Promise::Deferred in C callback function

[k2la]

I'm trouble about the following code:

void my_callback(result res, void *ctx) {
    Napi::Promise::Deferred *deferred = (Napi::Promise::Deferred *)ctx;
    Napi::Env env = deferred->Env(); // segmentation fault
    if (res == RESULT_OK) {
        deferred->Resolve(Napi::String::New(env, "Success"));
    } else {
        deferred->Reject(
          Napi::Error::New(env, std::string("Failed")).Value());
    }
}

Napi::Value MyClass::Do(const Napi::CallbackInfo &info) {
    Napi::Env env = info.Env();
    Napi::Promise::Deferred deferred = Napi::Promise::Deferred::New(env);

    do_async(my_callback, &deferred);
    return deferred.Promise();
}

This code:

1. MyClass::Do is called by Nodejs code directly.
2. After do_async ends in another thread, my_callback begins.
3. my_callback sees res and executes deferred->Resolve or deferred->Reject.

I want to run this code as above but segmentation fault occurs in my_callback.

With the following restriction, How should I use deferred without errors in my_callback?

• Declaration about my_callback cannot be changed.

FYI: That I tried in order to solve this problem.

[mhdawson]

I think we need to see the implementation of do_async. My first guess is that you might need to use https://github.com/nodejs/node-addon-api/blob/master/doc/async_worker.md

and ensure that my_callback is invoked in the OnOK method to ensure it runs on the main thread.

[mhdawson]

The key point above is that node-addon-api code interacting with JavaScript must run on the main thread, not on any other thread.

[k2la]

Thank you for your reply.

I think we need to see the implementation of do_async.

I want to use the following Producer::SendAsync as do_async.

void pulsarProducerSendCallbackProxy(pulsar_result res, pulsar_message_t *message, void *ctx) {
    Napi::Promise::Deferred *deferred = (Napi::Promise::Deferred *)ctx;
    Napi::Env env = deferred->Env(); // segmentation fault
    if (res == RESULT_OK) {
        deferred->Resolve(Napi::String::New(env, "Success"));
    } else {
        deferred->Reject(Napi::Error::New(env, std::string("Failed")).Value());
    }
}

Napi::Value Producer::SendAsync(const Napi::CallbackInfo &info) {
    Napi::Env env = info.Env();
    Napi::Promise::Deferred deferred = Napi::Promise::Deferred::New(env);

    pulsar_message_t *cMessage = Message::BuildMessage(info[0].As<Napi::Object>());
    pulsar_producer_send_async(this->cProducer, cMessage, pulsarProducerSendCallbackProxy, $deferred);
    return scope.Escape(deferred.Promise());
}

We register pulsarProducerSendCallbackProxy which operates deferred with pulsar_producer_send_async in Pulsar C++ client library.

pulsar_producer_send_async sends messages asynchronously by using boost::asio::async_write.

Reigstered pulsarProducerSendCallbackProxy executes as a handler by boost::asio::async_write.

Can we operate deferred by using a function registered with boost::asio::async_write ?

FYI: From pulsar_producer_send_async to boost::asio::async_write, following functions are called.

1. pulsar_producer_send_async
2. ProducerImpl::sendAsync
3. ProducerImpl::sendMessage
4. ClientConnection::sendMessage
5. ClientConnectino::asyncWrite
6. boost::asio::async_write

[mhdawson]

I think you'll need to use AsyncWorker or more likely https://nodejs.org/dist/latest-v11.x/docs/api/n-api.html#n_api_asynchronous_thread_safe_function_calls to ensure that you are only making the N-API calls on the main thread. We don't have a wrapper in node-addon-api yet but you can mix node-addon-api and the native C calls to N-API

[k2la]

I understood that.
Thank you. I'll do some more research.

[mhdawson]

Closing this out as the question was answered. Please let us know if that was no the right thing to do.