pulse: fill audio monitor buffer more aggressively

Previously we would wait for pulse to attempt to read from the monitor
source and obs buffered at least 5ms of audio data before we tried to
fill the buffer. In some cases this resulted in consistently triggering
underruns in pulse.

Instead we try to fill the buffer immediately as obs outputs audio data
and while the pa buffer is not full. We also stop trying to grow the
buffer to prevent underruns after we reach 1s of latency.

libobs/audio-monitoring/pulse/pulseaudio-output.c

@@ -20,7 +20,6 @@ struct audio_monitor {
 
 	struct circlebuf new_data;
 	audio_resampler_t *resampler;
-	size_t buffer_size;
 	size_t bytesRemaining;
 
 	bool ignore;
@@ -191,21 +190,20 @@ static void do_stream_write(void *param)
 	PULSE_DATA(param);
 	uint8_t *buffer = NULL;
 
-	while (data->new_data.size >= data->buffer_size &&
-	       data->bytesRemaining > 0) {
-		size_t bytesToFill = data->buffer_size;
-
-		if (bytesToFill > data->bytesRemaining)
+	while (data->new_data.size > 0 && data->bytesRemaining > 0) {
+		size_t bytesToFill = data->new_data.size;
+		if (data->bytesRemaining < bytesToFill)
 			bytesToFill = data->bytesRemaining;
 
 		pulseaudio_lock();
-		pa_stream_begin_write(data->stream, (void **)&buffer,
-				      &bytesToFill);
-		pulseaudio_unlock();
+		if (pa_stream_begin_write(data->stream, (void **)&buffer,
+					  &bytesToFill)) {
+			pulseaudio_unlock();
+			return;
+		}
 
 		circlebuf_pop_front(&data->new_data, buffer, bytesToFill);
 
-		pulseaudio_lock();
 		pa_stream_write(data->stream, buffer, bytesToFill, NULL, 0LL,
 				PA_SEEK_RELATIVE);
 		pulseaudio_unlock();
@@ -277,13 +275,30 @@ static void pulseaudio_underflow(pa_stream *p, void *userdata)
 	UNUSED_PARAMETER(p);
 	PULSE_DATA(userdata);
 
+	pa_sample_spec spec = {0};
+	spec.format = data->format;
+	spec.rate = (uint32_t)data->samples_per_sec;
+	spec.channels = data->channels;
+	uint64_t latency = pa_bytes_to_usec(data->attr.tlength, &spec);
+
 	pthread_mutex_lock(&data->playback_mutex);
-	if (obs_source_active(data->source))
+	if (obs_source_active(data->source) && latency < 1000000) {
+		data->attr.fragsize = (uint32_t)-1;
+		data->attr.maxlength = (uint32_t)-1;
+		data->attr.prebuf = (uint32_t)-1;
+		data->attr.minreq = (uint32_t)-1;
 		data->attr.tlength = (data->attr.tlength * 3) / 2;
-
-	pa_stream_set_buffer_attr(data->stream, &data->attr, NULL, NULL);
+		pa_stream_set_buffer_attr(data->stream, &data->attr, NULL,
+					  NULL);
+		data->bytesRemaining = data->attr.maxlength;
+	}
 	pthread_mutex_unlock(&data->playback_mutex);
 
+	if (latency >= 1000000) {
+		blog(LOG_WARNING, "source monitor reached max latency %ldms",
+		     latency / 1000);
+	}
+
 	pulseaudio_signal(0);
 }
 
@@ -473,9 +488,6 @@ static bool audio_monitor_init(struct audio_monitor *monitor,
 	monitor->attr.prebuf = (uint32_t)-1;
 	monitor->attr.tlength = pa_usec_to_bytes(25000, &spec);
 
-	monitor->buffer_size =
-		monitor->bytes_per_frame * pa_usec_to_bytes(5000, &spec);
-
 	pa_stream_flags_t flags = PA_STREAM_INTERPOLATE_TIMING |
 				  PA_STREAM_AUTO_TIMING_UPDATE;
 
@@ -493,6 +505,8 @@ static bool audio_monitor_init(struct audio_monitor *monitor,
 		return false;
 	}
 
+	monitor->bytesRemaining = monitor->attr.maxlength;
+
 	blog(LOG_INFO, "Started Monitoring in '%s'", monitor->device);
 	return true;
 }