Add WorkerPool::PostTaskAndReply and use in DHCP code.

base/threading/post_task_and_reply_impl.h

Index: base/threading/post_task_and_reply_impl.h
diff --git a/base/threading/post_task_and_reply_impl.h b/base/threading/post_task_and_reply_impl.h
new file mode 100644
index 0000000000000000000000000000000000000000..1327d63a1ead91117c8231d583183f044779e75d
--- /dev/null
+++ b/base/threading/post_task_and_reply_impl.h
@@ -0,0 +1,41 @@
+// Copyright (c) 2011 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.
+
+// This file contains the implementation shared by
+// MessageLoopProxy::PostTaskAndReply and WorkerPool::PostTaskAndReply.
+
+#ifndef BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_
+#define BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_
+#pragma once
+
+#include "base/bind.h"
+#include "base/location.h"
+
+namespace base {
+

[awong, 2011/10/05 17:27:01]

Put this in the base::internal namespace?

[Jói, 2011/10/06 14:20:31]

Yes, I like that.

+// Inherit from this in a class that implements PostTask appropriately
+// for sending to a destination thread.
+//
+// Note that 'reply' will always get posted back to your current
+// MessageLoop.
+//
+// If you're looking for a concrete implementation of
+// PostTaskAndReply, you probably want base::MessageLoopProxy, or you
+// may want base::WorkerPool.
+class PostTaskAndReplyImpl {

[awong, 2011/10/05 17:27:01]

I think it'd be simpler to just loft the concrete PostTaskAndReply() class into
this file.

Then in MessageLoopProxy(), and WorkerPool(), you can just new/post it as you
want. No need to indirect through an interface.

[Jói, 2011/10/06 14:20:31]

That way there's duplication of code and the possibility of having to update two
places in the future to fix a bug or introduce new functionality for
PostTaskAndReply.  My preference would be to leave as is.

[awong, 2011/10/06 19:53:52]

I think I'm still in favor of removing this template method pattern.  The shared
code is pretty short and is just the standard pattern of "delete if post fails."
 I don't see any reason it would diverge in the future, so the removal of an
inheritance relation + a virtual dispatch seems worth it.

+ public:
+  // Implementation for MessageLoopProxy::PostTaskAndReply and
+  // WorkerPool::PostTaskAndReply.
+  bool PostTaskAndReply(const tracked_objects::Location& from_here,
+                        const Closure& task,
+                        const Closure& reply);
+
+ protected:
+  virtual bool PostTask(const tracked_objects::Location& from_here,
+                        const Closure& task) = 0;
+};
+
+}  // namespace base
+
+#endif  // BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_