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

media/base/silent_sink_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 "media/base/silent_sink_suspender.h"
+
+#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_task_runner_handle.h"
+
+namespace media {
+
+SilentSinkSuspender::SilentSinkSuspender(
+    AudioRendererSink::RenderCallback* callback,
+    base::TimeDelta silence_timeout,
+    const AudioParameters& params,
+    const scoped_refptr<AudioRendererSink>& sink,
+    const scoped_refptr<base::SingleThreadTaskRunner>& worker)
+    : callback_(callback),
+      params_(params),
+      sink_(sink),
+      task_runner_(base::ThreadTaskRunnerHandle::Get()),
+      silence_timeout_(silence_timeout),
+      fake_sink_(worker, params_),
+      sink_transition_callback_(
+          base::Bind(&SilentSinkSuspender::TransitionSinks,
+                     base::Unretained(this))) {
+  DCHECK(params_.IsValid());
+  DCHECK(sink_);
+  DCHECK(callback_);
+  DCHECK(task_runner_->BelongsToCurrentThread());
+}
+
+SilentSinkSuspender::~SilentSinkSuspender() {
+  DCHECK(task_runner_->BelongsToCurrentThread());
+  fake_sink_.Stop();
+}
+
+int SilentSinkSuspender::Render(AudioBus* dest,
+                                uint32_t frames_delayed,
+                                uint32_t frames_skipped) {
+  // Lock required since AudioRendererSink::Pause() is not synchronous, we need
+  // to discard these calls during the transition to the fake sink.
+  base::AutoLock al(transition_lock_);
+  if (is_using_fake_sink_ && dest) {
+    // Audio should be silent at this point, if not, it will be handled once the
+    // transition to the fake sink is complete.
+    dest->Zero();
+    return dest->frames();
+  }
+
+  // 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(AudioBus::Create(params_));
+    dest = buffers_after_silence_.back().get();
+  } else if (!buffers_after_silence_.empty()) {
+    // Drain any non-silent transitional buffers before queuing more audio data.
+    // Note: These do not skew clocks derived from frame count since we don't
+    // issue Render() to the client when returning these buffers.
+    DCHECK(!is_using_fake_sink_);
+    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_ > silence_timeout_) {
+      is_transition_pending_ = true;
+      task_runner_->PostTask(
+          FROM_HERE, base::Bind(sink_transition_callback_.callback(), true));
+    }
+  }
+
+  return dest->frames();
+}
+
+void SilentSinkSuspender::OnRenderError() {
+  callback_->OnRenderError();
+}
+
+void SilentSinkSuspender::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) {
+    sink_->Pause();
+
+    // |sink_| may still be executing Render() at this point or even sometime
+    // after this point, so we must acquire the lock to make sure we don't have
+    // concurrent Render() execution. Once |is_using_fake_sink_| is set to true,
+    // calls from |sink_| will be dropped.
+    {
+      base::AutoLock al(transition_lock_);
+      is_transition_pending_ = false;
+      is_using_fake_sink_ = true;
+    }
+    fake_sink_.Start(
+        base::Bind(base::IgnoreResult(&SilentSinkSuspender::Render),
+                   base::Unretained(this), nullptr, 0, 0));
+  } else {
+    fake_sink_.Stop();
+
+    // Despite the fake sink having a synchronous Stop(), if this transition
+    // occurs too soon after pausing the real sink, we may have pending Render()
+    // calls from before the transition to the fake sink. As such, we need to
+    // hold the lock here to avoid any races.
+    {
+      base::AutoLock al(transition_lock_);
+      is_transition_pending_ = false;
+      is_using_fake_sink_ = false;
+    }
+    sink_->Play();
+  }
+}
+
+}  // namespace content