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
new file mode 100644
index 0000000000000000000000000000000000000000..b32944be56b02cdaf6e277f1d4959e3b68221b1e
--- /dev/null
+++ b/media/midi/task_service.h
@@ -0,0 +1,94 @@
+// 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 = int;
+  using InstanceId = int;
+
+  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.
+  bool BindInstance();

[yhirano, 2017/06/08 09:03:38]

In https://codereview.chromium.org/2923163003/ the return value is not used. Is
it needed? If it's needed, please add a comment for it.

[Takashi Toyoshima, 2017/06/08 10:30:11]

Yeah, I should fix caller side to check this returned value as current platform
dependent codes does.

+  bool UnbindInstance();
+
+  // Posts a task to run on a specified TaskRunner.
+  void PostStaticTask(RunnerId runner, base::OnceClosure task);
+
+  // Post a task to run on a specificed TaskRunner, and ensures that the bound

[yhirano, 2017/06/08 09:03:38]

Post"s"

[Takashi Toyoshima, 2017/06/08 10:30:11]

Done.

+  // instance should not quit UnbindInstance() while a bound task is running.
+  void PostBoundTask(RunnerId runner, base::OnceClosure task);

[yhirano, 2017/06/08 09:03:38]

You should add a comment (and DCHECK) that |runner| must not be negative.

If it's possible to say that |runner| must be positive, we can remove
reply_task_lock_ by setting kReplyRunnerId to 0.

[Takashi Toyoshima, 2017/06/08 10:30:11]

That would make code a little confusing due to "index != id", but let me try.

+  void PostBoundDelayedTask(RunnerId runner,
+                            base::OnceClosure task,
+                            base::TimeDelta delay);
+
+  // Posts a task to run on a thread that called BindInstance(), and ensures
+  // that the bound instance should not quit UnbindInstance() while thee task is

[yhirano, 2017/06/08 09:03:38]

s/thee/the/

[Takashi Toyoshima, 2017/06/08 10:30:11]

Done.

+  // running.
+  void PostBoundReplyTask(base::OnceClosure task);
+
+ private:
+  // Returns a SingleThreadTaskRunner reference. Each TaskRunner will be
+  // constructed on demand.
+  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner(RunnerId id);
+
+  // Helps to run a posted bound task on TaskRunner safely.
+  void RunTask(InstanceId instance_id, RunnerId runner, base::OnceClosure task);
+
+  // Keeps a TaskRunner for the thread that calls BindInstance() to post reply
+  // tasks.
+  scoped_refptr<base::SingleThreadTaskRunner> reply_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 a lock object to ensure that a reply task runs while the instance is
+  // bound.
+  base::Lock reply_task_lock_;
+
+  // 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_