if available bytes on the network is available, close connection.

[huster-songtao]

void loop() {
  delay(10);  // <- fixes some issues with WiFi stability

  if (mqtt.loop() == false) {
    Serial.println("[MQTT] mqtt.loop() == false.");
    connectMQTT();
  }

Board:XIAO ESP32C3

bool MQTTClient::loop() {
  if (!this->connected()) {
    Serial.println("[MQTT] return immediately if not connected.");
    return false;
  }

  // [MQTT] get available bytes on the network: 0
  int available = this->netClient->available();
  // Serial.print("[MQTT] get available bytes on the network: ");
  // Serial.println(available);

  // yield if data is available
  if (available > 0) {
    this->_lastError = lwmqtt_yield(&this->client, available, this->timeout);
    if (this->_lastError != LWMQTT_SUCCESS) {
      Serial.println("[MQTT] if available bytes on the network is available, close connection.");
      this->close();
      return false;
    }
  }

  // keep the connection alive
  this->_lastError = lwmqtt_keep_alive(&this->client, this->timeout);
  if (this->_lastError != LWMQTT_SUCCESS) {
    Serial.println("[MQTT] keep the connection alive, close connection.");
    this->close();
    return false;
  }

  return true;
}

The mqtt.loop() always print false.

[MQTT] if available bytes on the network is available, close connection. [MQTT] mqtt.loop() == false. [MQTT] connecting mqtt...

mqtt.loop() should be true after it connected.

[huster-songtao]

bool MQTTClient::loop() {
  if (!this->connected()) {
    Serial.println("[MQTT] return immediately if not connected.");
    return false;
  }

  // [MQTT] get available bytes on the network: 0
  int available = this->netClient->available();
  // Serial.print("[MQTT] get available bytes on the network: ");
  // Serial.println(available);

  // yield if data is available
  if (available > 0) {
    this->_lastError = lwmqtt_yield(&this->client, available, this->timeout);
    if (this->_lastError != LWMQTT_SUCCESS) {
      Serial.println("[MQTT] if available bytes on the network is available, close connection.");
      this->close();
      switch(this->_lastError)
      {
        case LWMQTT_SUCCESS: // = 0,
          Serial.println("[MQTT] LWMQTT_SUCCESS.");
          break;
        case LWMQTT_BUFFER_TOO_SHORT: // = -1,
          Serial.println("[MQTT] LWMQTT_BUFFER_TOO_SHORT.");
          break;
        case LWMQTT_VARNUM_OVERFLOW: // = -2,
          Serial.println("[MQTT] LWMQTT_VARNUM_OVERFLOW.");
          break;
        case LWMQTT_NETWORK_FAILED_CONNECT: // = -3,
          Serial.println("[MQTT] LWMQTT_NETWORK_FAILED_CONNECT.");
          break;
        case LWMQTT_NETWORK_TIMEOUT: // = -4,
          Serial.println("[MQTT] LWMQTT_NETWORK_TIMEOUT.");
          break;
        case LWMQTT_NETWORK_FAILED_READ: // = -5,
          Serial.println("[MQTT] LWMQTT_NETWORK_FAILED_READ.");
          break;
        case LWMQTT_NETWORK_FAILED_WRITE: // = -6,
          Serial.println("[MQTT] LWMQTT_NETWORK_FAILED_WRITE.");
          break;
        case LWMQTT_REMAINING_LENGTH_OVERFLOW: // = -7,
          Serial.println("[MQTT] LWMQTT_REMAINING_LENGTH_OVERFLOW.");
          break;
        case LWMQTT_REMAINING_LENGTH_MISMATCH: // = -8,
          Serial.println("[MQTT] LWMQTT_REMAINING_LENGTH_MISMATCH.");
          break;
        case LWMQTT_MISSING_OR_WRONG_PACKET: // = -9,
          Serial.println("[MQTT] LWMQTT_MISSING_OR_WRONG_PACKET.");
          break;
        case LWMQTT_CONNECTION_DENIED: // = -10,
          Serial.println("[MQTT] LWMQTT_CONNECTION_DENIED.");
          break;
        case LWMQTT_FAILED_SUBSCRIPTION: // = -11,
          Serial.println("[MQTT] LWMQTT_FAILED_SUBSCRIPTION.");
          break;
        case LWMQTT_SUBACK_ARRAY_OVERFLOW: // = -12,
          Serial.println("[MQTT] LWMQTT_SUBACK_ARRAY_OVERFLOW.");
          break;
        case LWMQTT_PONG_TIMEOUT: // = -13,
          Serial.println("[MQTT] LWMQTT_PONG_TIMEOUT.");
          break;
      }
      return false;
    }
  }

