OSXOutputPlugin: set the buffer time to be 100ms

[mk: the following text was copied from
https://github.com/MusicPlayerDaemon/MPD/pull/167]

For certain format (hi-res files) and normal buffer size hardware, The
hardware may at once consume most of the buffers. However, in Delay()
function, MPD is supposed to wait for 25 ms after the next try. it
will create a hiccup. The negative impact is much major than
increasing the latency.

I understand larger buffers come at a price. That's why in my earlier
commit last year I significantly reduced it. However, the buffer size
in CoreAudio is set according to the hardware, which is super small
latency. For instance, the system audio of 2015 generation of macbook
pro has maximum buffer size of 4096 samples, which is just 0.09s for
44.1k framerate, or 0.04s for 96k frames --- . compare to the 0.5 sec
latency alsa plugin has, even if we quadruple it, it's still super
tiny.

NEWS

@@ -1,4 +1,6 @@
 ver 0.20.13 (not yet released)
+* output
+  - osx: set up ring buffer to hold at least 100ms
 * database
   - simple: don't purge mount points on update/rescan
   - simple: fix "mount" bug caused by bad compiler optimization

src/output/plugins/OSXOutputPlugin.cxx

@@ -36,9 +36,10 @@
 
 #include <memory>
 
+static constexpr unsigned MPD_OSX_BUFFER_TIME_MS = 100;
+
 struct OSXOutput {
 	AudioOutput base;
-
 	/* configuration settings */
 	OSType component_subtype;
 	/* only applicable with kAudioUnitSubType_HALOutput */
@@ -693,7 +694,9 @@ osx_output_open(AudioOutput *ao, AudioFormat &audio_format)
 					 errormsg);
 	}
 
-	od->ring_buffer = new boost::lockfree::spsc_queue<uint8_t>(buffer_frame_size);
+	size_t ring_buffer_size = std::max<size_t>(buffer_frame_size,
+						   MPD_OSX_BUFFER_TIME_MS * audio_format.GetFrameSize() * audio_format.sample_rate / 1000);
+	od->ring_buffer = new boost::lockfree::spsc_queue<uint8_t>(ring_buffer_size);
 
 	status = AudioOutputUnitStart(od->au);
 	if (status != 0) {
@@ -717,7 +720,7 @@ osx_output_delay(AudioOutput *ao) noexcept
 	OSXOutput *od = (OSXOutput *)ao;
 	return od->ring_buffer->write_available()
 		? std::chrono::steady_clock::duration::zero()
-		: std::chrono::milliseconds(25);
+		: std::chrono::milliseconds(MPD_OSX_BUFFER_TIME_MS / 4);
 }
 
 int