Erase MessageHandler for topics without any registered handlers

lib/mqtt_abstraction_impl.cpp

@@ -364,9 +364,10 @@ void MQTTAbstractionImpl::on_mqtt_message(const Message& message) {
         }
         lock.unlock();
 
+        // It can happen that we unsubscribe from a topic and have removed the message handler but the MQTT unsubscribe
+        // didn't complete yet and we still receive messages on this topic that we can just ignore
         if (!found) {
-            EVLOG_AND_THROW(
-                EverestInternalError(fmt::format("Internal error: topic '{}' should have a matching handler!", topic)));
+            EVLOG_verbose << fmt::format("Topic '{}' should have a matching handler!", topic);
         }
     } catch (boost::exception& e) {
         EVLOG_critical << fmt::format("Caught MQTT on_message boost::exception:\n{}",
@@ -451,15 +452,17 @@ void MQTTAbstractionImpl::register_handler(const std::string& topic, std::shared
     if (this->message_handlers.count(topic) == 0) {
         this->message_handlers.emplace(std::piecewise_construct, std::forward_as_tuple(topic), std::forward_as_tuple());
     }
-    this->message_handlers[topic].add_handler(handler);
 
-    // only subscribe for this topic if we aren't already and the mqtt client is connected
-    // if we are not connected the on_mqtt_connect() callback will subscribe to the topic
-    if (this->mqtt_is_connected && this->message_handlers[topic].count_handlers() == 1) {
+    const auto subscription_necessary =
+        (this->mqtt_is_connected && this->message_handlers.at(topic).count_handlers() == 0);
+
+    this->message_handlers.at(topic).add_handler(handler);
+
+    if (subscription_necessary) {
         EVLOG_verbose << fmt::format("Subscribing to {}", topic);
         this->subscribe(topic, qos);
     }
-    EVLOG_verbose << fmt::format("#handler[{}] = {}", topic, this->message_handlers[topic].count_handlers());
+    EVLOG_verbose << fmt::format("#handler[{}] = {}", topic, this->message_handlers.at(topic).count_handlers());
 }
 
 void MQTTAbstractionImpl::unregister_handler(const std::string& topic, const Token& token) {
@@ -484,6 +487,7 @@ void MQTTAbstractionImpl::unregister_handler(const std::string& topic, const Tok
             EVLOG_verbose << fmt::format("Unsubscribing from {}", topic);
             this->unsubscribe(topic);
         }
+        this->message_handlers.erase(topic);
     }
 
     const std::string handler_count = (number_of_handlers == 0) ? "None" : std::to_string(number_of_handlers);