Make OS audio buffer size limits visible.

media/audio/mac/audio_manager_mac.cc

Index: media/audio/mac/audio_manager_mac.cc
diff --git a/media/audio/mac/audio_manager_mac.cc b/media/audio/mac/audio_manager_mac.cc
index 09b6433e631d819fc6bc87ab72f4c55b5cc672d1..1a892e9a851d3bd42d3dec9c8ee8c46510c8399d 100644
--- a/media/audio/mac/audio_manager_mac.cc
+++ b/media/audio/mac/audio_manager_mac.cc
@@ -34,10 +34,6 @@ namespace media {
 // Maximum number of output streams that can be open simultaneously.
 static const int kMaxOutputStreams = 50;
 
-// Define bounds for for low-latency input and output streams.
-static const int kMinimumInputOutputBufferSize = 128;
-static const int kMaximumInputOutputBufferSize = 4096;
-
 // Default sample-rate on most Apple hardware.
 static const int kFallbackSampleRate = 44100;
 
@@ -838,9 +834,13 @@ AudioParameters AudioManagerMac::GetPreferredOutputStreamParameters(
   // stream must be able to FIFO requests appropriately when this happens.
   int buffer_size = ChooseBufferSize(false, hardware_sample_rate);
   if (has_valid_input_params) {

[o1ka, 2017/06/09 10:52:39]

combine into a conditional assignment?
int buffer_size = has_valid_input_params ? .. : ..

[Andrew MacPherson, 2017/06/12 11:23:32]

Done.

-    buffer_size =
-        std::min(kMaximumInputOutputBufferSize,
-                 std::max(input_params.frames_per_buffer(), buffer_size));
+    // If passed in via the input_params we allow buffer sizes to go as low as
+    // the the kMinimumInputOutputBufferSize, ignoring what ChooseBufferSize()
+    // normally returns.
+    buffer_size = std::min(

[o1ka, 2017/06/09 10:52:39]

Add a test to media/audio/audio_manager_unittest.cc to make sure
AudioManager::MakeAudioOutputStreamProxy behaves as we expect?

[Andrew MacPherson, 2017/06/12 11:23:32]

No problem, as soon as we agree that this approach is the right one I'll add a
few tests here. 

+        static_cast<int>(limits::kMaximumInputOutputBufferSize),
+        std::max(input_params.frames_per_buffer(),
+                 static_cast<int>(limits::kMinimumInputOutputBufferSize)));
   }
 
   int hardware_channels;
@@ -895,8 +895,8 @@ int AudioManagerMac::ChooseBufferSize(bool is_input, int sample_rate) {
   // TODO(xians): Remove this workaround after WebAudio supports user defined
   // buffer size.  See https://github.com/WebAudio/web-audio-api/issues/348
   // for details.
-  int buffer_size = is_input ?
-      kMinimumInputOutputBufferSize : 2 * kMinimumInputOutputBufferSize;
+  int buffer_size = is_input ? limits::kMinimumInputOutputBufferSize
+                             : 2 * limits::kMinimumInputOutputBufferSize;
   const int user_buffer_size = GetUserBufferSize();
   if (user_buffer_size) {
     buffer_size = user_buffer_size;
@@ -904,9 +904,9 @@ int AudioManagerMac::ChooseBufferSize(bool is_input, int sample_rate) {
     // The default buffer size is too small for higher sample rates and may lead
     // to glitching.  Adjust upwards by multiples of the default size.
     if (sample_rate <= 96000)
-      buffer_size = 2 * kMinimumInputOutputBufferSize;
+      buffer_size = 2 * limits::kMinimumInputOutputBufferSize;
     else if (sample_rate <= 192000)
-      buffer_size = 4 * kMinimumInputOutputBufferSize;
+      buffer_size = 4 * limits::kMinimumInputOutputBufferSize;
   }
 
   return buffer_size;