Tokio timeout stuck on a loop.

[galeaspablo]

This is a simplified version of my code,

async fn empty_await() -> i32 {
    0
}

#[tokio::main(flavor="multi_thread", worker_threads = 6)]
async fn main() {
    let this_should_timeout_but_does_not = tokio::time::timeout(
        Duration::from_millis(1),
        async move {
            let mut x = 0;
            loop {
                empty_await().await;
                x += 1;
                println!("x = {}", x);
            }
        }
    ).await.unwrap();

    println!("line never reached");
}

The output of this program is

....
x = 6533492
x = 6533493
x = 6533494
x = 6533495
x = 6533496
x = 6533497
x = 6533498
x = 6533499
x = 6533500

It will keep going until I kill the process. Do I fundamentally misunderstand tokio and async? Isn't the timeout meant to stop the infinite loop? I read here that calling await gives the runtime a chance to swap tasks (hence the empty_await).

[galeaspablo]

This one panics as expected. How come the other one doesn't?

#[tokio::main(flavor="multi_thread", worker_threads = 6)]
async fn main() {
    let this_should_timeout_but_does_not = tokio::time::timeout(
        Duration::from_millis(1),
        async move {
            let mut x = 0;
            loop {
                tokio::time::sleep(Duration::from_millis(5)).await;
                x += 1;
                println!("x = {}", x);
            }
        }
    ).await.unwrap();

    println!("line never reached");
}

[Noah-Kennedy]

So, the future created by the async move block never actually yields because the inner future you await is always ready.

Rewrite empty wait as such:

async fn empty_await() -> i32 {
    tokio::task::yield_now().await;
    0
}

Many tokio async functions and futures use an internal budgeting system to force futures which would otherwise be ready to occasionally yield in order to mitigate this issue.

[galeaspablo]

Makes sense. Works as expected now :) Thank you

[Noah-Kennedy]

No problem!