output/jack: connect to server on MPD startup

.. and keep up the JACK connection while MPD runs.  Allocate the ring
buffers on the first open, and free them at MPD exit.

NEWS

@@ -33,6 +33,7 @@ ver 0.16 (20??/??/??)
   - pulse: renamed context to "Music Player Daemon"
   - pulse: connect to server on MPD startup, implement pause
   - jack: don't disconnect during pause
+  - jack: connect to server on MPD startup
 * mixers:
   - removed support for legacy mixer configuration
   - reimplemented software volume as mixer+filter plugin

src/output/jack_output_plugin.c

@@ -75,43 +75,6 @@ jack_output_quark(void)
 	return g_quark_from_static_string("jack_output");
 }
 
-static void
-mpd_jack_client_free(struct jack_data *jd)
-{
-	assert(jd != NULL);
-
-	if (jd->client != NULL) {
-		jack_deactivate(jd->client);
-		jack_client_close(jd->client);
-		jd->client = NULL;
-	}
-
-	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ringbuffer); ++i) {
-		if (jd->ringbuffer[i] != NULL) {
-			jack_ringbuffer_free(jd->ringbuffer[i]);
-			jd->ringbuffer[i] = NULL;
-		}
-	}
-}
-
-static void
-mpd_jack_free(struct jack_data *jd)
-{
-	assert(jd != NULL);
-
-	for (unsigned i = 0; i < G_N_ELEMENTS(jd->output_ports); ++i)
-		g_free(jd->output_ports[i]);
-
-	g_free(jd);
-}
-
-static void
-mpd_jack_finish(void *data)
-{
-	struct jack_data *jd = data;
-	mpd_jack_free(jd);
-}
-
 static int
 mpd_jack_process(jack_nframes_t nframes, void *arg)
 {
@@ -185,12 +148,69 @@ mpd_jack_info(const char *msg)
 }
 #endif
 
+/**
+ * Disconnect the JACK client.
+ */
+static void
+mpd_jack_disconnect(struct jack_data *jd)
+{
+	assert(jd != NULL);
+	assert(jd->client != NULL);
+
+	jack_deactivate(jd->client);
+	jack_client_close(jd->client);
+	jd->client = NULL;
+}
+
+/**
+ * Connect the JACK client and performs some basic setup
+ * (e.g. register callbacks).
+ */
+static bool
+mpd_jack_connect(struct jack_data *jd, GError **error_r)
+{
+	assert(jd != NULL);
+
+	jd->shutdown = false;
+
+	if ((jd->client = jack_client_new(jd->name)) == NULL) {
+		g_set_error(error_r, jack_output_quark(), 0,
+			    "Failed to connect to JACK server");
+		return false;
+	}
+
+	jack_set_process_callback(jd->client, mpd_jack_process, jd);
+	jack_on_shutdown(jd->client, mpd_jack_shutdown, jd);
+
+	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ports); ++i) {
+		jd->ports[i] = jack_port_register(jd->client, port_names[i],
+						  JACK_DEFAULT_AUDIO_TYPE,
+						  JackPortIsOutput, 0);
+		if (jd->ports[i] == NULL) {
+			g_set_error(error_r, jack_output_quark(), 0,
+				    "Cannot register output port \"%s\"",
+				    port_names[i]);
+			mpd_jack_disconnect(jd);
+			return false;
+		}
+	}
+
+	return true;
+}
+
+static bool
+mpd_jack_test_default_device(void)
+{
+	return true;
+}
+
 static void *
 mpd_jack_init(G_GNUC_UNUSED const struct audio_format *audio_format,
 	      const struct config_param *param, GError **error_r)
 {
 	struct jack_data *jd;
 	const char *value;
+	GError *error = NULL;
 
 	jd = g_new(struct jack_data, 1);
 	jd->name = config_get_block_string(param, "name", "mpd_jack");
@@ -220,56 +240,86 @@ mpd_jack_init(G_GNUC_UNUSED const struct audio_format *audio_format,
 	jd->ringbuffer_size =
 		config_get_block_unsigned(param, "ringbuffer_size", 32768);
 
+	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ringbuffer); ++i)
+		jd->ringbuffer[i] = NULL;
+
 	jack_set_error_function(mpd_jack_error);
 
 #ifdef HAVE_JACK_SET_INFO_FUNCTION
 	jack_set_info_function(mpd_jack_info);
 #endif
 
