Break out WebAudio suspension code into new class. Add tests.

content/renderer/media/webaudio_suspender.cc

+// Copyright 2016 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 "content/renderer/media/webaudio_suspender.h"
+
+#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_task_runner_handle.h"
+
+namespace content {
+
+WebAudioSuspender::WebAudioSuspender(
+    media::AudioRendererSink::RenderCallback* callback,
+    const media::AudioParameters& params,
+    const scoped_refptr<media::AudioRendererSink>& sink,
+    const scoped_refptr<base::SingleThreadTaskRunner>& worker)
+    : callback_(callback),
+      params_(params),
+      sink_(sink),
+      task_runner_(base::ThreadTaskRunnerHandle::Get()),
+      fake_sink_(worker, params_),
+      sink_transition_callback_(base::Bind(&WebAudioSuspender::TransitionSinks,
+                                           base::Unretained(this))) {
+  DCHECK(params_.IsValid());
+  DCHECK(sink_);
+  DCHECK(callback_);
+  DCHECK(task_runner_->BelongsToCurrentThread());
+}
+
+WebAudioSuspender::~WebAudioSuspender() {
+  DCHECK(task_runner_->BelongsToCurrentThread());
+  fake_sink_.Stop();
+}
+
+int WebAudioSuspender::Render(media::AudioBus* dest,
+                              uint32_t frames_delayed,
+                              uint32_t frames_skipped) {
+  // When we're using the |fake_sink_| a null destination will be sent; we store
+  // the audio data for a future transition out of silence.
+  if (!dest) {
+    DCHECK(is_using_fake_sink_);
+    DCHECK_EQ(frames_delayed, 0u);
+    DCHECK_EQ(frames_skipped, 0u);
+    // If we have no buffers or a transition is pending, one or more extra
+    // Render() calls have occurred in before TransitionSinks() can run, so we
+    // store this data for the eventual transition.
+    if (buffers_after_silence_.empty() || is_transition_pending_)
+      buffers_after_silence_.push_back(media::AudioBus::Create(params_));
+    dest = buffers_after_silence_.back().get();
+  }
+
+  // Drain any non-silent transitional buffers before queuing more audio data.
+  // Note: This may cause skew in the WebAudio clock if too many buffers occur.
+  if (!is_using_fake_sink_ && !buffers_after_silence_.empty()) {
+    buffers_after_silence_.front()->CopyTo(dest);
+    buffers_after_silence_.pop_front();
+    return dest->frames();
+  }
+
+  // Pass-through to client and request rendering.
+  callback_->Render(dest, frames_delayed, frames_skipped);
+
+  // Check for silence or real audio data and transition if necessary.
+  if (!dest->AreFramesZero()) {
+    first_silence_time_ = base::TimeTicks();
+    if (is_using_fake_sink_) {
+      is_transition_pending_ = true;
+      task_runner_->PostTask(
+          FROM_HERE, base::Bind(sink_transition_callback_.callback(), false));
+      return dest->frames();
+    }
+  } else if (!is_using_fake_sink_) {
+    const base::TimeTicks now = base::TimeTicks::Now();
+    if (first_silence_time_.is_null())
+      first_silence_time_ = now;
+    if (now - first_silence_time_ > silent_timeout_) {

[o1ka, 2016/09/23 10:34:35]

can be "else if"

[DaleCurtis, 2016/09/23 20:24:19]

Not if silent timeout is 0 or negative (testing).

[o1ka, 2016/09/26 12:21:55]

Acknowledged.

+      is_transition_pending_ = true;
+      task_runner_->PostTask(
+          FROM_HERE, base::Bind(sink_transition_callback_.callback(), true));
+    }
+  }
+
+  return dest->frames();
+}
+
+void WebAudioSuspender::OnRenderError() {
+  callback_->OnRenderError();
+}
+
+void WebAudioSuspender::TransitionSinks(bool use_fake_sink) {
+  DCHECK(task_runner_->BelongsToCurrentThread());
+
+  // Ignore duplicate requests which can occur if the transition takes too long
+  // and multiple Render() events occur.
+  if (use_fake_sink && is_using_fake_sink_)
+    return;
+  if (!use_fake_sink && !is_using_fake_sink_)
+    return;
+
+  // It's safe to modify |is_using_fake_sink_| in between Pause() and
+  // Play() since no sink is actively calling into Render() during that time.

[o1ka, 2016/09/23 10:34:35]

ARS interface does not guarantee that Pause() is synchronous. AOD does it
asynchronously
(https://cs.chromium.org/chromium/src/media/audio/audio_output_device.cc?q=Aud...),
and we still use AOD (Vanellope is under finch, going for beta only)

[DaleCurtis, 2016/09/23 20:24:19]

Ah thanks for pointing that out. I'd forgotten. Added a lock and rejiggered
things a bit.

+  if (use_fake_sink) {
+    sink_->Pause();
+    is_transition_pending_ = false;
+    is_using_fake_sink_ = true;

[o1ka, 2016/09/23 10:34:34]

The above means that these two flags may be concurrently modified here on
|task_runner_| thread and accessed on audio rendering thread at the same time, I
suspect.

[DaleCurtis, 2016/09/23 20:24:19]

Locked!

+    fake_sink_.Start(base::Bind(base::IgnoreResult(&WebAudioSuspender::Render),
+                                base::Unretained(this), nullptr, 0, 0));
+  } else {
+    fake_sink_.Stop();
+    is_transition_pending_ = false;
+    is_using_fake_sink_ = false;
+    sink_->Play();
+  }
+}
+
+}  // namespace content