Add an initial revision of an audio server.

services/media/audio/audio_output.h

Index: services/media/audio/audio_output.h
diff --git a/services/media/audio/audio_output.h b/services/media/audio/audio_output.h
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+// Copyright 2015 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 SERVICES_MEDIA_AUDIO_AUDIO_OUTPUT_H_
+#define SERVICES_MEDIA_AUDIO_AUDIO_OUTPUT_H_
+
+#include <deque>
+#include <memory>
+#include <set>
+
+#include "base/callback.h"
+#include "base/synchronization/lock.h"
+#include "base/threading/sequenced_worker_pool.h"
+#include "mojo/services/media/common/cpp/local_time.h"
+#include "services/media/audio/audio_pipe.h"
+#include "services/media/audio/audio_track_impl.h"
+#include "services/media/audio/fwd_decls.h"
+
+namespace mojo {
+namespace media {
+namespace audio {
+
+class AudioOutput {
+ public:
+  virtual ~AudioOutput();
+
+  // AddTrack/RemoveTrack
+  //
+  // Adds or removes a track to/from the set of current set of tracks serviced
+  // by this output.  Called only from the main message loop.  Obtains the
+  // processing_lock and may block for the time it takes the derived class to
+  // run its processing task if the task is in progress when the method was
+  // called.
+  MediaResult AddTrackLink(AudioTrackToOutputLinkPtr link);
+  MediaResult RemoveTrackLink(const AudioTrackToOutputLinkPtr& link);
+
+ protected:
+  explicit AudioOutput(AudioOutputManager* manager);
+
+  //////////////////////////////////////////////////////////////////////////////
+  //
+  // Methods which may be implemented by derived classes to customize behavior.
+  //
+  //////////////////////////////////////////////////////////////////////////////
+
+  // Init
+  //
+  // Called during startup on the AudioServer's main message loop thread.  No
+  // locks are being held at this point.  Derived classes should allocate their
+  // hardware resources and initialize any internal state.  Return
+  // MediaResult::OK if everything is good and the output is ready to do work.
+  virtual MediaResult Init();
+
+  // Cleanup
+  //
+  // Called at shutdown on the AudioServer's main message loop thread to allow
+  // derived classes to clean up any allocated resources.  All pending
+  // processing callbacks have either been nerfed or run till completion.  All
+  // AudioTrack tracks have been disconnected.  No locks are being held.
+  virtual void Cleanup();
+
+  // Process
+  //
+  // Called from within the context of the processing lock any time a scheduled
+  // processing callback fires.  One callback will be automatically scheduled at
+  // the end of initialization.  After that, derived classes are responsible for
+  // scheduling all subsequent callbacks to keep the engine running.
+  //
+  // Note:  Process callbacks execute on one of the threads from the
+  // AudioOutputManager's base::SequencedWorkerPool.  While successive callbacks
+  // may not execute on the same thread, they are guaranteed to execute in a
+  // serialized fashion.
+  virtual void Process() = 0;
+
+  // InitializeLink
+  //
+  // Called on the AudioServer's main message loop any time a track is being
+  // added to this output.  Outputs should allocate and initialize any
+  // bookkeeping they will need to perform mixing on behalf of the newly added
+  // track.
+  //
+  // @return MediaResult::OK if initialization succeeded, or an appropriate
+  // error code otherwise.
+  virtual MediaResult InitializeLink(const AudioTrackToOutputLinkPtr& link);
+
+  //////////////////////////////////////////////////////////////////////////////
+  //
+  // Methods which may used by derived classes from within the context of a
+  // processing callback.  Note; since these methods are intended to be called
+  // from the within a process callback, the processing_lock will always be held
+  // when they are called.
+  //
+
+  // ScheduleCallback
+  //
+  // Schedule a processing callback at the specified absolute time on the local
+  // clock.
+  void ScheduleCallback(LocalTime when);
+
+  // ShutdownSelf
+  //
+  // Kick off the process of shooting ourselves in the head.  Note, after this
+  // method has been called, no new callbacks may be scheduled.  As soon as the
+  // main message loop finds out about our shutdown request, it will complete
+  // the process of shutting us down, unlinking us from our tracks and calling
+  // the Cleanup method.
+  void ShutdownSelf();
+
+  // shutting_down
+  //
+  // Check the shutting down flag.  Only the base class may modify the flag, but
+  // derived classes are free to check it at any time.
+  inline bool shutting_down() const { return shutting_down_; }
+
+  // TODO(johngro): Order this by priority.  Figure out how we are going to be
+  // able to quickly find a track with a specific priority in order to optimize
+  // changes of priority.  Perhaps uniquify the priorities by assigning a
+  // sequence number to the lower bits (avoiding collisions when assigning new
+  // priorities will be the trick).
+  //
+  // Right now, we have no priorities, so this is just a set of track/output
+  // links.
+  AudioTrackToOutputLinkSet links_;
+  AudioOutputManager* manager_;
+
+ private:
+  // It's always nice when you manager is also your friend.  Seriously though,
+  // the AudioOutputManager gets to call Init and Shutown, no one else
+  // (including derived classes) should be able to.
+  friend class AudioOutputManager;
+
+  // Thunk used to schedule delayed processing tasks on our task_runner.
+  static void ProcessThunk(AudioOutputWeakPtr weak_output);
+
+  // Called from the AudioOutputManager after an output has been created.
+  // Gives derived classes a chance to set up hardware, then sets up the
+  // machinery needed for scheduling processing tasks and schedules the first
+  // processing callback immediately in order to get the process running.
+  MediaResult Init(const AudioOutputPtr& self,
+                   scoped_refptr<base::SequencedTaskRunner> task_runner);
+
+  // Called from Shutdown (main message loop) and ShutdowSelf (processing
+  // context).  Starts the process of shutdown, preventing new processing tasks
+  // from being scheduled, and nerfing any tasks in flight.
+  //
+  // @return true if this call just kicked off the process of shutting down,
+  // false otherwise.
+  bool BeginShutdown();
+
+  // Called from the AudioOutputManager on the main message loop
+  // thread.  Makes certain that the process of shutdown has started,
+  // synchronizes with any processing tasks which were executing at the time,
+  // then finishes the shutdown process by unlinking from all tracks and
+  // cleaning up all resources.
+  void Shutdown();
+
+  base::Lock processing_lock_;
+  base::Lock shutdown_lock_;
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+  AudioOutputWeakPtr weak_self_;
+
+  // TODO(johngro): Eliminate the shutting down flag and just use the
+  // task_runner_'s nullness for this test?
+  volatile bool shutting_down_ = false;
+  volatile bool shut_down_ = false;
+};
+
+}  // namespace audio
+}  // namespace media
+}  // namespace mojo
+
+#endif  // SERVICES_MEDIA_AUDIO_AUDIO_OUTPUT_H_
+