Implementation of PostTaskAndReply() in MessageLoopProxy and BrowserThread.

base/message_loop_proxy.cc

-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// 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.
 
 #include "base/message_loop_proxy.h"
 
+#include "base/bind.h"
+
 namespace base {
 
+namespace {
+
+// This relay class remembers the MessageLoop that it was created on, and
+// ensures that both the |task| and |reply| Closures are deleted on this same
+// thread. Also, |task| is guaranteed to be deleted before |reply| is run or
+// deleted.
+//
+// If this is not possible because the originating MessageLoop is no longer
+// available, the the |task| and |reply| Closures are leaked.  Leaking is
+// considered preferable to having a thread-safetey violations caused by
+// invoking the Closure destructor on the wrong thread.
+class PostTaskAndReplyRelay {
+ public:
+  PostTaskAndReplyRelay(const tracked_objects::Location& from_here,
+                        const Closure& task, const Closure& reply)
+      : from_here_(from_here),
+        origin_loop_(MessageLoopProxy::current()) {
+    task_ = task;
+    reply_ = reply;
+  }
+
+  ~PostTaskAndReplyRelay() {
+    DCHECK(origin_loop_->BelongsToCurrentThread());
+    task_.Reset();
+    reply_.Reset();
+  }
+
+  void Run() {
+    task_.Run();
+    origin_loop_->PostTask(
+        from_here_,
+        Bind(&PostTaskAndReplyRelay::RunReplyAndSelfDestruct,
+             base::Unretained(this)));
+  }
+
+ private:
+  void RunReplyAndSelfDestruct() {
+    DCHECK(origin_loop_->BelongsToCurrentThread());
+
+    // Force |task_| to be released before |reply_| is to ensure that no one
+    // accidentally depends on |task_| keeping one of its arguments alive while
+    // |reply_| is executing.
+    task_.Reset();
+
+    reply_.Run();
+
+    // Cue mission impossible theme.
+    delete this;
+  }
+
+  tracked_objects::Location from_here_;
+  scoped_refptr<MessageLoopProxy> origin_loop_;
+  Closure reply_;
+  Closure task_;
+};
+
+}  // namespace
+
 MessageLoopProxy::MessageLoopProxy() {
 }
 
 MessageLoopProxy::~MessageLoopProxy() {
 }
 
+bool MessageLoopProxy::PostTaskAndReply(
+    const tracked_objects::Location& from_here,
+    const Closure& task,
+    const Closure& reply) {
+  PostTaskAndReplyRelay* relay =
+      new PostTaskAndReplyRelay(from_here, task, reply);
+  if (!PostTask(from_here, Bind(&PostTaskAndReplyRelay::Run,
+                                Unretained(relay)))) {
+    delete relay;
+    return false;
+  }
+
+  return true;
+}
+
 void MessageLoopProxy::OnDestruct() const {
   delete this;
 }
 
 }  // namespace base