How to call Python function in a Tokio request handler

[owenthereal]

👋

I'm exploring pyo3 to see if it's a good fit for what I'm doing. I would like to get a Python script like this to work with pyo3. I have pushed the example code here for the sake of discussion.

I would like that:

• The pyo3func.start_function starts a gRPC server in Rust using Tonic (basically an async gRPC server in Tokio)
• When there is a gRPC request to the server, the Tonic handler calls a Python function and return the result from the Python function as the gRPC response.

However, I found that the Python::with_gil line hangs forever in the Tonic/Tokio async handler when there is a request. I'm wondering whether you could help me understand how to make a Python function call in an async Rust handler.

Another issue is that after the Tonic server starts, pressing ctrl+c can not stop the running server. I think it's related to that I block the pyfunction. I'm wondering whether there is a better way to start an async Tonic/Tokio server in pyfunction that kill signals can reach the Rust server.

I'm fairly new to Pyo3 and I may need some hand-holding 😄 . It would be perfect if you have some example codes that I could take a look at for similar use cases. Thank you in advance!

[mejrs]

I think it's related to that I block the pyfunction

Yes, the tokio runtime is multithreaded, and you have this function hold the GIL the entire time, so any other thread will deadlock when they need to acquire it. So you should probably release it before starting the runtime:

#[pyfunction]
fn start_function(py: Python, f: Py<PyFunction>) -> PyResult<()> {
    let addr = "127.0.0.1:50051".parse().unwrap();
    let func = Function {
        callback: Arc::new(f),
    };
    //              👇 may need `move` here
    py.allow_threads(||{
        let rt = tokio::runtime::Runtime::new().unwrap();
        let serve = Server::builder()
            .add_service(function_server::FunctionServer::new(func))
            .serve(addr);
        rt.block_on(serve).unwrap();
    });

    Ok(())
}

Also, the Arc<Py<...>> is not needed, you can just use Py<...>

[owenthereal]

@mejrs:

Thanks for the reply! Yes, adding py.allow_threads gets the example to work! And Arc<Py<...>> is not needed (only Py<...>).

Another issue is that after the Tonic server starts, pressing ctrl+c can not stop the running server

I'm still having trouble sending sigterm to the running server with ctrl+c to stop the server from running. Would you have any recommendations or any examples on how to solve it?

[mejrs]

You'll probably want to look at https://tokio.rs/tokio/topics/shutdown for that. There is also Python::check_signals but given that you probably aren't in Python-land very long I'd just use the tokio handler.