RPC_Response, Dangling Reference

[brhmrsln]

Hello all, "process_rpc_message" is calling the callback function "processTemperatureChange". The callback function must return RPC_Response object. Now lets consider the example [0010-esp8266_esp32_rpc]. İn 0010-esp8266_esp32_rpc.ino file, function loop() we are registering the callback functions:

    const std::array<RPC_Callback, 2U> callbacks = {
      RPC_Callback{ RPC_TEMPERATURE_METHOD,    processTemperatureChange },
      RPC_Callback{ RPC_SWITCH_METHOD,         processSwitchChange }
    };
    // Perform a subscription. All consequent data processing will happen in
    // processTemperatureChange() and processSwitchChange() functions,
    // as denoted by callbacks array.
    if (!tb.RPC_Subscribe(callbacks.cbegin(), callbacks.cend())) {
#if THINGSBOARD_ENABLE_PROGMEM
      Serial.println(F("Failed to subscribe for RPC"));
#else
      Serial.println("Failed to subscribe for RPC");
#endif
      return;
    }

and one of the callback function is :

RPC_Response processTemperatureChange(const RPC_Data &data) {
#if THINGSBOARD_ENABLE_PROGMEM
  Serial.println(F("Received the set temperature RPC method"));
#else
  Serial.println("Received the set temperature RPC method");
#endif

  // Process data
  const float example_temperature = data[RPC_TEMPERATURE_KEY];

#if THINGSBOARD_ENABLE_PROGMEM
  Serial.print(F("Example temperature: "));
#else
  Serial.print("Example temperature: ");
#endif
  Serial.println(example_temperature);

  // Just an response example
  StaticJsonDocument<JSON_OBJECT_SIZE(1)> doc;
  doc[RPC_RESPONSE_KEY] = 42;
  return RPC_Response(doc);
}

In the callback function 'processTemperatureChange', The 'StaticJsonDocument' object is created locally and passed as an argument to RPC_Response constructor. The called RPC_Response constructor is the one with the parameter JsonVariant.

explicit RPC_Response(JsonVariant variant);

Now the local 'StaticJsonDocument' object is impilicitly converted to 'JsonVariant', which means 'JsonVariant' is referencing the local 'StaticJsonDocument' object. The 'process_rpc_message' gets the callback function's("processTemperatureChange") return value as a response.

RPC_Response response;
...
response = rpc.Call_Callback<Logger>(param);

As soon as callback function is finished, local object 'StaticJsonDocument' lifetime is over and the returned value RPC_Response is now referencing a released source 'StaticJsonDocument'. Undefined behaviour. The solution can be: RPC_Response class can store the JsonDocument so that we can move the locally created 'StaticJsonDocument' object and then use it when sending the response to the server. Thanks in advance.

[MathewHDYT]

Already known thanks to issue #167, is fixed in the aforementioned pull request. As soon as it has been merged the issue will be resolved.

[brhmrsln]

Thank you for the quick response.