make(chan *op.Ops),
+		frameAck:    make(chan struct{}),
 	}
 	w.callbacks.w = w
 	go w.run(opts)
@@ -118,6 +120,7 @@ func (w *Window) Queue() *Queue {
 // the window state from previous calls.
 func (w *Window) update(frame *op.Ops) {
 	w.frames <- frame
+	<-w.frameAck
 }
 
 func (w *Window) draw(size image.Point, frame *op.Ops) {
@@ -279,17 +282,27 @@ func (w *Window) run(opts *window.Options) {
 					}
 				}
 				var frame *op.Ops
-				// Wait for either a frame or the ack event,
-				// which meant that the client didn't draw.
+				// Wait for either a frame or an ack event to go
+				// through, which means that the client didn't give us
+				// a frame.
+				gotFrame := false
 				select {
 				case frame = <-w.frames:
+					gotFrame = true
 				case w.out <- ackEvent:
 				}
 				if err != nil {
+					if gotFrame {
+						w.frameAck <- struct{}{}
+					}
 					w.destroy(err)
 					return
 				}
-				w.draw(e2.Size, frame)
+				if gotFrame {
+					w.draw(e2.Size, frame)
+					// We're done with frame, let the client continue.
+					w.frameAck <- struct{}{}
+				}
 				if e2.Sync {
 					if err := w.gpu.Flush(); err != nil {
 						w.gpu.Release()