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TMessagesProj/jni/voip/webrtc/base/sequenced_task_runner.h

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+// 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_TASK_RUNNER_H_
+#define BASE_SEQUENCED_TASK_RUNNER_H_
+
+#include <memory>
+
+#include "base/base_export.h"
+#include "base/callback.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 Location& from_here, OnceClosure task);
+
+  virtual bool PostNonNestableDelayedTask(const Location& from_here,
+                                          OnceClosure 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 Location& from_here, const T* object) {
+    return DeleteOrReleaseSoonInternal(from_here, &DeleteHelper<T>::DoDelete,
+                                       object);
+  }
+
+  template <class T>
+  bool DeleteSoon(const Location& from_here, std::unique_ptr<T> object) {
+    return DeleteSoon(from_here, object.release());
+  }
+
+  // Submits a non-nestable task to release the given object.
+  //
+  // ReleaseSoon makes sure that the object it the scoped_refptr points to gets
+  // properly released on the correct thread.
+  // We apply ReleaseSoon to the rvalue as the side-effects can be unclear to
+  // the caller if an lvalue is used. That being so, the scoped_refptr should
+  // always be std::move'd.
+  // Example use:
+  //
+  // scoped_refptr<T> foo_scoped_refptr;
+  // ...
+  // task_runner->ReleaseSoon(std::move(foo_scoped_refptr));
+  template <class T>
+  void ReleaseSoon(const Location& from_here, scoped_refptr<T>&& object) {
+    if (!object)
+      return;
+
+    DeleteOrReleaseSoonInternal(from_here, &ReleaseHelper<T>::DoRelease,
+                                object.release());
+  }
+
+  // Returns true iff tasks posted to this TaskRunner are sequenced
+  // with this call.
+  //
+  // 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.
+  virtual bool RunsTasksInCurrentSequence() const = 0;
+
+ protected:
+  ~SequencedTaskRunner() override = default;
+
+ private:
+  bool DeleteOrReleaseSoonInternal(const Location& from_here,
+                                   void (*deleter)(const void*),
+                                   const void* object);
+};
+
+// Sample usage with std::unique_ptr :
+// std::unique_ptr<Foo, base::OnTaskRunnerDeleter> ptr(
+//     new Foo, base::OnTaskRunnerDeleter(my_task_runner));
+//
+// For RefCounted see base::RefCountedDeleteOnSequence.
+struct BASE_EXPORT OnTaskRunnerDeleter {
+  explicit OnTaskRunnerDeleter(scoped_refptr<SequencedTaskRunner> task_runner);
+  ~OnTaskRunnerDeleter();
+
+  OnTaskRunnerDeleter(OnTaskRunnerDeleter&&);
+  OnTaskRunnerDeleter& operator=(OnTaskRunnerDeleter&&);
+
+  // For compatibility with std:: deleters.
+  template <typename T>
+  void operator()(const T* ptr) {
+    if (ptr)
+      task_runner_->DeleteSoon(FROM_HERE, ptr);
+  }
+
+  scoped_refptr<SequencedTaskRunner> task_runner_;
+};
+
+}  // namespace base
+
+#endif  // BASE_SEQUENCED_TASK_RUNNER_H_