Introduce TaskRunner::RunsTasksInCurrentSequence()

base/task_runner.h

 // 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.
 
 #ifndef BASE_TASK_RUNNER_H_
 #define BASE_TASK_RUNNER_H_
 
 #include <stddef.h>
 
 #include "base/base_export.h"
@@ skipping 54 matching lines @@
 
   // Like PostTask, but tries to run the posted task only after
   // |delay_ms| has passed.
   //
   // It is valid for an implementation to ignore |delay_ms|; that is,
   // to have PostDelayedTask behave the same as PostTask.
   virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
                                OnceClosure task,
                                base::TimeDelta delay) = 0;
 
-  // Returns true if the current thread is a thread on which a task
-  // may be run, and false if no task will be run on the current
-  // thread.
+  // Drepecated: favor RunsTasksInCurrentSequence().
+  // TODO(http://crbug.com/665062): mass redirect callers and remove this.
+  bool RunsTasksOnCurrentThread() const;
+
+  // Returns true iff tasks posted to this TaskRunner are sequenced
+  // with this call.
   //
-  // It is valid for an implementation to always return true, or in
-  // general to use 'true' as a default value.
-  virtual bool RunsTasksOnCurrentThread() const = 0;
+  // In particular:
+  // - Returns true if this is a SequencedTaskRunner to which the
+  //   current task was posted.
+  // - Returns true if this is a SequencedTaskRunner bound to the
+  //   same sequence as the SequencedTaskRunner to which the current
+  //   task was posted.
+  // - Returns true if this is a SingleThreadTaskRunner bound to
+  //   the current thread.
+  // TODO(http://crbug.com/665062):
+  // This API doesn't make sense for parallel TaskRunners.
+  // Introduce alternate static APIs for documentation purposes of "this runs
+  // in pool X", have RunsTasksInCurrentSequence() return false for parallel
+  // TaskRunners, and ultimately move this method down to SequencedTaskRunner.

[gab, 2017/04/19 18:37:56]

Indent last 4 lines by two extra spaces to make it clear they're part of TODO
block.

+  virtual bool RunsTasksInCurrentSequence() const = 0;
 
   // Posts |task| on the current TaskRunner.  On completion, |reply|
   // is posted to the thread that called PostTaskAndReply().  Both
   // |task| and |reply| are guaranteed to be deleted on the thread
   // from which PostTaskAndReply() is invoked.  This allows objects
   // that must be deleted on the originating thread to be bound into
   // the |task| and |reply| Closures.  In particular, it can be useful
   // to use WeakPtr<> in the |reply| Closure so that the reply
   // operation can be canceled. See the following pseudo-code:
   //
@@ skipping 49 matching lines @@
   virtual void OnDestruct() const;
 };
 
 struct BASE_EXPORT TaskRunnerTraits {
   static void Destruct(const TaskRunner* task_runner);
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_RUNNER_H_