Receive long MQTT payload

esphome/components/mqtt/mqtt_client.cpp

@@ -25,9 +25,17 @@ void MQTTClientComponent::setup() {
   ESP_LOGCONFIG(TAG, "Setting up MQTT...");
   this->mqtt_client_.onMessage([this](char *topic, char *payload, AsyncMqttClientMessageProperties properties,
                                       size_t len, size_t index, size_t total) {
-    std::string payload_s(payload, len);
-    std::string topic_s(topic);
-    this->on_message(topic_s, payload_s);
+    if (index == 0)
+      this->payload_buffer_.reserve(total);
+
+    // append new payload, may contain incomplete MQTT message
+    this->payload_buffer_.append(payload, len);
+
+    // MQTT fully received
+    if (len + index == total) {
+      this->on_message(topic, this->payload_buffer_);
+      this->payload_buffer_.clear();
+    }
   });
   this->mqtt_client_.onDisconnect([this](AsyncMqttClientDisconnectReason reason) {
     this->state_ = MQTT_CLIENT_DISCONNECTED;

esphome/components/mqtt/mqtt_client.h

@@ -250,6 +250,7 @@ class MQTTClientComponent : public Component {
   };
   std::string topic_prefix_{};
   MQTTMessage log_message_;
+  std::string payload_buffer_;
   int log_level_{ESPHOME_LOG_LEVEL};
 
   std::vector<MQTTSubscription> subscriptions_;