python的异步C++库

Question

I'm struggling with something that hopefuly someone here will be able to guide me through please.

I have an asynchronous python code(asyncio), and I wrote simple asynchronous client in C++ that is very similiar to this one: https://www.boost.org/doc/libs/1_74_0/doc/html/boost_asio/example/cpp03/timeouts/async_tcp_client.cpp . For testing purposes I have server, which sends message every x seconds to client, when client connects. My plan is to be able to use this client from my python code but the problem is that currently it blocks my event loop and I don't really know what to do with it. Is there a way to write library that would be awaitable from python code?

Example C++ library:

void start_client(std::string ip, std::string port)
{
    try
    {
        boost::asio::io_context io_context;
        tcp::resolver r(io_context);
        // https://www.boost.org/doc/libs/1_74_0/doc/html/boost_asio/example/cpp03/timeouts/async_tcp_client.cpp
        client c(io_context);

        c.start(r.resolve(ip, port));

        io_context.run();  // this will block :(
        
    }
    catch (std::exception& e)
    {
        std::cerr << "Exception: " << e.what() << "\n";
    }
}


#include <pybind11/pybind11.h>


PYBIND11_MODULE(async_client, m) {
    m.def("start_client", &start_client);
}

and also some minimal reproducible python code:

import asyncio
import async_client 


async def client():
    async_client.start_client('0.0.0.0', '3000')  # this will block :(


async def test():
    while True:
        print('This will not get displayed!')
        await asyncio.sleep(3)


async def main():
    await asyncio.gather(client(), test())

if __name__ == '__main__':
    asyncio.run(main())

I already tried releasing GIL and running io_context in a diffrent thread, but I still need some endless process so it won't jump out of scope and it still blocks.

m.def("start_client", &start_client, pybind11::call_guard<pybind11::gil_scoped_release>() );

Any tips appreciated, thank you.

Answer 1

To make start_client not block the event loop, you need to release the GIL, and also hand it off to a thread pool (executor) managed by asyncio for that purpose:

async def client():
    loop = asyncio.get_event_loop()
    await loop.run_in_executor(
        None, async_client.start_client, '0.0.0.0', '3000'
    )

Ideally one would connect asyncio and boost_asio event loops, so that threads are not needed, but doing so is likely to be a major undertaking.