Support PostTaskAndReply from a sequenced task.

base/threading/post_task_and_reply_impl.h

 // 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
-// TaskRunner::PostTaskAndReply and WorkerPool::PostTaskAndReply.
-
 #ifndef BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_
 #define BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_
 
+#include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/location.h"
 
 namespace base {
 namespace internal {
 
-// 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::SingleThreadTaskRunner, or you
-// may want base::WorkerPool.
-class PostTaskAndReplyImpl {
- public:
-  virtual ~PostTaskAndReplyImpl() = default;
+using PostTaskCallback =
+    Callback<bool(const tracked_objects::Location&, const Closure& task)>;
 
-  // Implementation for TaskRunner::PostTaskAndReply and
-  // WorkerPool::PostTaskAndReply.
-  bool PostTaskAndReply(const tracked_objects::Location& from_here,
-                        const Closure& task,
-                        const Closure& reply);
-
- private:
-  virtual bool PostTask(const tracked_objects::Location& from_here,
-                        const Closure& task) = 0;
-};
+// Posts |task| via |post_task_callback|. Then, when |task| completes its
+// execution, posts |reply| to the sequence from which this function was
+// invoked. Can only be called when SequencedTaskRunnerHandle::IsSet().

[gab, 2016/07/25 17:57:44]

Indeed, this was also the conclusion from
https://groups.google.com/a/chromium.org/d/topic/chromium-dev/cd9hldEhIEc/dis...
on which we decided that PostTaskAndReply should move from TaskRunner to
SequencedTaskRunner, is this something you'd want to take on?

(this has to be true already today, in fact today people must only be calling it
from SingleThreadTaskRunner or ThreadTaskRunnerHandle::Get() would blow up --
the API change might trip a few callers using overly generic TaskRunner handles
for their SingleThreadTaskRunners though and making this call through them...
not sure..!)

Add a TODO here in my name (or yours if you want to do this -- but I'd prefer to
prioritize MessageLoop deprecation):

// TODO(gab): Move TaskRunner::PostTaskAndReply() to SequencedTaskRunner.

[fdoray, 2016/07/25 20:40:04]

PostTaskAndReply() doesn't have to be called on a SequencedTaskRunner. The only
requirement is that there is a registered ThreadTaskRunnerHandle (or a
SequencedTaskRunnerHandle with this CL).

This works fine:

scoped_refptr<SequencedTaskRunner> reply_runner = ...;
SequencedTaskRunnerHandle sequenced_task_runner_handle(reply_task_runner);
scoped_refptr<TaskRunner> parallel_task_runner = ...;
parallel_task_runner->PostTaskAndReply(...);

The task will run on |parallel_task_runner| and the reply on |reply_runner|.

+bool BASE_EXPORT PostTaskAndReply(const tracked_objects::Location& from_here,
+                                  const Closure& task,
+                                  const Closure& reply,
+                                  const PostTaskCallback& post_task_callback);
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_