From e6f8e73705dc310cb8b188fd509b543fe3ad7cf9 Mon Sep 17 00:00:00 2001
From: Gareth Cooper <gareth@garethcooper.com>
Date: Thu, 8 Apr 2021 13:26:34 +0100
Subject: [PATCH] Receive long MQTT payload (#1590)

---
 esphome/components/mqtt/mqtt_client.cpp | 14 +++++++++++---
 esphome/components/mqtt/mqtt_client.h   |  1 +
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/esphome/components/mqtt/mqtt_client.cpp b/esphome/components/mqtt/mqtt_client.cpp
index 190df0b2f3..b8743fc142 100644
--- a/esphome/components/mqtt/mqtt_client.cpp
+++ b/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;
diff --git a/esphome/components/mqtt/mqtt_client.h b/esphome/components/mqtt/mqtt_client.h
index 885f4a9a96..e4f7c479b2 100644
--- a/esphome/components/mqtt/mqtt_client.h
+++ b/esphome/components/mqtt/mqtt_client.h
@@ -259,6 +259,7 @@ 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_;