output/jack: clear ring buffers before activating

After playback has stopped, the ring buffers may still contain
samples.  These will be played when playback is started the next
time.  We should clear the buffers each time.

NEWS

@@ -36,6 +36,7 @@ ver 0.16 (20??/??/??)
   - jack: don't disconnect during pause
   - jack: connect to server on MPD startup
   - jack: added option "client_name"
+  - jack: clear ring buffers before activating
   - wildcards allowed in audio_format configuration
   - alsa: don't recover on CANCEL
   - consistently lock audio output objects

src/output/jack_output_plugin.c

@@ -335,11 +335,16 @@ mpd_jack_start(struct jack_data *jd, GError **error_r)
 	   persist until MPD exits.  It's too unsafe to delete them
 	   because we can never know when mpd_jack_process() gets
 	   called */
-	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ringbuffer); ++i)
+	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ringbuffer); ++i) {
 		if (jd->ringbuffer[i] == NULL)
 			jd->ringbuffer[i] =
 				jack_ringbuffer_create(jd->ringbuffer_size);
 
+		/* clear the ring buffer to be sure that data from
+		   previous playbacks are gone */
+		jack_ringbuffer_reset(jd->ringbuffer[i]);
+	}
+
 	if ( jack_activate(jd->client) ) {
 		g_set_error(error_r, jack_output_quark(), 0,
 			    "cannot activate client");