Prefer ScopedNestableTaskAllower over manual save/restore

content/browser/download/drag_download_file.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 "content/browser/download/drag_download_file.h"
 
 #include "base/bind.h"
 #include "base/file_util.h"
 #include "base/message_loop.h"
 #include "content/browser/download/download_stats.h"
@@ skipping 205 matching lines @@
   // Only do the check on Windows where two threads are involved.
 #if defined(OS_WIN)
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 #endif
 }
 
 #if defined(OS_WIN)
 void DragDownloadFile::StartNestedMessageLoop() {
   AssertCurrentlyOnDragThread();
 
-  bool old_state = MessageLoop::current()->NestableTasksAllowed();
-  MessageLoop::current()->SetNestableTasksAllowed(true);
+  MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
   is_running_nested_message_loop_ = true;
   MessageLoop::current()->Run();

[jar (doing other things), 2012/02/11 03:24:38]

This looks suspiciously like a bug... but I'm not sure.

I suspect we should have line 229 that does:
is_running_nested_message_loop_ = false.

without it, I'm wary that a call in line 234 will post a spurious Quit()
message.


Alternatively, I'd be OK with asserting (here) that it was set to false via line
235.  ...but looking at the logic, I'm confused about whether it is evyr
possible to reach lien 234 without having is_running_nested_message_loop_ .  A
brief read of the code (perhaps too brief) suggests that 231 is only called when
in a nested message loop... but I'm not sure at all.

[dhollowa, 2012/02/13 17:44:26]

Done.  I've added DCHECK to verify the logic.  If this trips we might consider
using |MessageLoop::IsNested| instead.

-  MessageLoop::current()->SetNestableTasksAllowed(old_state);
 }
 
 void DragDownloadFile::QuitNestedMessageLoop() {
   AssertCurrentlyOnDragThread();
 
   if (is_running_nested_message_loop_) {
     is_running_nested_message_loop_ = false;
     MessageLoop::current()->Quit();
   }
 }
 #endif