Synchronously check return value from JoinHandle in Tokio

[manforowicz]

I spawn a tokio task, and take its return value once it has finished:

fn main() {
    let rt = tokio::runtime::Runtime::new().unwrap();

    let mut join_handle = rt.spawn(async {
        tokio::time::sleep(std::time::Duration::from_secs(5)).await;
        "Done"
    });

    // GUI paint loop
    loop {
        // Paint a GUI on every frame...

        // If task finished, display it in the GUI
        if join_handle.is_finished() {
            let return_value = rt.block_on(&mut join_handle).unwrap();
            println!("Return value: {}", &return_value);
        }
    }
}

This successfully prints the return value from join_handle, but then it panics:

Return value: Done
thread 'main' panicked at tokio-1.39.2/src/runtime/task/core.rs:375:22:
JoinHandle polled after completion

Why does tokio panic, and how can I get around it?

[theemathas]

You tried to block_on() the same future twice. Apparently that's not supported.

[manforowicz]

You're right, I can't believe I didn't spot this basic mistake . Here's a trivially corrected version of my code:

fn main() {
    let rt = tokio::runtime::Runtime::new().unwrap();

    let mut join_handle_option = Some(rt.spawn(async {
        tokio::time::sleep(std::time::Duration::from_secs(5)).await;
        "Done"
    }));

    // GUI paint loop
    loop {
        // Paint a GUI on every frame...

        // If task finished, display it in the GUI
        if let Some(join_handle) = &mut join_handle_option {
            if join_handle.is_finished() {
                let return_value = rt.block_on(join_handle).unwrap();
                println!("Return value: {}", &return_value);
                join_handle_option = None;
            }
        }
    }
}

I don't think this question is very helpful, so feel free to delete it (I don't have permissions to).