app,gpu,internal/egl: lock OS thread when making OpenGL context current

OpenGL stores the current context in thread-local memory, but commit
4f5baa9a51 removed a runtime.LockOSThread from app.Window that ensured
the goroutine that drives the context stays on the operating thread that
has the context current. This change restores the thread lock.

As a bonus, this change makes the OpenGL contexts responsible for locking
the thread at MakeCurrent, thereby removing LockOSThread calls from GPU
backend-agnostic code.

Fixes: https://todo.sr.ht/~eliasnaur/gio/334
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/gl_ios.go

@@ -31,6 +31,7 @@ import "C"
 import (
 	"errors"
 	"fmt"
+	"runtime"
 
 	"gioui.org/gpu"
 	"gioui.org/internal/gl"
@@ -101,6 +102,9 @@ func (c *context) Present() error {
 }
 
 func (c *context) Lock() error {
+	// OpenGL contexts are implicit and thread-local. Lock the OS thread.
+	runtime.LockOSThread()
+
 	if C.gio_makeCurrent(c.ctx) == 0 {
 		return errors.New("[EAGLContext setCurrentContext] failed")
 	}

app/gl_macos.go

@@ -7,6 +7,9 @@ package app
 
 import (
 	"errors"
+	"runtime"
+
+	"unsafe"
 
 	"gioui.org/gpu"
 	"gioui.org/internal/gl"
@@ -35,8 +38,6 @@ static void glFlush(PFN_glFlush f) {
 */
 import "C"
 
-import "unsafe"
-
 type glContext struct {
 	c    *gl.Functions
 	ctx  C.CFTypeRef
@@ -95,6 +96,9 @@ func (c *glContext) Present() error {
 }
 
 func (c *glContext) Lock() error {
+	// OpenGL contexts are implicit and thread-local. Lock the OS thread.
+	runtime.LockOSThread()
+
 	C.gio_lockContext(c.ctx)
 	C.gio_makeCurrentContext(c.ctx)
 	return nil

gpu/headless/driver_test.go

@@ -165,7 +165,6 @@ func newDriver(t *testing.T) driver.Device {
 	if err != nil {
 		t.Skipf("no context available: %v", err)
 	}
-	runtime.LockOSThread()
 	if err := ctx.MakeCurrent(); err != nil {
 		t.Fatal(err)
 	}

gpu/headless/headless.go

@@ -8,7 +8,6 @@ import (
 	"errors"
 	"image"
 	"image/color"
-	"runtime"
 
 	"gioui.org/gpu"
 	"gioui.org/gpu/internal/driver"
@@ -146,8 +145,6 @@ func (w *Window) Screenshot(img *image.RGBA) error {
 func contextDo(ctx context, f func() error) error {
 	errCh := make(chan error)
 	go func() {
-		runtime.LockOSThread()
-		defer runtime.UnlockOSThread()
 		if err := ctx.MakeCurrent(); err != nil {
 			errCh <- err
 			return

internal/egl/egl.go

@@ -135,6 +135,9 @@ func (c *Context) ReleaseCurrent() {
 }
 
 func (c *Context) MakeCurrent() error {
+	// OpenGL contexts are implicit and thread-local. Lock the OS thread.
+	runtime.LockOSThread()
+
 	if c.eglSurf == nilEGLSurface && !c.eglCtx.surfaceless {
 		return errors.New("no surface created yet EGL_KHR_surfaceless_context is not supported")
 	}