app: avoid relocking contexts every frame

Signed-off-by: Joe Julian <me@joejulian.name>

app/gl_ios.go

@@ -115,9 +115,6 @@ func (c *context) Unlock() {
 }
 
 func (c *context) Refresh() error {
-	if C.gio_makeCurrent(c.ctx) == 0 {
-		return errors.New("[EAGLContext setCurrentContext] failed")
-	}
 	if !c.init {
 		c.init = true
 		c.frameBuffer = c.c.CreateFramebuffer()

app/gl_macos.go

@@ -111,8 +111,6 @@ func (c *glContext) Unlock() {
 }
 
 func (c *glContext) Refresh() error {
-	c.Lock()
-	defer c.Unlock()
 	C.gio_updateContext(c.ctx)
 	return nil
 }

app/os.go

@@ -165,6 +165,8 @@ type frameEvent struct {
 	Sync bool
 }
 
+// The caller must hold the context lock while using API, Refresh,
+// RenderTarget, or Present.
 type context interface {
 	API() gpu.API
 	RenderTarget() (gpu.RenderTarget, error)

app/window.go

@@ -46,6 +46,10 @@ type Window struct {
 
 	ctx context
 	gpu gpu.GPU
+	// ctxNeedsLock tracks whether the rendering context must be made
+	// current again before the next GPU operation. Refresh paths, surface
+	// loss, and explicit unlocks all invalidate the current binding.
+	ctxNeedsLock bool
 	// timer tracks the delayed invalidate goroutine.
 	timer struct {
 		// quit is shuts down the goroutine.
@@ -146,9 +150,14 @@ func (w *Window) validateAndProcess(size image.Point, sync bool, frame *op.Ops,
 				if err != nil {
 					return err
 				}
+				w.ctxNeedsLock = true
 				sync = true
 			}
 		}
+		if err := w.lockContext(); err != nil {
+			w.destroyGPU()
+			return err
+		}
 		if sync && w.ctx != nil {
 			if err := w.ctx.Refresh(); err != nil {
 				if errors.Is(err, errOutOfDate) {
@@ -162,9 +171,8 @@ func (w *Window) validateAndProcess(size image.Point, sync bool, frame *op.Ops,
 				}
 				return err
 			}
-		}
-		if w.ctx != nil {
-			if err := w.ctx.Lock(); err != nil {
+			w.unlockContext()
+			if err := w.lockContext(); err != nil {
 				w.destroyGPU()
 				return err
 			}
@@ -172,7 +180,7 @@ func (w *Window) validateAndProcess(size image.Point, sync bool, frame *op.Ops,
 		if w.gpu == nil && !w.nocontext {
 			gpu, err := gpu.New(w.ctx.API())
 			if err != nil {
-				w.ctx.Unlock()
+				w.unlockContext()
 				w.destroyGPU()
 				return err
 			}
@@ -180,7 +188,7 @@ func (w *Window) validateAndProcess(size image.Point, sync bool, frame *op.Ops,
 		}
 		if w.gpu != nil {
 			if err := w.frame(frame, size); err != nil {
-				w.ctx.Unlock()
+				w.unlockContext()
 				if errors.Is(err, errOutOfDate) {
 					// GPU surface needs refreshing.
 					sync = true
@@ -200,7 +208,6 @@ func (w *Window) validateAndProcess(size image.Point, sync bool, frame *op.Ops,
 		var err error
 		if w.gpu != nil {
 			err = w.ctx.Present()
-			w.ctx.Unlock()
 		}
 		return err
 	}
@@ -503,16 +510,37 @@ func (c *callbacks) ActionAt(p f32.Point) (system.Action, bool) {
 	return c.w.queue.ActionAt(p)
 }
 
+func (w *Window) lockContext() error {
+	if w.ctx == nil || !w.ctxNeedsLock {
+		return nil
+	}
+	if err := w.ctx.Lock(); err != nil {
+		return err
+	}
+	w.ctxNeedsLock = false
+	return nil
+}
+
+func (w *Window) unlockContext() {
+	if w.ctx == nil || w.ctxNeedsLock {
+		return
+	}
+	w.ctx.Unlock()
+	w.ctxNeedsLock = true
+}
+
 func (w *Window) destroyGPU() {
 	if w.gpu != nil {
-		w.ctx.Lock()
-		w.gpu.Release()
-		w.ctx.Unlock()
+		if err := w.lockContext(); err == nil {
+			w.gpu.Release()
+			w.unlockContext()
+		}
 		w.gpu = nil
 	}
 	if w.ctx != nil {
 		w.ctx.Release()
 		w.ctx = nil
+		w.ctxNeedsLock = false
 	}
 }
 
@@ -655,10 +683,11 @@ func (w *Window) processEvent(e event.Event) bool {
 		w.coalesced.destroy = &e2
 	case ViewEvent:
 		if !e2.Valid() && w.gpu != nil {
-			w.ctx.Lock()
-			w.gpu.Release()
+			if err := w.lockContext(); err == nil {
+				w.gpu.Release()
+				w.unlockContext()
+			}
 			w.gpu = nil
-			w.ctx.Unlock()
 		}
 		w.coalesced.view = &e2
 	case ConfigEvent: