Fix: handle MQTT message fragmentation

MQTT messages might arrive in parts if their payload is too big. for that
reason, we need to be prepared to re-assemble fragmented messages
on a topic before handing them over to the subscriber.

include/MqttSettings.h

@@ -6,6 +6,8 @@
 #include <Ticker.h>
 #include <espMqttClient.h>
 #include <mutex>
+#include <map>
+#include <vector>
 
 class MqttSettingsClass {
 public:
@@ -36,6 +38,7 @@ private:
 
     MqttClient* _mqttClient = nullptr;
     Ticker _mqttReconnectTimer;
+    std::map<String, std::vector<uint8_t>> _fragments;
     MqttSubscribeParser _mqttSubscribeParser;
     std::mutex _clientLock;
 };

src/MqttSettings.cpp

@@ -86,9 +86,33 @@ void MqttSettingsClass::onMqttDisconnect(espMqttClientTypes::DisconnectReason re
 
 void MqttSettingsClass::onMqttMessage(const espMqttClientTypes::MessageProperties& properties, const char* topic, const uint8_t* payload, const size_t len, const size_t index, const size_t total)
 {
-    ESP_LOGD(TAG, "Received MQTT message on topic: %s", topic);
+    ESP_LOGD(TAG, "Received MQTT message on topic '%s' (Bytes %zu-%zu/%zu)",
+        topic, index + 1, (index + len), total);
 
-    _mqttSubscribeParser.handle_message(properties, topic, payload, len);
+    // shortcut for most MQTT messages, which are not fragmented
+    if (index == 0 && len == total) {
+        return _mqttSubscribeParser.handle_message(properties, topic, payload, len);
+    }
+
+    auto& fragment = _fragments[String(topic)];
+
+    // first fragment of a new message
+    if (index == 0) {
+        fragment.clear();
+        fragment.reserve(total);
+    }
+
+    fragment.insert(fragment.end(), payload, payload + len);
+
+    if (fragment.size() < total) {
+        return;
+    } // wait for last fragment
+
+    ESP_LOGD(TAG, "Fragmented MQTT message reassembled for topic '%s'", topic);
+
+    _mqttSubscribeParser.handle_message(properties, topic, fragment.data(), total);
+
+    _fragments.erase(String(topic));
 }
 
 void MqttSettingsClass::performConnect()