GPU: Keep track of the last real context and real surface made current.

ui/gl/gl_surface_osmesa.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/logging.h"
 #include "ui/gl/gl_bindings.h"
 #include "ui/gl/gl_context.h"
 #include "ui/gl/gl_surface_osmesa.h"
 
 namespace gfx {
 
 GLSurfaceOSMesa::GLSurfaceOSMesa(unsigned format, const gfx::Size& size)
     : format_(format),
       size_(size) {
 }
 
 bool GLSurfaceOSMesa::Initialize() {
   return Resize(size_);
 }
 
 void GLSurfaceOSMesa::Destroy() {
   buffer_.reset();
 }
 
 bool GLSurfaceOSMesa::Resize(const gfx::Size& new_size) {
-  GLContext* current_context = GLContext::GetCurrent();
+  GLContext* current_context = GLContext::GetRealCurrent();

[no sievers, 2013/05/24 21:48:06]

Wait, why does this have to get the real context here then?
And isn't that a problem if a virtual context was current, that we will now
return from here with the real context being set current for both
GLContext::GetCurrent() and GLContext::GetRealCurrent()?

Is this just to keep the virtual API from potentially skipping the context
switch below in line 52? Would it?

[jonathan.backer, 2013/05/27 16:37:36]

AFAICT, this code it about resizing a buffer that may be in use. If it's in use,
we need to release, resize, rebind.

In practice, this surface could be current with a real context or a virtual
context. But it's the real binding that matters (not the virtual binding). Note,
a surface being current virtually does not always mean that it's current for
real (there's a 'lite' path in VirtualGLApi::MakeCurrent).
Yes. This is a problem. Thank you. I'm going to fix this with a
ScopedMakeCurrent.
No. The reset in line 52 is a bug (I should restore whatever was previously
current --- including virtual contexts).

   bool was_current = current_context && current_context->IsCurrent(this);
   if (was_current)
     current_context->ReleaseCurrent(this);
 
   // Preserve the old buffer.
   scoped_ptr<int32[]> old_buffer(buffer_.release());
 
   // Allocate a new one.
   buffer_.reset(new int32[new_size.GetArea()]);
   memset(buffer_.get(), 0, new_size.GetArea() * sizeof(buffer_[0]));
@@ skipping 36 matching lines @@
 
 unsigned GLSurfaceOSMesa::GetFormat() {
   return format_;
 }
 
 GLSurfaceOSMesa::~GLSurfaceOSMesa() {
   Destroy();
 }
 
 }  // namespace gfx