Add an initial revision of an audio server.

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_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_);
+}
+
+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 simultaniously, 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.

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

FWIW, we might actually find it worthwhile to push all packets through the audio
output itself as usual even when flushed.  Just mark them as flushed so that
they can skip the main processing stages.

That way there will be one consistent path by which packets are queued,
processed, and retired.  I've found this kind of pattern useful for other
pipelines and it's no more expensive in the end.

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

As discussed F2F yesterday, this will add a ton of statefulness to the front end
of the flush operation.  We can consider it later on, but I still feel that I
can make number a strong argument for handling it like this, including (but not
limited to)

1) Performance.
2) Centralization of implementation (and reducing the responsibilities of all
the various output implementations)
3) General complexity and maintainability.

+  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).reset();
+  }
+
+  queue->clear();
+}
+
+}  // namespace audio
+}  // namespace media
+}  // namespace mojo
+