Introduce PushPullFIFO class and remove other FIFOs

third_party/WebKit/Source/platform/audio/PushPullFIFO.h

Index: third_party/WebKit/Source/platform/audio/PushPullFIFO.h
diff --git a/third_party/WebKit/Source/platform/audio/PushPullFIFO.h b/third_party/WebKit/Source/platform/audio/PushPullFIFO.h
new file mode 100644
index 0000000000000000000000000000000000000000..4927074fb3b7d818ca87417125c47da5fd3864d0
--- /dev/null
+++ b/third_party/WebKit/Source/platform/audio/PushPullFIFO.h
@@ -0,0 +1,87 @@
+// Copyright 2017 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 PushPullFIFO_h
+#define PushPullFIFO_h
+
+#include "platform/audio/AudioBus.h"
+#include "public/platform/WebCommon.h"
+#include "wtf/Allocator.h"
+
+namespace blink {
+
+// A configuration data container for PushPullFIFO unit test.
+struct PushPullFIFOStateForTest {
+  const size_t fifoLength;
+  const unsigned numberOfChannels;
+  const size_t framesAvailable;
+  const size_t indexRead;
+  const size_t indexWrite;
+  const unsigned overflowCount;
+  const unsigned underflowCount;
+};
+
+// PushPullFIFO class is an intermediate audio sample storage between
+// Blink-WebAudio and the renderer. The renderer's hardware callback buffer size
+// varies on the platform, but the WebAudio always operates by 128 frames of
+// render quantum (RQ); thus FIFO is needed.

[Raymond Toy, 2017/01/30 20:59:28]

"128 frames of render quantum" is awkward.  Rephrase.

"thus FIFO is needed" => "thus FIFO is needed to handle the general case" or
something like that.

[hongchan, 2017/02/01 18:07:28]

Done.

+//  - The maximum FIFO length is 32768 frames (256 RQs).

[Raymond Toy, 2017/01/30 20:59:28]

Recall that we encountered an Android device with callback size of 45K or so. 
Are we going to disallow working on such a device?

Also, shouldn't that max constant be defined in this file?

[hongchan, 2017/02/01 18:07:27]

Done.

+//  - The push size is 128 frames (1 RQ), fixed.
+//  - The pull size can be flexible.
+//  - In case of overflow (FIFO full while push), the existing frames in FIFO
+//    will be overwritten and the read index will be forcibly moved to where the
+//    write index is located.

[Raymond Toy, 2017/01/30 20:59:28]

That doesn't really tell me what happens with the data that will be read. 
Please explain what data will be read.

[hongchan, 2017/02/01 18:07:28]

Done.

+//  - In case of underflow (FIFO empty while pull), the remaining space in the
+//    output bus will be filled with silence. Thus it will fulfill the request

[Raymond Toy, 2017/01/30 20:59:28]

What does "remaining space" mean here?  The FIFO is empty.

[hongchan, 2017/02/01 18:07:28]

"remaining space in the output bus" - that means the bus that will be passed to
the audio device. Rephrased the comment a bit.

+//    from the consumer without causing error, but with a glitch.

[hongchan, 2017/01/27 22:39:14]

Here's the contract of FIFO.

+class BLINK_PLATFORM_EXPORT PushPullFIFO {
+  USING_FAST_MALLOC(PushPullFIFO);
+  WTF_MAKE_NONCOPYABLE(PushPullFIFO);
+
+ public:
+  // If |fifoLength| is greater than the maximum value allowed, it will be
+  // clamped.

[Raymond Toy, 2017/01/30 20:59:28]

Clamped to what?

[hongchan, 2017/02/01 18:07:28]

Done.

+  explicit PushPullFIFO(unsigned numberOfChannels, size_t fifoLength);
+  ~PushPullFIFO();
+
+  // Pushing audio data is done by the WebAudio render-backing thread. The
+  // priority of this thread is lower than the actual audio device thread,

[Raymond Toy, 2017/01/30 20:59:28]

How do we know the priorities already?

[hongchan, 2017/02/01 18:07:28]

The comments are edited. No mention of multi-threading in this CL.

+  // thus the thread can be blocked when the audio device thread is pulling
+  // data from the FIFO.
+  void push(const AudioBus* inputBus);
+
+  // Pulling audio data is done by the audio device thread. In the case of
+  // underflow, the rest will be filled up with zeros (thus glitches). Pulling
+  // an empty FIFO is a valid operation, and the consumer will get a block of
+  // silence.
+  void pull(AudioBus* outputBus, size_t framesRequested);
+
+  size_t framesAvailable() const { return m_framesAvailable; }
+  size_t length() const { return m_fifoLength; }
+  unsigned numberOfChannels() const { return m_fifoBus->numberOfChannels(); }
+  AudioBus* bus() const { return m_fifoBus.get(); }
+
+  // For unit test. Get the current configuration that consists of FIFO length,
+  // number of channels, read/write index position and under/overflow count.
+  const PushPullFIFOStateForTest getStateForTest() const;
+
+ private:
+  // The size of the FIFO.
+  const size_t m_fifoLength = 0;
+
+  RefPtr<AudioBus> m_fifoBus;

[hongchan, 2017/01/27 22:39:14]

The order is changed because of the change in initializers.

+
+  // The number of frames in the FIFO actually available for pulling.
+  size_t m_framesAvailable;
+
+  size_t m_indexRead;
+  size_t m_indexWrite;
+
+  unsigned m_overflowCount;
+  unsigned m_underflowCount;
+};
+
+}  // namespace blink
+
+#endif  // PushPullFIFO_h