Move sequenced_task_runner to base/task

base/sequenced_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_SEQUENCED_TASKRUNNER_H_
-#define BASE_SEQUENCED_TASKRUNNER_H_
-
-#include "base/base_export.h"
-#include "base/sequenced_task_runner_helpers.h"
-#include "base/task_runner.h"
-
-namespace base {
-
-// A SequencedTaskRunner is a subclass of TaskRunner that provides
-// additional guarantees on the order that tasks are started, as well
-// as guarantees on when tasks are in sequence, i.e. one task finishes
-// before the other one starts.
-//
-// Summary
-// -------
-// Non-nested tasks with the same delay will run one by one in FIFO
-// order.
-//
-// Detailed guarantees
-// -------------------
-//
-// SequencedTaskRunner also adds additional methods for posting
-// non-nestable tasks.  In general, an implementation of TaskRunner
-// may expose task-running methods which are themselves callable from
-// within tasks.  A non-nestable task is one that is guaranteed to not
-// be run from within an already-running task.  Conversely, a nestable
-// task (the default) is a task that can be run from within an
-// already-running task.
-//
-// The guarantees of SequencedTaskRunner are as follows:
-//
-//   - Given two tasks T2 and T1, T2 will start after T1 starts if:
-//
-//       * T2 is posted after T1; and
-//       * T2 has equal or higher delay than T1; and
-//       * T2 is non-nestable or T1 is nestable.
-//
-//   - If T2 will start after T1 starts by the above guarantee, then
-//     T2 will start after T1 finishes and is destroyed if:
-//
-//       * T2 is non-nestable, or
-//       * T1 doesn't call any task-running methods.
-//
-//   - If T2 will start after T1 finishes by the above guarantee, then
-//     all memory changes in T1 and T1's destruction will be visible
-//     to T2.
-//
-//   - If T2 runs nested within T1 via a call to the task-running
-//     method M, then all memory changes in T1 up to the call to M
-//     will be visible to T2, and all memory changes in T2 will be
-//     visible to T1 from the return from M.
-//
-// Note that SequencedTaskRunner does not guarantee that tasks are run
-// on a single dedicated thread, although the above guarantees provide
-// most (but not all) of the same guarantees.  If you do need to
-// guarantee that tasks are run on a single dedicated thread, see
-// SingleThreadTaskRunner (in single_thread_task_runner.h).
-//
-// Some corollaries to the above guarantees, assuming the tasks in
-// question don't call any task-running methods:
-//
-//   - Tasks posted via PostTask are run in FIFO order.
-//
-//   - Tasks posted via PostNonNestableTask are run in FIFO order.
-//
-//   - Tasks posted with the same delay and the same nestable state
-//     are run in FIFO order.
-//
-//   - A list of tasks with the same nestable state posted in order of
-//     non-decreasing delay is run in FIFO order.
-//
-//   - A list of tasks posted in order of non-decreasing delay with at
-//     most a single change in nestable state from nestable to
-//     non-nestable is run in FIFO order. (This is equivalent to the
-//     statement of the first guarantee above.)
-//
-// Some theoretical implementations of SequencedTaskRunner:
-//
-//   - A SequencedTaskRunner that wraps a regular TaskRunner but makes
-//     sure that only one task at a time is posted to the TaskRunner,
-//     with appropriate memory barriers in between tasks.
-//
-//   - A SequencedTaskRunner that, for each task, spawns a joinable
-//     thread to run that task and immediately quit, and then
-//     immediately joins that thread.
-//
-//   - A SequencedTaskRunner that stores the list of posted tasks and
-//     has a method Run() that runs each runnable task in FIFO order
-//     that can be called from any thread, but only if another
-//     (non-nested) Run() call isn't already happening.
-class BASE_EXPORT SequencedTaskRunner : public TaskRunner {
- public:
-  // The two PostNonNestable*Task methods below are like their
-  // nestable equivalents in TaskRunner, but they guarantee that the
-  // posted task will not run nested within an already-running task.
-  //
-  // A simple corollary is that posting a task as non-nestable can
-  // only delay when the task gets run.  That is, posting a task as
-  // non-nestable may not affect when the task gets run, or it could
-  // make it run later than it normally would, but it won't make it
-  // run earlier than it normally would.
-
-  // TODO(akalin): Get rid of the boolean return value for the methods
-  // below.
-
-  bool PostNonNestableTask(const tracked_objects::Location& from_here,
-                           const Closure& task);
-
-  virtual bool PostNonNestableDelayedTask(
-      const tracked_objects::Location& from_here,
-      const Closure& task,
-      base::TimeDelta delay) = 0;
-
-  // Submits a non-nestable task to delete the given object.  Returns
-  // true if the object may be deleted at some point in the future,
-  // and false if the object definitely will not be deleted.
-  template <class T>
-  bool DeleteSoon(const tracked_objects::Location& from_here,
-                  const T* object) {
-    return
-        subtle::DeleteHelperInternal<T, bool>::DeleteViaSequencedTaskRunner(
-            this, from_here, object);
-  }
-
-  // Submits a non-nestable task to release the given object.  Returns
-  // true if the object may be released at some point in the future,
-  // and false if the object definitely will not be released.
-  template <class T>
-  bool ReleaseSoon(const tracked_objects::Location& from_here,
-                   T* object) {
-    return
-        subtle::ReleaseHelperInternal<T, bool>::ReleaseViaSequencedTaskRunner(
-            this, from_here, object);
-  }
-
- protected:
-  virtual ~SequencedTaskRunner() {}
-
- private:
-  template <class T, class R> friend class subtle::DeleteHelperInternal;
-  template <class T, class R> friend class subtle::ReleaseHelperInternal;
-
-  bool DeleteSoonInternal(const tracked_objects::Location& from_here,
-                          void(*deleter)(const void*),
-                          const void* object);
-
-  bool ReleaseSoonInternal(const tracked_objects::Location& from_here,
-                           void(*releaser)(const void*),
-                           const void* object);
-};
-
-}  // namespace base
-
-#endif  // BASE_SEQUENCED_TASKRUNNER_H_