MQTT -- add code to deal with empty payloads by adding a dummy payload to empty messages by default.

common.h

@@ -151,6 +151,7 @@ typedef struct {
   int mqtt_publish_parsed;
   int mqtt_publish_cover;
   int mqtt_enable_remote;
+  char *mqtt_empty_payload_substitute;
 #endif
   uint8_t hw_addr[8]; // only needs 6 but 8 is handy when converting this to a number
   int port;

mqtt.c

@@ -93,12 +93,20 @@ void on_connect(struct mosquitto *mosq, __attribute__((unused)) void *userdata,
 }
 
 // helper function to publish under a topic and automatically append the main topic
-void mqtt_publish(char *topic, char *data, uint32_t length) {
+void mqtt_publish(char *topic, char *data_in, uint32_t length_in) {
+  char *data = data_in;
+  uint32_t length = length_in;
+  
+  if ((length == 0) && (config.mqtt_empty_payload_substitute != NULL)) {
+    length = strlen(config.mqtt_empty_payload_substitute);
+    data = config.mqtt_empty_payload_substitute;
+  }
+  
   char fulltopic[strlen(config.mqtt_topic) + strlen(topic) + 3];
   snprintf(fulltopic, strlen(config.mqtt_topic) + strlen(topic) + 2, "%s/%s", config.mqtt_topic,
            topic);
   debug(2, "[MQTT]: publishing under %s", fulltopic);
-
+  
   int rc;
   if ((rc = mosquitto_publish(global_mosq, NULL, fulltopic, length, data, 0, 0)) !=
       MOSQ_ERR_SUCCESS) {

scripts/shairport-sync.conf

@@ -224,6 +224,12 @@ metadata =
 // How to enable the MQTT-metadata/remote-service
 // For this section to be operative, Shairport Sync must be built with the following configuration flag:
 // --with-mqtt-client
+
+// Note that, for compatability with many MQTT brokers and applications,
+// every message that has no extra data is given a
+// payload consisting of the string "--".
+// You can change this or you can enable empty payloads -- see below.
+
 mqtt =
 {
 //	enabled = "no"; // set this to yes to enable the mqtt-metadata-service
@@ -238,8 +244,10 @@ mqtt =
 //	topic = NULL; //MQTT topic where this instance of shairport-sync should publish. If not set, the general.name value is used.
 //	publish_raw = "no"; //whether to publish all available metadata under the codes given in the 'metadata' docs.
 //	publish_parsed = "no"; //whether to publish a small (but useful) subset of metadata under human-understandable topics
+//	empty_payload_substitute = "--"; // MQTT messages with empty payloads often are invisible or have special significance to MQTT brokers and readers.
+//    To avoid empty payload problems, the string here is used instead of any empty payload. Set it to the empty string -- "" -- to leave the payload empty.
 //	Currently published topics:artist,album,title,genre,format,songalbum,volume,client_ip,
-//	Additionally, empty messages at the topics play_start,play_end,play_flush,play_resume are published
+//	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
 //	enable_remote = "no"; //whether to remote control via MQTT. RC is available under `topic`/remote.
 //	Available commands are "command", "beginff", "beginrew", "mutetoggle", "nextitem", "previtem", "pause", "playpause", "play", "stop", "playresume", "shuffle_songs", "volumedown", "volumeup"

shairport.c

@@ -1181,6 +1181,14 @@ int parse_options(int argc, char **argv) {
       die("You need to have metadata.include_cover_art enabled in order to use mqtt.publish_cover");
     }
     config_set_lookup_bool(config.cfg, "mqtt.enable_remote", &config.mqtt_enable_remote);
+    if (config_lookup_string(config.cfg, "mqtt.empty_payload_substitute", &str)) {
+      if (strlen(str) == 0)
+        config.mqtt_empty_payload_substitute = NULL;
+      else
+        config.mqtt_empty_payload_substitute = strdup(str);
+    } else {
+      config.mqtt_empty_payload_substitute = strdup("--");
+    }
 #ifndef CONFIG_AVAHI
     if (config.mqtt_enable_remote) {
       die("You have enabled MQTT remote control which requires shairport-sync to be built with "
@@ -1191,7 +1199,7 @@ int parse_options(int argc, char **argv) {
 #endif
 
 #ifdef CONFIG_AIRPLAY_2
-    int64_t aid;
+    long long aid;
 
     // replace the airplay_device_id with this, if provided
     if (config_lookup_int64(config.cfg, "general.airplay_device_id", &aid)) {
@@ -1510,6 +1518,11 @@ void exit_function() {
       if (config.service_name)
         free(config.service_name);
 
+#ifdef CONFIG_MQTT
+      if (config.mqtt_empty_payload_substitute)
+        free(config.mqtt_empty_payload_substitute);
+#endif
+
 #ifdef CONFIG_CONVOLUTION
       if (config.convolution_ir_file)
         free(config.convolution_ir_file);