Allow audio system to handle synchronized low-latency audio I/O

content/browser/renderer_host/media/audio_sync_reader.cc

Index: content/browser/renderer_host/media/audio_sync_reader.cc
===================================================================
--- content/browser/renderer_host/media/audio_sync_reader.cc	(revision 156123)
+++ content/browser/renderer_host/media/audio_sync_reader.cc	(working copy)
@@ -17,12 +17,25 @@
 const int kMinIntervalBetweenReadCallsInMs = 10;
 #endif
 
+using media::AudioBus;
+
 AudioSyncReader::AudioSyncReader(base::SharedMemory* shared_memory,
-                                 const media::AudioParameters& params)
-    : shared_memory_(shared_memory) {
+                                 const media::AudioParameters& params,
+                                 int input_channels)
+    : shared_memory_(shared_memory),
+      input_channels_(input_channels) {
   packet_size_ = media::PacketSizeInBytes(shared_memory_->created_size());
-  DCHECK_EQ(packet_size_, media::AudioBus::CalculateMemorySize(params));
-  audio_bus_ = media::AudioBus::WrapMemory(params, shared_memory->memory());
+  DCHECK_EQ(packet_size_, AudioBus::CalculateMemorySize(params));
+  output_bus_ = AudioBus::WrapMemory(params, shared_memory->memory());
+
+  if (input_channels_ > 0) {
+    // The input storage is after the output storage.
+    int output_memory_size = AudioBus::CalculateMemorySize(params);
+    int frames = params.frames_per_buffer();
+    char* input_data =
+        static_cast<char*>(shared_memory_->memory()) + output_memory_size;
+    input_bus_ = AudioBus::WrapMemory(input_channels_, frames, input_data);
+  }
 }
 
 AudioSyncReader::~AudioSyncReader() {
@@ -45,7 +58,7 @@
   }
 }
 
-int AudioSyncReader::Read(media::AudioBus* audio_bus) {
+int AudioSyncReader::Read(AudioBus* source, AudioBus* dest) {
 #if defined(OS_WIN)
   // HACK: yield if reader is called too often.
   // Problem is lack of synchronization between host and renderer. We cannot be
@@ -62,6 +75,14 @@
   previous_call_time_ = base::Time::Now();
 #endif
 
+  // Copy optional synchronized live audio input for consumption by renderer
+  // process.
+  if (source && input_bus_.get()) {
+    DCHECK_EQ(source->channels(), input_bus_->channels());
+    DCHECK_LE(source->frames(), input_bus_->frames());
+    source->CopyTo(input_bus_.get());
+  }
+
   // Retrieve the actual number of bytes available from the shared memory.  If
   // the renderer has not completed rendering this value will be invalid (still
   // the marker stored in UpdatePendingBytes() above) and must be sanitized.
@@ -75,20 +96,21 @@
   // value for a couple reasons.  One, it might still be the unknown data size
   // marker.  Two, shared memory comes from a potentially untrusted source.
   int frames =
-      size / (sizeof(*audio_bus_->channel(0)) * audio_bus_->channels());
+      size / (sizeof(*output_bus_->channel(0)) * output_bus_->channels());
   if (frames < 0)
     frames = 0;
-  else if (frames > audio_bus_->frames())
-    frames = audio_bus_->frames();
+  else if (frames > output_bus_->frames())
+    frames = output_bus_->frames();
 
   // Copy data from the shared memory into the caller's AudioBus.
-  audio_bus_->CopyTo(audio_bus);
+  output_bus_->CopyTo(dest);
 
   // Zero out any unfilled frames in the destination bus.
-  audio_bus->ZeroFramesPartial(frames, audio_bus->frames() - frames);
+  dest->ZeroFramesPartial(frames, dest->frames() - frames);
 
-  // Zero out the entire buffer.
-  memset(shared_memory_->memory(), 0, packet_size_);
+  // Zero out the entire output buffer to avoid stuttering/repeating-buffers
+  // in the anomalous case if the renderer is unable to keep up with real-time.
+  output_bus_->Zero();
 
   // Store unknown length of data into buffer, in case renderer does not store
   // the length itself. It also helps in decision if we need to yield.