Deadlock when deactivating the client after the server has shut down

[kunerd]

Hi,

I'm trying to make my GUI app a jack client and would like to handle the case of a server shutdown with an automatic retry. Unfortunately, I either ran into a problem, where my working thread hangs, or I if I start the async client in another thread, I get a SEGFAULT as soon as I try to stop it.
It's very likely, that I'm doing something wrong here.

I have build a stripped down minimal example here:
https://github.com/kunerd/rust_jack_threading/tree/segfault

The main branch contains my previous attempt without the additional thread.

I really would appreciate some help on this.

[kunerd]

After a bit of digging I noticed that I can reproduce the problem with a stripped down version, too.
https://github.com/kunerd/rust_jack_threading/tree/deadlock_stripped_version

First, I thought it's still a problem with my shutdown callback impl, but I can get it to deadlock without this, too. I could figure out that async_client.deactivate() hangs if it gets called, after the server has shutdown. So this is probably a bug.

[kunerd]

The issue is resolved, if I explicitly drop the mutex lock before the error is returned in this line:
https://github.com/RustAudio/rust-jack/blob/main/src/client/async_client.rs#L117

[kunerd]

This is definitely a bug.

Steps to reproduce:

1. start jack server
2. run the following minimal application

use std::time::Duration;

fn main() {
    let (client, _status) =
        jack::Client::new("deadlock_client", jack::ClientOptions::default()).unwrap();

    let client = client
        .activate_async(Notifications, ProcessHandler {})
        .unwrap();

    println!("async client running...");

    // shutdown server
    std::thread::sleep(Duration::from_secs(5));

    println!("sleep ended...");

    // deadlock
    client.deactivate().unwrap();
}

struct Notifications;
struct ProcessHandler;

impl jack::NotificationHandler for Notifications {}

impl jack::ProcessHandler for ProcessHandler {
    fn process(&mut self, _: &jack::Client, _process_scope: &jack::ProcessScope) -> jack::Control {
        jack::Control::Continue
    }
}

or full project here: https://github.com/kunerd/rust_jack_threading/tree/deadlock_bug_report
3. shutdown server during the sleep

expected

Application should shutdown without hanging.

actual

Application hangs forever.

[fstxz]

I came across the same issue when I was trying to cleanly exit my app when jack shuts down. I decided to use std::process:exit() for now.

[kunerd]

@fstxz I have created a fork and fixed the issue by using a manual drop. But I don't think that this is a good solution, so I won't upstream it for now. I haven't looked further into why the lib uses a global mutex, probably there is a reason for this.

[kunerd]

I have tested my application with 0.13.3 and it works fine, now. Don't know why this ticket hasn't been auto closed.