[Chromoting] Introducing refcount-based life time management of the message loops in the service (d…

remoting/base/auto_thread_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 REMOTING_BASE_AUTO_THREAD_TASK_RUNNER_H_
+#define REMOTING_BASE_AUTO_THREAD_TASK_RUNNER_H_
+
+#include "base/basictypes.h"
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/message_loop.h"
+#include "base/single_thread_task_runner.h"
+
+class MessageLoop;
+
+namespace remoting {
+
+// A wrapper around |SingleThreadTaskRunner| that provides automatic lifetime
+// management, by invoking a caller-supplied callback when no more references
+// remain, that can stop the wrapped task runner. The caller may also provide a
+// reference to a parent |AutoThreadTaskRunner| to express dependencies between
+// child and parent thread lifetimes.
+class AutoThreadTaskRunner : public base::SingleThreadTaskRunner {
+ public:
+  // Constructs an instance of |AutoThreadTaskRunner| wrapping |task_runner|.
+  // |stop_callback| is called (on arbitraty thread) when the last reference to
+  // the object is dropped.
+  explicit AutoThreadTaskRunner(
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+  AutoThreadTaskRunner(scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+                       const base::Closure& stop_callback);
+
+  // TODO(alexeypa): Remove the |parent| reference once Thread class supports
+  // stopping the thread's message loop and joining the thread separately.
+  // See http://crbug.com/145856.
+  //
+  // Background: currently the only legitimate way of stopping a thread is to
+  // call Thread::Stop() from the same thread that started it. Thread::Stop()
+  // will stop the thread message loop by posting a private task and join
+  // the thread. Thread::Stop() verifies that it is called from the right thread
+  // and that the message loop has been stopped by the private task mentioned
+  // above.
+  //
+  // Due to NPAPI/PPAPI limitations tasks cannot be posted to the main message
+  // loop when the host plugin is being shut down. This presents a challenge
+  // since Thread::Stop() cannot be called from an arbitrary thread. To work
+  // around this we keep a reference to the parent task runner (typically
+  // the one that started the thread) to keep it alive while the worker thread
+  // is in use. Thread::Stop() is called to stop the worker thread when
+  // the parent task runner exits.
+  AutoThreadTaskRunner(scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+                       scoped_refptr<AutoThreadTaskRunner> parent);
+  AutoThreadTaskRunner(scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+                       scoped_refptr<AutoThreadTaskRunner> parent,
+                       const base::Closure& stop_callback);
+
+  // SingleThreadTaskRunner implementation
+  virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
+                               const base::Closure& task,
+                               base::TimeDelta delay) OVERRIDE;
+  virtual bool PostNonNestableDelayedTask(
+      const tracked_objects::Location& from_here,
+      const base::Closure& task,
+      base::TimeDelta delay) OVERRIDE;
+  virtual bool RunsTasksOnCurrentThread() const OVERRIDE;
+
+ private:
+  friend class base::DeleteHelper<AutoThreadTaskRunner>;
+
+  virtual ~AutoThreadTaskRunner();
+
+  // An reference to the parent message loop to keep it alive while
+  // |task_runner_| is running.
+  scoped_refptr<AutoThreadTaskRunner> parent_;
+
+  // This callback quits |task_runner_|. It can be run on any thread.
+  base::Closure stop_callback_;
+
+  // The wrapped task runner.
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+
+  DISALLOW_COPY_AND_ASSIGN(AutoThreadTaskRunner);
+};
+
+}  // namespace remoting
+
+#endif  // REMOTING_BASE_AUTO_THREAD_TASK_RUNNER_H_