Fix: Resubscribe when mqtt reconnects

In case ospd-openvas disconnects from the MQTT
broker it will reconnect, but without subscribing
to the topics again. This will be fixed with this PR

ospd_openvas/messaging/mqtt.py

@@ -86,7 +86,13 @@ def publish(self, message: Message) -> None:
 
 class MQTTSubscriber(Subscriber):
     def __init__(self, client: MQTTClient):
-        self._client = client
+        self.client = client
+        # Save the active subscriptions on subscribe() so we can resubscribe
+        # after reconnect
+        self.subscriptions: dict = {}
+
+        self.client.on_connect = self.on_connect
+        self.client.user_data_set(self.subscriptions)
 
     def subscribe(
         self, message_class: Type[Message], callback: Callable[[Message], None]
@@ -96,8 +102,21 @@ def subscribe(
 
         logger.debug("Subscribing to topic %s", message_class.topic)
 
-        self._client.subscribe(message_class.topic, qos=QOS_AT_LEAST_ONCE)
-        self._client.message_callback_add(message_class.topic, func)
+        self.client.subscribe(message_class.topic, qos=QOS_AT_LEAST_ONCE)
+        self.client.message_callback_add(message_class.topic, func)
+
+        self.subscriptions[message_class.topic] = func
+
+    @staticmethod
+    def on_connect(_client, _userdata, _flags, rc, _properties):
+        if rc == 0:
+            # If we previously had active subscription we subscribe to them
+            # again because they got lost after a broker disconnect.
+            # Userdata is set in __init__() and filled in subscribe()
+            if _userdata:
+                for topic, func in _userdata.items():
+                    _client.subscribe(topic, qos=QOS_AT_LEAST_ONCE)
+                    _client.message_callback_add(topic, func)
 
     @staticmethod
     def _handle_message(