Web MIDI: implement TaskService

media/midi/task_service.h

Index: media/midi/task_service.h
diff --git a/media/midi/task_service.h b/media/midi/task_service.h
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+// Copyright 2017 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 MEDIA_MIDI_TASK_SERVICE_H_
+#define MEDIA_MIDI_TASK_SERVICE_H_
+
+#include "base/callback_forward.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
+#include "base/synchronization/lock.h"
+#include "base/threading/thread.h"
+#include "base/time/time.h"
+#include "media/midi/midi_export.h"
+
+namespace midi {
+
+// TaskService manages TaskRunners that can be used in midi and provides
+// functionalities to ensure thread safety.
+class MIDI_EXPORT TaskService final {
+ public:
+  using RunnerId = size_t;
+  using InstanceId = int;
+
+  static constexpr RunnerId kDefaultRunnerId = 0;
+
+  TaskService();
+  ~TaskService();
+
+  // Issues an InstanceId internally to post tasks via PostBoundTask() and
+  // PostDelayedBoundTask() with the InstanceId. Once UnbindInstance() is
+  // called, tasks posted via these methods with unbind InstanceId won't be
+  // invoked any more.
+  // Returns true if call is bound or unbound correctly. Otherwise returns
+  // false, that happens when the BindInstance() is called twice without
+  // unbinding the previous instance.
+  bool BindInstance();
+  bool UnbindInstance();
+
+  // Posts a task to run on a specified TaskRunner. |runner_id| should be
+  // kDefaultRunnerId or a positive number. If kDefaultRunnerId is specified
+  // the task runs on the thread on which BindInstance() is called. Other number
+  // will run the task on a dedicated thread that is bound to the |runner_id|.
+  void PostStaticTask(RunnerId runner_id, base::OnceClosure task);
+
+  // Posts a task to run on a specified TaskRunner, and ensures that the bound
+  // instance should not quit UnbindInstance() while a bound task is running.
+  // See PostStaticTask() for |runner_id|.
+  void PostBoundTask(RunnerId runner, base::OnceClosure task);
+  void PostBoundDelayedTask(RunnerId runner_id,
+                            base::OnceClosure task,
+                            base::TimeDelta delay);
+
+ private:
+  // Returns a SingleThreadTaskRunner reference. Each TaskRunner will be
+  // constructed on demand.
+  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner(RunnerId runner_id);
+
+  // Helps to run a posted bound task on TaskRunner safely.
+  void RunTask(InstanceId instance_id,
+               RunnerId runner_id,
+               base::OnceClosure task);
+
+  // Keeps a TaskRunner for the thread that calls BindInstance() as a default
+  // task runner to run posted tasks.
+  scoped_refptr<base::SingleThreadTaskRunner> default_task_runner_;
+
+  // Holds threads to host SingleThreadTaskRunners.
+  std::vector<std::unique_ptr<base::Thread>> threads_;
+
+  // Holds lock objects to ensure that tasks run while the instance is bound.
+  std::vector<std::unique_ptr<base::Lock>> thread_task_locks_;
+
+  // Holds InstanceId for the next bound instance.
+  InstanceId next_instance_id_;
+
+  // Holds InstanceId for the current bound instance.
+  InstanceId bound_instance_id_;
+
+  // Protects |next_instance_id_| and |bound_instance_id_|.
+  base::Lock instance_lock_;
+
+  // Protects all other members.
+  base::Lock lock_;
+
+  // If multiple locks should be obtained simultaneously, we should acquire them
+  // in the order below so to avoid deadklocks.
+  // |instance_lock_| -> |lock_| -> |(one of) thread_task_locks_|.
+
+  DISALLOW_COPY_AND_ASSIGN(TaskService);
+};
+
+};  // namespace midi
+
+#endif  // MEDIA_MIDI_TASK_SERVICE_H_