Add an initial revision of an audio server.

services/media/audio/audio_track_to_output_link.h

Index: services/media/audio/audio_track_to_output_link.h
diff --git a/services/media/audio/audio_track_to_output_link.h b/services/media/audio/audio_track_to_output_link.h
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index 0000000000000000000000000000000000000000..f9d31ec1d2a6302540175e4c2549856553cadadf
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+++ b/services/media/audio/audio_track_to_output_link.h
@@ -0,0 +1,128 @@
+// 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_TRACK_TO_OUTPUT_LINK_H_
+#define SERVICES_MEDIA_AUDIO_AUDIO_TRACK_TO_OUTPUT_LINK_H_
+
+#include <deque>
+#include <memory>
+
+#include "base/synchronization/lock.h"
+#include "services/media/audio/audio_pipe.h"
+#include "services/media/audio/fwd_decls.h"
+
+namespace mojo {
+namespace media {
+namespace audio {
+
+// AudioTrackToOutputLink is a small class which tracks the relationship between
+// an audio track and an audio output.  Tracks and outputs are expected to hold
+// strong pointers to the state in the collections they use to track their
+// peers.
+//
+// When either a track or an output ceases to exist, its collection will clear
+// releasing the reference to the shared state.  When the other half of the
+// relationship realizes that its peer has gone away (typically by failing to
+// promote the weak reference to its peer held in the share state object), it
+// can purge the state object strong pointer from its collection triggering the
+// final cleanup of the shared state.
+//
+// Because the final cleanup of the shared state can be triggered either from an
+// output manager mixer thread, or from the audio service's main message loop,
+// it must be safe to destruct all of the shared state from any thread in the
+// system.  No assumptions may be made about threading when destructing.

[jeffbrown, 2015/11/04 23:43:34]

This could be amended by making the cleanup signal come from only one place. 
Might be worth exploring in a future patch.

[johngro, 2015/11/06 02:20:26]

Acknowledged.

+//
+// The AudioTrackToOutputLink object holds a queue of pending audio packet
+// references sourced from the AudioTrack to be rendered on the audio output.
+// The references are safe to release either from an output manager thread, or
+// from the audio service's main message loop thread (which drives track
+// behavior).
+//
+// Finally, both the Output may have a pointer to a Bookkeeping object in order
+// to manage bookkeeping tasks specific to the Track/Output relationship.  The
+// following rules must be obeyed at all times...
+//
+// + Derrived classes of the Bookkeeping object created by the Output must be
+//   safe to destroy either thread.  During destruction, no potentially blocking
+//   operations may be performed.  No heavy operations (such as logging) should
+//   be performed.
+// + Only the output is permitted to access the output bookkeeping.  The track
+//   must make no attempts to modify the bookkeeping or its pointer.
+// + Outputs must hold a strong reference to the shared link object object
+//   whenever they are accessing their bookkeeping object.  The link object is
+//   considered to be the owner of the Bookkeeping, users must never hold a
+//   naked pointer to their bookkeeping if the link could possibly destruct.
+//
+class AudioTrackToOutputLink {
+ public:
+  struct Bookkeeping {
+    virtual ~Bookkeeping();
+  };
+
+  using BookkeepingPtr = std::unique_ptr<Bookkeeping>;
+  using PacketQueue    = std::deque<AudioPipe::AudioPacketRefPtr>;
+  using PacketQueuePtr = std::unique_ptr<PacketQueue>;
+
+  static AudioTrackToOutputLinkPtr New(AudioTrackImplWeakPtr track,
+                                       AudioOutputWeakPtr output);
+  virtual ~AudioTrackToOutputLink();
+
+  // Accessors for the track and output pointers.  Automatically attempts to
+  // promote the weak pointer to a strong pointer.
+  //
+  // TODO(johngro):  Given the way outputs are currently shut down, there is
+  // actually no need for the link to hold a weak pointer to output.  By the
+  // time it destructs, All references to it are guaranteed to have been removed
+  // from all tracks in the context of the main event loop.  Consider converting
+  // this from a weak pointer to a strong pointer.
+  AudioTrackImplPtr GetTrack() { return track_.lock(); }
+  AudioOutputPtr GetOutput() { return output_.lock(); }
+
+  // AudioTrack PendingQueue operations.  Never call these from the AudioOutput.
+  void PushToPendingQueue(const AudioPipe::AudioPacketRefPtr& pkt);
+  void FlushPendingQueue();
+
+  // AudioOutput PendingQueue operations.  Never call these from the AudioTrack.
+  // When consuming audio, AudioOutputs must always pair their calls to
+  // LockPendingQueueFront and UnlockPendingQueueFront, passing the pointer to
+  // the reference to the front of the queue they obtained in the process (even
+  // if the front of the queue was nullptr).
+  //
+  // Doing so ensures that AudioTracks which are attempting to flush the pending
+  // queue are forced to wait if the front of the queue is involved in a mixing
+  // operation.  This, in turn, guarantees that audio packets are always
+  // returned to the user in the order which they were queued in without forcing
+  // AudioTracks to wait to queue new data if a mix operation is in progress.
+  AudioPipe::AudioPacketRefPtr LockPendingQueueFront(bool* was_flushed);
+  void UnlockPendingQueueFront(AudioPipe::AudioPacketRefPtr* pkt,
+                               bool release_packet);
+
+  // Bookkeeping access.
+  //
+  BookkeepingPtr& output_bookkeeping() { return output_bookkeeping_; }
+
+ private:
+  void ReleaseQueue(const PacketQueuePtr& queue);
+
+  AudioTrackToOutputLink(AudioTrackImplWeakPtr track,
+                         AudioOutputWeakPtr output);
+
+  AudioTrackImplWeakPtr track_;
+  AudioOutputWeakPtr output_;
+  BookkeepingPtr output_bookkeeping_;
+
+  base::Lock flush_lock_;
+  base::Lock pending_queue_lock_;
+  PacketQueuePtr pending_queue_;
+  bool flushed_ = true;
+#if !(defined(NDEBUG) && !defined(DCHECK_ALWAYS_ON))
+  std::atomic<bool> flush_lock_held_;
+#endif
+};
+
+}  // namespace audio
+}  // namespace media
+}  // namespace mojo
+
+#endif  // SERVICES_MEDIA_AUDIO_AUDIO_TRACK_TO_OUTPUT_LINK_H_