mqtt: add Subscribe method; server subscribes and logs incoming messages

cmd/server/main.go

@@ -43,6 +43,15 @@ func main() {
 		} else {
 			mq = mqc
 			mqttclient.SetDefault(mqc)
+
+			// subscribe to the configured topic and log incoming messages
+			if cfg.MQTT.Topic != "" {
+				if err := mq.Subscribe(cfg.MQTT.Topic, func(t string, p []byte) {
+					log.Printf("mqtt recv on %s: %s", t, string(p))
+				}); err != nil {
+					log.Printf("warning: mqtt subscribe failed: %v", err)
+				}
+			}
 			// publish a startup message (non-blocking)
 			go func() {
 				msg := fmt.Sprintf("lambdaiot-core started on %s", addr)