Fix cancellation snappiness

Screen reader users report getting late cancellation, or even mixtures
of speech. This is because the default buffering parameters of alsa or
pulseaudio are relatively large.

This change sets alsa and pulseaudio buffer sizes to 10ms worth of audio,
which is the human snappiness perception limit.

src/alsa.c

@@ -99,6 +99,7 @@ alsa_object_open(struct audio_object *object,
 
 	snd_pcm_hw_params_t *params = NULL;
 	snd_pcm_hw_params_malloc(&params);
+	snd_pcm_uframes_t bufsize = (rate * channels * LATENCY) / 1000;
 
 	int err = 0;
 	if ((err = snd_pcm_open(&self->handle, self->device ? self->device : "default", SND_PCM_STREAM_PLAYBACK, 0)) < 0)
@@ -113,6 +114,8 @@ alsa_object_open(struct audio_object *object,
 		goto error;
 	if ((err = snd_pcm_hw_params_set_channels(self->handle, params, channels)) < 0)
 		goto error;
+	if ((err = snd_pcm_hw_params_set_buffer_size_near(self->handle, params, &bufsize)) < 0)
+		goto error;
 	if ((err = snd_pcm_hw_params(self->handle, params)) < 0)
 		goto error;
 	if ((err = snd_pcm_prepare(self->handle)) < 0)

src/audio_priv.h

@@ -52,6 +52,10 @@ struct audio_object
 	                         int error);
 };
 
+/* We try to aim for 10ms cancelation latency, which will be perceived as
+ * "snappy" by users */
+#define LATENCY 10
+
 #if defined(_WIN32) || defined(_WIN64)
 
 #include <windows.h>

src/pulseaudio.c

@@ -74,14 +74,21 @@ pulseaudio_object_open(struct audio_object *object,
 	}
 
 	int error = 0;
+	pa_buffer_attr battr;
+
+	battr.fragsize = (uint32_t) -1;
+	battr.maxlength = (uint32_t) -1;
+	battr.minreq = (uint32_t) -1;
+	battr.prebuf = (uint32_t) -1;
+	battr.tlength = pa_bytes_per_second(&self->ss) * LATENCY / 1000;
 	self->s = pa_simple_new(NULL,
 	                        self->application_name,
 	                        PA_STREAM_PLAYBACK,
 	                        self->device,
 	                        self->description,
 	                        &self->ss,
 	                        NULL,
-	                        NULL,
+	                        &battr,
 	                        &error);
 	return error;
 }