Add MQTT retain flag support

MQTT.md

@@ -48,6 +48,7 @@ mqtt =
 //	publish_raw = "no"; // Whether to publish all available metadata under the codes given in the 'metadata' docs.
 	publish_parsed = "yes"; // Whether to publish a small (but useful) subset of metadata under human-understandable topics.
 	publish_cover = "yes"; // Whether to publish the cover over MQTT in binary form. This may lead to a bit of load on the broker.
+//	publish_retain = "no"; // Whether to set the retain flag on published MQTT messages. When enabled, the broker stores the last message for each topic so new subscribers receive the most recent value immediately.
 //	enable_remote = "no"; // Whether to remote control via MQTT. RC is available under `topic`/remote.
 };
 ```

common.h

@@ -230,6 +230,7 @@ typedef struct {
   int mqtt_publish_raw;
   int mqtt_publish_parsed;
   int mqtt_publish_cover;
+  int mqtt_publish_retain;
   int mqtt_enable_remote;
   int mqtt_enable_autodiscovery;
   char *mqtt_autodiscovery_prefix;

mqtt.c

@@ -243,7 +243,8 @@ void mqtt_publish(char *topic, char *data_in, uint32_t length_in) {
   debug(2, "[MQTT]: publishing under %s", fulltopic);
 
   int rc;
-  if ((rc = mosquitto_publish(global_mosq, NULL, fulltopic, length, data, 0, 0)) !=
+  if ((rc = mosquitto_publish(global_mosq, NULL, fulltopic, length, data, 0,
+                              config.mqtt_publish_retain)) !=
       MOSQ_ERR_SUCCESS) {
     switch (rc) {
     case MOSQ_ERR_NO_CONN:

scripts/shairport-sync.conf

@@ -364,6 +364,7 @@ mqtt =
 //	Currently published topics:artist,album,title,genre,format,songalbum,volume,client_ip,
 //	Additionally, messages at the topics play_start,play_end,play_flush,play_resume are published
 //	publish_cover = "no"; //whether to publish the cover over mqtt in binary form. This may lead to a bit of load on the broker
+//	publish_retain = "no"; //whether to set the retain flag on published MQTT messages. When enabled, the broker stores the last message for each topic.
 //	enable_autodiscovery = "no"; //whether to publish an autodiscovery message to automatically appear in Home Assistant
 //	autodiscovery_prefix = "homeassistant"; //string to prepend to autodiscovery topic
 //	enable_remote = "no"; //whether to remote control via MQTT. RC is available under `topic`/remote.

shairport.c

@@ -1440,6 +1440,7 @@ int parse_options(int argc, char **argv) {
     config_set_lookup_bool(config.cfg, "mqtt.publish_raw", &config.mqtt_publish_raw);
     config_set_lookup_bool(config.cfg, "mqtt.publish_parsed", &config.mqtt_publish_parsed);
     config_set_lookup_bool(config.cfg, "mqtt.publish_cover", &config.mqtt_publish_cover);
+    config_set_lookup_bool(config.cfg, "mqtt.publish_retain", &config.mqtt_publish_retain);
     if (config.mqtt_publish_cover && !config.get_coverart) {
       die("You need to have metadata.include_cover_art enabled in order to use mqtt.publish_cover");
     }
@@ -3131,6 +3132,8 @@ int main(int argc, char **argv) {
         config.mqtt_publish_parsed ? "" : " not");
   debug(option_print_level, "mqtt will%s publish cover Art.",
         config.mqtt_publish_cover ? "" : " not");
+  debug(option_print_level, "mqtt will%s set retain flag.",
+        config.mqtt_publish_retain ? "" : " not");
   debug(option_print_level, "mqtt remote control is %sabled.",
         config.mqtt_enable_remote ? "en" : "dis");
   debug(option_print_level, "mqtt autodiscovery is %sabled.",