Implementation of PostTaskAndReply() in MessageLoopProxy and BrowserThread.

base/task.h

Index: base/task.h
diff --git a/base/task.h b/base/task.h
index 460c8c2e5c9a4dca9318da1c45db848baa7b8007..acc909832e71fab82b52577ca23e89e482c1ed6a 100644
--- a/base/task.h
+++ b/base/task.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "base/base_export.h"
+#include "base/callback.h"
 #include "base/debug/alias.h"
 #include "base/memory/raw_scoped_refptr_mismatch_checker.h"
 #include "base/memory/weak_ptr.h"
@@ -15,6 +16,7 @@
 
 namespace base {
 const size_t kDeadTask = 0xDEAD7A53;
+class MessageLoopProxy;
 }
 
 // Task ------------------------------------------------------------------------
@@ -602,6 +604,36 @@ class TaskClosureAdapter : public RefCounted<TaskClosureAdapter> {
 
 }  // namespace subtle
 
+namespace internal {
+
+// 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.
+//
+// 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);
+
+  ~PostTaskAndReplyRelay();
+
+  void Run();
+
+ private:
+  void RunReplyAndSelfDestruct();
+
+  tracked_objects::Location from_here_;
+  scoped_refptr<MessageLoopProxy> origin_loop_;
+  Closure task_;
+  Closure reply_;
+};
+
+}  // namespace internal
+
 }  // namespace base
 
 #endif  // BASE_TASK_H_