+	if (!mpd_jack_connect(jd, &error)) {
+		/* this is non-fatal - it's enough to connect when
+		   playback starts */
+		g_warning("%s", error->message);
+		g_error_free(error);
+	}
+
 	return jd;
 }
 
-static bool
-mpd_jack_test_default_device(void)
+static void
+mpd_jack_finish(void *data)
 {
-	return true;
+	struct jack_data *jd = data;
+
+	if (jd->client != NULL)
+		mpd_jack_disconnect(jd);
+
+	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ringbuffer); ++i) {
+		if (jd->ringbuffer[i] != NULL) {
+			jack_ringbuffer_free(jd->ringbuffer[i]);
+			jd->ringbuffer[i] = NULL;
+		}
+	}
+
+	for (unsigned i = 0; i < G_N_ELEMENTS(jd->output_ports); ++i)
+		g_free(jd->output_ports[i]);
+
+	g_free(jd);
+}
+
+/**
+ * Stops the playback on the JACK connection.
+ */
+static void
+mpd_jack_stop(struct jack_data *jd)
+{
+	assert(jd != NULL);
+
+	if (jd->client == NULL)
+		return;
+
+	if (jd->shutdown)
+		/* the connection has failed; close it */
+		mpd_jack_disconnect(jd);
+	else
+		/* the connection is alive: just stop playback */
+		jack_deactivate(jd->client);
 }
 
 static bool
-mpd_jack_connect(struct jack_data *jd, GError **error_r)
+mpd_jack_start(struct jack_data *jd, GError **error_r)
 {
 	const char *output_ports[2], **jports;
 
-	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ringbuffer); ++i)
-		jd->ringbuffer[i] =
-			jack_ringbuffer_create(jd->ringbuffer_size);
-
-	jd->shutdown = false;
-
-	if ((jd->client = jack_client_new(jd->name)) == NULL) {
-		g_set_error(error_r, jack_output_quark(), 0,
-			    "Failed to connect to JACK server");
+	if (jd->client == NULL && !mpd_jack_connect(jd, error_r))
 		return false;
-	}
 
-	jack_set_process_callback(jd->client, mpd_jack_process, jd);
-	jack_on_shutdown(jd->client, mpd_jack_shutdown, jd);
-
-	for (unsigned i = 0; i < G_N_ELEMENTS(jd->ports); ++i) {
-		jd->ports[i] = jack_port_register(jd->client, port_names[i],
-						  JACK_DEFAULT_AUDIO_TYPE,
-						  JackPortIsOutput, 0);
-		if (jd->ports[i] == NULL) {
-			g_set_error(error_r, jack_output_quark(), 0,
-				    "Cannot register output port \"%s\"",
-				    port_names[i]);
-			return false;
-		}
-	}
+	/* allocate the ring buffers on the first open(); these
+	   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)
+		if (jd->ringbuffer[i] == NULL)
+			jd->ringbuffer[i] =
+				jack_ringbuffer_create(jd->ringbuffer_size);
 
 	if ( jack_activate(jd->client) ) {
 		g_set_error(error_r, jack_output_quark(), 0,
 			    "cannot activate client");
+		mpd_jack_stop(jd);
 		return false;
 	}
 
@@ -281,6 +331,7 @@ mpd_jack_connect(struct jack_data *jd, GError **error_r)
 		if (jports == NULL) {
 			g_set_error(error_r, jack_output_quark(), 0,
 				    "no ports found");
+			mpd_jack_stop(jd);
 			return false;
 		}
 
@@ -310,6 +361,7 @@ mpd_jack_connect(struct jack_data *jd, GError **error_r)
 			if (jports != NULL)
 				free(jports);
 
+			mpd_jack_stop(jd);
 			return false;
 		}
 	}
@@ -329,10 +381,8 @@ mpd_jack_open(void *data, struct audio_format *audio_format, GError **error_r)
 
 	jd->pause = false;
 
-	if (!mpd_jack_connect(jd, error_r)) {
-		mpd_jack_client_free(jd);
+	if (!mpd_jack_start(jd, error_r))
 		return false;
-	}
 
 	set_audioformat(jd, audio_format);
 	jd->audio_format = *audio_format;
@@ -345,7 +395,7 @@ mpd_jack_close(G_GNUC_UNUSED void *data)
 {
 	struct jack_data *jd = data;
 
-	mpd_jack_client_free(jd);
+	mpd_jack_stop(jd);
 }
 
 static inline jack_default_audio_sample_t