input/alsa: set period_size=buffer_size/4

This way, we have four periods instead of the default of two.  With
only two periods, we don't get woken up often enough, and we
frequently encounter buffer overruns.  With four periods, we have more
time to breathe, and the buffer overruns magically disappear.

NEWS

@@ -1,6 +1,7 @@
 ver 0.20.2 (not yet released)
 * input
   - alsa: fix crash bug
+  - alsa: fix buffer overruns
 * decoder
   - flac: add options "probesize" and "analyzeduration"
 * resampler

src/input/plugins/AlsaInputPlugin.cxx

@@ -57,15 +57,6 @@ static constexpr unsigned int default_rate = 44100; // cd quality
 static constexpr size_t ALSA_MAX_BUFFERED = default_rate * default_channels * 2;
 static constexpr size_t ALSA_RESUME_AT = ALSA_MAX_BUFFERED / 2;
 
-/**
- * This value should be the same as the read buffer size defined in
- * PcmDecoderPlugin.cxx:pcm_stream_decode().
- * We use it to calculate how many audio frames to buffer in the alsa driver
- * before reading from the device. snd_pcm_readi() blocks until that many
- * frames are ready.
- */
-static constexpr size_t read_buffer_size = 4096;
-
 class AlsaInputStream final
 	: public AsyncInputStream,
 	  MultiSocketMonitor, DeferredMonitor {
@@ -344,18 +335,22 @@ ConfigureCapture(snd_pcm_t *capture_handle,
 		    (unsigned)period_size_min, (unsigned)period_size_max,
 		    period_time_min, period_time_max);
 
-	/* period needs to be big enough so that poll() doesn't fire too often,
+	/* choose the maximum possible buffer_size ... */
-	 * but small enough that buffer overruns don't occur if Read() is not
+	snd_pcm_hw_params_set_buffer_size(capture_handle, hw_params,
-	 * invoked often enough.
+					  buffer_size_max);
-	 * the calculation here is empirical; however all measurements were
+
-	 * done using 44100:16:2. When we extend this plugin to support
+	/* ... and calculate the period_size to have four periods in
-	 * other audio formats then this may need to be revisited */
+	   one buffer; this way, we get woken up often enough to avoid
-	snd_pcm_uframes_t period = read_buffer_size * 2;
+	   buffer overruns, but not too often */
-	int direction = -1;
+	snd_pcm_uframes_t buffer_size;
-	if ((err = snd_pcm_hw_params_set_period_size_near(capture_handle, hw_params,
+	if (snd_pcm_hw_params_get_buffer_size(hw_params, &buffer_size) == 0) {
-							  &period, &direction)) < 0)
+		snd_pcm_uframes_t period_size = buffer_size / 4;
-		throw FormatRuntimeError("Cannot set period size (%s)",
+		int direction = -1;
-					 snd_strerror(err));
+		if ((err = snd_pcm_hw_params_set_period_size_near(capture_handle, hw_params,
+								  &period_size, &direction)) < 0)
+			throw FormatRuntimeError("Cannot set period size (%s)",
+						 snd_strerror(err));
+	}
 
 	if ((err = snd_pcm_hw_params(capture_handle, hw_params)) < 0)
 		throw FormatRuntimeError("Cannot set parameters (%s)",