From 33146961f4355fe5d1daad8241936bbcf81022cb Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Wed, 27 May 2020 16:12:57 +0200
Subject: [PATCH] app/internal/window: [macOS] avoid racy access to window
 width, height, scale

The macOS redraw callback is not invoked on the main thread, so its
access to window fields must be synchronized.

An alternative would be to schedule the asynchronous redraws on the main
thread, but I believe frame callbacks are performance-sensitive enough
to warrant the extra locking complexity.

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 app/internal/window/os_macos.go | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/app/internal/window/os_macos.go b/app/internal/window/os_macos.go
index d1ffdbf1..fbc8b224 100644
--- a/app/internal/window/os_macos.go
+++ b/app/internal/window/os_macos.go
@@ -48,9 +48,12 @@ func init() {
 }
 
 type window struct {
-	view          C.CFTypeRef
-	w             Callbacks
-	stage         system.Stage
+	view  C.CFTypeRef
+	w     Callbacks
+	stage system.Stage
+
+	// mu protect the following fields
+	mu            sync.Mutex
 	scale         float32
 	width, height float32
 }
@@ -204,8 +207,12 @@ func gio_onMouse(view C.CFTypeRef, cdir C.int, cbtns C.NSUInteger, x, y, dx, dy
 //export gio_onDraw
 func gio_onDraw(view C.CFTypeRef) {
 	w := mustView(view)
-	w.scale = float32(C.gio_getViewBackingScale(w.view))
-	w.width, w.height = float32(C.gio_viewWidth(w.view)), float32(C.gio_viewHeight(w.view))
+	scale := float32(C.gio_getViewBackingScale(w.view))
+	width, height := float32(C.gio_viewWidth(w.view)), float32(C.gio_viewHeight(w.view))
+	w.mu.Lock()
+	w.scale = scale
+	w.width, w.height = width, height
+	w.mu.Unlock()
 	w.draw(true)
 }
 
@@ -216,13 +223,15 @@ func gio_onFocus(view C.CFTypeRef, focus C.BOOL) {
 }
 
 func (w *window) draw(sync bool) {
-	wf, hf := w.width, w.height
+	w.mu.Lock()
+	wf, hf, scale := w.width, w.height, w.scale
+	w.mu.Unlock()
 	if wf == 0 || hf == 0 {
 		return
 	}
-	width := int(wf*w.scale + .5)
-	height := int(hf*w.scale + .5)
-	cfg := configFor(w.scale)
+	width := int(wf*scale + .5)
+	height := int(hf*scale + .5)
+	cfg := configFor(scale)
 	cfg.now = time.Now()
 	w.setStage(system.StageRunning)
 	w.w.Event(FrameEvent{