  // keep the connection alive
  this->_lastError = lwmqtt_keep_alive(&this->client, this->timeout);
  if (this->_lastError != LWMQTT_SUCCESS) {
    Serial.println("[MQTT] keep the connection alive, close connection.");
    this->close();
    switch(this->_lastError)
      {
        case LWMQTT_SUCCESS: // = 0,
          Serial.println("[MQTT] LWMQTT_SUCCESS.");
          break;
        case LWMQTT_BUFFER_TOO_SHORT: // = -1,
          Serial.println("[MQTT] LWMQTT_BUFFER_TOO_SHORT.");
          break;
        case LWMQTT_VARNUM_OVERFLOW: // = -2,
          Serial.println("[MQTT] LWMQTT_VARNUM_OVERFLOW.");
          break;
        case LWMQTT_NETWORK_FAILED_CONNECT: // = -3,
          Serial.println("[MQTT] LWMQTT_NETWORK_FAILED_CONNECT.");
          break;
        case LWMQTT_NETWORK_TIMEOUT: // = -4,
          Serial.println("[MQTT] LWMQTT_NETWORK_TIMEOUT.");
          break;
        case LWMQTT_NETWORK_FAILED_READ: // = -5,
          Serial.println("[MQTT] LWMQTT_NETWORK_FAILED_READ.");
          break;
        case LWMQTT_NETWORK_FAILED_WRITE: // = -6,
          Serial.println("[MQTT] LWMQTT_NETWORK_FAILED_WRITE.");
          break;
        case LWMQTT_REMAINING_LENGTH_OVERFLOW: // = -7,
          Serial.println("[MQTT] LWMQTT_REMAINING_LENGTH_OVERFLOW.");
          break;
        case LWMQTT_REMAINING_LENGTH_MISMATCH: // = -8,
          Serial.println("[MQTT] LWMQTT_REMAINING_LENGTH_MISMATCH.");
          break;
        case LWMQTT_MISSING_OR_WRONG_PACKET: // = -9,
          Serial.println("[MQTT] LWMQTT_MISSING_OR_WRONG_PACKET.");
          break;
        case LWMQTT_CONNECTION_DENIED: // = -10,
          Serial.println("[MQTT] LWMQTT_CONNECTION_DENIED.");
          break;
        case LWMQTT_FAILED_SUBSCRIPTION: // = -11,
          Serial.println("[MQTT] LWMQTT_FAILED_SUBSCRIPTION.");
          break;
        case LWMQTT_SUBACK_ARRAY_OVERFLOW: // = -12,
          Serial.println("[MQTT] LWMQTT_SUBACK_ARRAY_OVERFLOW.");
          break;
        case LWMQTT_PONG_TIMEOUT: // = -13,
          Serial.println("[MQTT] LWMQTT_PONG_TIMEOUT.");
          break;
      }
    return false;
  }

  return true;
}

I got the error: LWMQTT_BUFFER_TOO_SHORT so

    // Publish message
    String payload;
    serializeJson(doc, payload);// {"clientid":"84:FC:E6:00:89:18","D10":"LOW"}
    // bool publish(const String &topic, const String &payload)
    Serial.print("[MQTT] payload.length() = ");
    Serial.println(payload.length());

I got logs below: [MQTT] Payload:{"id":"ESP32C364E83389CDC0","D0":false,"D1":false,"D2":false,"D3":false,"D4":false,"D5":false,"D6":false,"D7":true,"D8":false,"D9":true,"D10":true} [MQTT] payload.length() = 147

I know, the buffer is too short, because the default value is 128, MQTTClient mqtt; change to MQTTClient mqtt(256);

so the bug above is fixed.