services/media/audio/audio_track_to_output_link.cc

// 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.

#include "base/logging.h"
#include "services/media/audio/audio_output.h"
#include "services/media/audio/audio_track_impl.h"
#include "services/media/audio/audio_track_to_output_link.h"

namespace mojo {
namespace media {
namespace audio {

AudioTrackToOutputLink::Bookkeeping::~Bookkeeping() {}

AudioTrackToOutputLink::AudioTrackToOutputLink(AudioTrackImplWeakPtr track,
                                               AudioOutputWeakPtr output)
  : track_(track),
    output_(output),
    pending_queue_(new PacketQueue) {
#if !(defined(NDEBUG) && !defined(DCHECK_ALWAYS_ON))
  flush_lock_held_ = false;
#endif
}

AudioTrackToOutputLink::~AudioTrackToOutputLink() {
  ReleaseQueue(pending_queue_);
}

void AudioTrackToOutputLink::UpdateGain() {
  AudioTrackImplPtr track(GetTrack());
  AudioOutputPtr    output(GetOutput());

  // If either side of this relationship is going away, then we are shutting
  // down.  Don't bother to re-calculate the amplitude scale factor.
  if (!track || !output) { return; }

  // Obtain the track gain and, if it is at or below the muted threshold, force
  // the track to be muted and get out.
  double db_gain = track->DbGain();
  if (db_gain <= AudioTrack::kMutedGain) {
    gain_.ForceMute();
    return;
  }

  // Add in the output gain and clamp to the maximum allowed total gain.
  db_gain += output->DbGain();
  if (db_gain > AudioTrack::kMaxGain) {
    db_gain = AudioTrack::kMaxGain;
  }

  gain_.Set(db_gain);
}

AudioTrackToOutputLinkPtr AudioTrackToOutputLink::New(
    AudioTrackImplWeakPtr track,
    AudioOutputWeakPtr output) {
  return AudioTrackToOutputLinkPtr(new AudioTrackToOutputLink(track, output));
}

void AudioTrackToOutputLink::PushToPendingQueue(
    const AudioPipe::AudioPacketRefPtr& pkt) {
  base::AutoLock lock(pending_queue_lock_);
  pending_queue_->emplace_back(pkt);
}

void AudioTrackToOutputLink::FlushPendingQueue() {
  // Create a new (empty) queue before obtaining any locks.  This will allow us
  // to quickly swap the empty queue for the current queue and get out of all
  // the locks, and then release the packets at our leisure instead of
  // potentially holding off a high priority mixing thread while releasing
  // packets.
  //
  // Note: the safety of this technique depends on Flush only ever being called
  // from the AudioTrack, and the AudioTrack's actions being serialized on the
  // AudioServer's message loop thread.  If multiple flushes are allowed to be
  // invoked simultaneously, or if a packet is permitted to be added to the
  // queue while a flush operation is in progress, it is possible to return
  // packets to the user in an order different than the one that they were
  // queued in.
  PacketQueuePtr new_queue(new PacketQueue);

  {
    base::AutoLock lock(flush_lock_);
    {
      // TODO(johngro): Assuming that it is impossible to push a new packet
      // while a flush is in progress, it's pretty easy to show that this lock
      // can never be contended.  Because of this, we could consider removing
      // this lock operation (although, flush is a relatively rare operation, so
      // the extra overhead is pretty insignificant.
      base::AutoLock lock(pending_queue_lock_);
      pending_queue_.swap(new_queue);
    }
    flushed_ = true;
  }

  ReleaseQueue(new_queue);
}

AudioPipe::AudioPacketRefPtr AudioTrackToOutputLink::LockPendingQueueFront(
    bool* was_flushed) {
#if !(defined(NDEBUG) && !defined(DCHECK_ALWAYS_ON))
  // No one had better be holding the flush lock right now.  If someone is, then
  // we either have multiple threads hitting this operation at the same time
  // (should be impossible), or an audio output forgot to unlock the front of
  // the queue before attempting to lock it again.
  bool was_held = false;
  flush_lock_held_.compare_exchange_strong(was_held, true);
  DCHECK(!was_held);
#endif
  flush_lock_.Acquire();

  DCHECK(was_flushed);
  *was_flushed = flushed_;
  flushed_ = false;

  {
    base::AutoLock lock(pending_queue_lock_);
    if (pending_queue_->size()) {
      return pending_queue_->front();
    } else {
      return nullptr;
    }
  }
}

void AudioTrackToOutputLink::UnlockPendingQueueFront(
    AudioPipe::AudioPacketRefPtr* pkt,
    bool release_packet) {
  {
    base::AutoLock lock(pending_queue_lock_);

    // Assert that the user either got no packet when they locked the queue
    // (because the queue was empty), or that they got the front of the queue
    // and that the front of the queue has not changed.
    DCHECK(pkt);
    DCHECK((*pkt == nullptr) ||
           (pending_queue_->size() && (*pkt == pending_queue_->front())));

    if (*pkt) {
      *pkt = nullptr;
      if (release_packet) {
        pending_queue_->pop_front();
      }
    }
  }

  flush_lock_.Release();
#if !(defined(NDEBUG) && !defined(DCHECK_ALWAYS_ON))
  bool was_held = true;
  flush_lock_held_.compare_exchange_strong(was_held, false);
  DCHECK(was_held);
#endif
}

void AudioTrackToOutputLink::ReleaseQueue(const PacketQueuePtr& queue) {
  if (!queue) {
    return;
  }

  for (auto iter = queue->begin(); iter != queue->end(); ++iter) {
    (*iter)->SetResult(MediaConsumer::SendResult::FLUSHED);
    (*iter).reset();
  }

  queue->clear();
}

}  // namespace audio
}  // namespace media
}  // namespace mojo