Atomically access RealtimeAnalyser write_index_

The write index for the buffer that holds the time domain data is read
from the main thread and written by the audio thread.  Atomically
access these and also read (and write) them exactly once in each
routine so that they don't get out of sync within each routine.

Can't add a trylock to protect access in WriteInput because if the
audio thread can't get the lock, there will be strange gaps in the
saved data.  It's ok if the main thread reads a slightly old value of
the index; the data is just a bit stale, which is fine for the
AnalyserNode.

Bug: 786829
Test: test case from bug passes
Change-Id: Id8d5658b9610beb0d78c2c16d9eac579406e4c2b
Reviewed-on: https://chromium-review.googlesource.com/790945Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519767}

third_party/WebKit/Source/modules/webaudio/RealtimeAnalyser.cpp

@@ -86,16 +86,17 @@ void RealtimeAnalyser::WriteInput(AudioBus* bus, size_t frames_to_process) {
   if (!is_bus_good)
     return;
 
+  unsigned write_index = GetWriteIndex();
   // FIXME : allow to work with non-FFTSize divisible chunking
   bool is_destination_good =
-      write_index_ < input_buffer_.size() &&
-      write_index_ + frames_to_process <= input_buffer_.size();
+      write_index < input_buffer_.size() &&
+      write_index + frames_to_process <= input_buffer_.size();
   DCHECK(is_destination_good);
   if (!is_destination_good)
     return;
 
   // Perform real-time analysis
-  float* dest = input_buffer_.Data() + write_index_;
+  float* dest = input_buffer_.Data() + write_index;
 
   // Clear the bus and downmix the input according to the down mixing rules.
   // Then save the result in the m_inputBuffer at the appropriate place.
@@ -104,9 +105,10 @@ void RealtimeAnalyser::WriteInput(AudioBus* bus, size_t frames_to_process) {
   memcpy(dest, down_mix_bus_->Channel(0)->Data(),
          frames_to_process * sizeof(*dest));
 
-  write_index_ += frames_to_process;
-  if (write_index_ >= kInputBufferSize)
-    write_index_ = 0;
+  write_index += frames_to_process;
+  if (write_index >= kInputBufferSize)
+    write_index = 0;
+  SetWriteIndex(write_index);
 }
 
 namespace {
@@ -143,7 +145,7 @@ void RealtimeAnalyser::DoFFTAnalysis() {
 
   // Take the previous fftSize values from the input buffer and copy into the
   // temporary buffer.
-  unsigned write_index = write_index_;
+  unsigned write_index = GetWriteIndex();
   if (write_index < fft_size) {
     memcpy(temp_p, input_buffer + write_index - fft_size + kInputBufferSize,
            sizeof(*temp_p) * (fft_size - write_index));
@@ -290,7 +292,7 @@ void RealtimeAnalyser::GetFloatTimeDomainData(
     float* input_buffer = input_buffer_.Data();
     float* destination = destination_array->Data();
 
-    unsigned write_index = write_index_;
+    unsigned write_index = GetWriteIndex();
 
     for (unsigned i = 0; i < len; ++i) {
       // Buffer access is protected due to modulo operation.
@@ -319,7 +321,7 @@ void RealtimeAnalyser::GetByteTimeDomainData(DOMUint8Array* destination_array) {
     float* input_buffer = input_buffer_.Data();
     unsigned char* destination = destination_array->Data();
 
-    unsigned write_index = write_index_;
+    unsigned write_index = GetWriteIndex();
 
     for (unsigned i = 0; i < len; ++i) {
       // Buffer access is protected due to modulo operation.

third_party/WebKit/Source/modules/webaudio/RealtimeAnalyser.h

@@ -79,6 +79,11 @@ class RealtimeAnalyser final {
   AudioFloatArray input_buffer_;
   unsigned write_index_;
 
+  unsigned GetWriteIndex() const { return AcquireLoad(&write_index_); }
+  void SetWriteIndex(unsigned new_index) {
+    ReleaseStore(&write_index_, new_index);
+  }
+
   // Input audio is downmixed to this bus before copying to m_inputBuffer.
   scoped_refptr<AudioBus> down_mix_bus_;