fix(gateway): use os._exit instead of SystemExit for MQTT error handling

SystemExit only stops the paho-mqtt background thread, leaving the main
asyncio loop running silently. os._exit kills the entire process so
systemd can restart the gateway automatically.

gateway/gateway.py

@@ -78,15 +78,17 @@ class Gateway:
                 log.info("Successfully connected to MQTT broker")
             else:
                 log.error(f"Failed to connect to MQTT broker — reason code: {reason_code}")
+                os._exit(1)
 
-        # Callback triggered on disconnection — exit so systemd restarts the gateway.
-        # This handles cases where the broker becomes unreachable after the initial
-        # connection (e.g. network instability). Systemd will restart the gateway
-        # after RestartSec=10 seconds, retrying the connection automatically.
+        # Callback triggered on disconnection — kill the process so systemd
+        # restarts the gateway automatically after RestartSec=10 seconds.
+        # os._exit(1) is used instead of SystemExit because on_disconnect runs
+        # in the paho-mqtt background thread — SystemExit would only stop that
+        # thread, leaving the main asyncio loop running silently.
         def on_disconnect(client, userdata, flags, reason_code, properties):
             log.error(f"Disconnected from MQTT broker — reason code: {reason_code}")
             log.error("Exiting — systemd will restart the gateway")
-            raise SystemExit(1)
+            os._exit(1)
 
         # Callback to confirm message delivery to broker
         def on_publish(client, userdata, mid, reason_code, properties):
@@ -168,6 +170,9 @@ class Gateway:
     def publish(self, mac: str, data: dict):
         """Publish decoded sensor data to MQTT broker.
         Topic: {gateway_id}/{thingy_mac}/update
+
+        Checks the return code of mqttc.publish() and exits if an error
+        is detected, so systemd can restart the gateway automatically.
         """
         topic = f"{self.gateway_id}/{mac}/update"
         payload = {
@@ -184,7 +189,15 @@ class Gateway:
         if "battery" in data:
             payload["battery"] = data["battery"]
 
-        self.mqttc.publish(topic, json.dumps(payload))
+        result = self.mqttc.publish(topic, json.dumps(payload))
+
+        # Check if publish was successful — rc=0 means success
+        # Any other value indicates an error (e.g. connection lost)
+        if result.rc != mqtt.MQTT_ERR_SUCCESS:
+            log.error(f"Failed to publish to {topic} — rc: {result.rc}")
+            log.error("Exiting — systemd will restart the gateway")
+            os._exit(1)
+
         log.info(f"Published to {topic} : {payload}")
 
     def on_device_found(self, device, adv_data):