tokio::time::timeout does not always return an Error when the timeout is reached or exceeded.

[rthomas]

Version
1.21.2

Platform
Darwin Ryans-Air 21.6.0 Darwin Kernel Version 21.6.0: Mon Aug 22 20:20:07 PDT 2022; root:xnu-8020.140.49~2/RELEASE_ARM64_T8110 arm64

Description
tokio::time::timeout does not always return an Error when the timeout is reached.

When calling long-running sync code in an async block, or async functions that do not yield, then timeout will not return an error, even if the timeout has been exceeded.

use std::time::Instant;
use tokio::time::Duration;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    println!("start");

    tokio::time::timeout(Duration::from_secs(2), async {
        expensive_sync_fn();
        other_async_fn().await;
    })
    .await?;

    println!("done");
    Ok(())
}

fn expensive_sync_fn() {
    let start = Instant::now();
    loop {
        if Instant::now().duration_since(start).as_secs() > 3 {
            break;
        }
    }
}

async fn other_async_fn() {
    println!("other async fn.");
    // Uncommenting this will cause the timeout to return a deadline exceeded error.
    // tokio::time::sleep(Duration::from_millis(10)).await;
}

What I expected to happen is that timeout would return an error due to the timeout being exceeded, even if it could not interrupt the current execution, on completion of the block it should check and return the error.

If this is not possible, then the documentation should be amended to state this behaviour.

[Darksonn]

With the current behavior, timeout(fut) is cancel-safe when fut is, but if we made this change, that would no longer be the case.

[rthomas]

Thanks @Darksonn I may raise a PR against the docs to highlight this case, as it was unexpected when I was using timeout.

[Darksonn]

Yes, please open a PR.

[rthomas]

Closing due to referenced PR.