app/headless: remove OpenGL assumptions

To prepare package headless for multiple backends, refactor the common headless
driver to no longer assume an OpenGL context. Instead, introduce a headless
backend type and the OpenGL implementation, glBackend.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless.go

@@ -5,27 +5,22 @@
 package headless
 
 import (
-	"fmt"
 	"image"
 	"runtime"
 
-	"gioui.org/app/internal/glimpl"
-	"gioui.org/app/internal/srgb"
 	"gioui.org/gpu"
-	"gioui.org/gpu/gl"
 	"gioui.org/op"
 )
 
 // Window is a headless window.
 type Window struct {
 	size image.Point
-	ctx  context
-	fbo  *srgb.SRGBFBO
+	ctx  backend
 	gpu  *gpu.GPU
 }
 
-type context interface {
-	Functions() *glimpl.Functions
+type backend interface {
+	Screenshot(width, height int, pixels []byte) error
 	Backend() (gpu.Backend, error)
 	MakeCurrent() error
 	ReleaseCurrent()
@@ -34,7 +29,7 @@ type context interface {
 
 // NewWindow creates a new headless window.
 func NewWindow(width, height int) (*Window, error) {
-	ctx, err := newContext()
+	ctx, err := newContext(width, height)
 	if err != nil {
 		return nil, err
 	}
@@ -43,30 +38,16 @@ func NewWindow(width, height int) (*Window, error) {
 		ctx:  ctx,
 	}
 	err = contextDo(ctx, func() error {
-		f := ctx.Functions()
-		fbo, err := srgb.NewSRGBFBO(f)
-		if err != nil {
-			ctx.Release()
-			return err
-		}
-		if err := fbo.Refresh(width, height); err != nil {
-			fbo.Release()
-			ctx.Release()
-			return err
-		}
 		backend, err := ctx.Backend()
 		if err != nil {
-			fbo.Release()
 			ctx.Release()
 			return err
 		}
 		gpu, err := gpu.New(backend)
 		if err != nil {
-			fbo.Release()
 			ctx.Release()
 			return err
 		}
-		w.fbo = fbo
 		w.gpu = gpu
 		return err
 	})
@@ -83,10 +64,6 @@ func (w *Window) Release() {
 			w.gpu.Release()
 			w.gpu = nil
 		}
-		if w.fbo != nil {
-			w.fbo.Release()
-			w.fbo = nil
-		}
 		if w.ctx != nil {
 			w.ctx.Release()
 			w.ctx = nil
@@ -109,16 +86,8 @@ func (w *Window) Frame(frame *op.Ops) {
 // Screenshot returns an image with the content of the window.
 func (w *Window) Screenshot() (*image.RGBA, error) {
 	img := image.NewRGBA(image.Rectangle{Max: w.size})
-	if len(img.Pix) != w.size.X*w.size.Y*4 {
-		panic("unexpected RGBA size")
-	}
 	contextDo(w.ctx, func() error {
-		f := w.ctx.Functions()
-		f.ReadPixels(0, 0, w.size.X, w.size.Y, gl.RGBA, gl.UNSIGNED_BYTE, img.Pix)
-		if glErr := f.GetError(); glErr != gl.NO_ERROR {
-			return fmt.Errorf("glReadPixels failed: %d", glErr)
-		}
-		return nil
+		return w.ctx.Screenshot(w.size.X, w.size.Y, img.Pix)
 	})
 	// Flip image in y-direction. OpenGL's origin is in the lower
 	// left corner.
@@ -134,7 +103,7 @@ func (w *Window) Screenshot() (*image.RGBA, error) {
 	return img, nil
 }
 
-func contextDo(ctx context, f func() error) error {
+func contextDo(ctx backend, f func() error) error {
 	errCh := make(chan error)
 	go func() {
 		runtime.LockOSThread()

app/headless/headless_darwin.go

@@ -22,7 +22,7 @@ type nsContext struct {
 	prepared bool
 }
 
-func newContext() (context, error) {
+func newGLContext() (glContext, error) {
 	ctx := C.gio_headless_newContext()
 	return &nsContext{ctx: ctx, c: new(glimpl.Functions)}, nil
 }

app/headless/headless_egl.go

@@ -8,6 +8,6 @@ import (
 	"gioui.org/app/internal/egl"
 )
 
-func newContext() (context, error) {
+func newGLContext() (glContext, error) {
 	return egl.NewContext(egl.EGL_DEFAULT_DISPLAY)
 }

app/headless/headless_gl.go

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"fmt"
+	"runtime"
+
+	"gioui.org/app/internal/glimpl"
+	"gioui.org/app/internal/srgb"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
+)
+
+type glContext interface {
+	Functions() *glimpl.Functions
+	Backend() (gpu.Backend, error)
+	MakeCurrent() error
+	ReleaseCurrent()
+	Release()
+}
+
+type glBackend struct {
+	glContext
+	srgb *srgb.SRGBFBO
+}
+
+func newContext(width, height int) (backend, error) {
+	glctx, err := newGLContext()
+	if err != nil {
+		return nil, err
+	}
+	// Create the back buffer FBO after locking the thread and making
+	// the context current.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+	if err := glctx.MakeCurrent(); err != nil {
+		glctx.Release()
+		return nil, err
+	}
+	defer glctx.ReleaseCurrent()
+	fbo, err := srgb.NewSRGBFBO(glctx.Functions())
+	if err != nil {
+		glctx.Release()
+		return nil, err
+	}
+	if err := fbo.Refresh(width, height); err != nil {
+		fbo.Release()
+		glctx.Release()
+		return nil, err
+	}
+	return &glBackend{glctx, fbo}, nil
+}
+
+func (b *glBackend) Screenshot(width, height int, pixels []byte) error {
+	if len(pixels) != width*height*4 {
+		panic("unexpected RGBA size")
+	}
+	f := b.Functions()
+	f.ReadPixels(0, 0, width, height, gl.RGBA, gl.UNSIGNED_BYTE, pixels)
+	if glErr := f.GetError(); glErr != gl.NO_ERROR {
+		return fmt.Errorf("glReadPixels failed: %d", glErr)
+	}
+	return nil
+}
+
+func (b *glBackend) Release() {
+	if b.srgb != nil {
+		b.srgb.Release()
+		b.srgb = nil
+	}
+	b.glContext.Release()
+}

app/headless/headless_js.go

@@ -16,7 +16,7 @@ type jsContext struct {
 	f   *glimpl.Functions
 }
 
-func newContext() (*jsContext, error) {
+func newGLContext() (glContext, error) {
 	version := 2
 	doc := js.Global().Get("document")
 	cnv := doc.Call("createElement", "canvas")