Is block_in_place in a LocalSet supported?

[dspyz-matician]

I notice that whether or not I get an error when doing this seems arbitrary. Using run_until results in an error. But using spawn_local seems to be allowed.

In a large codebase, we've found what looks like a rare bug in tokio where there are no available worker threads despite the fact that no spawned tasks are being polled (although can't seem to produce a MWE yet). And it might be because this pattern is meant to be unsupported but the fact that it doesn't panic is a bug (I'm imagining in rare circumstances, the resulting undefined behavior "accidentally eats" a worker thread)? Or it might be completely unrelated.

use std::time::Duration;

use tokio::task::LocalSet;

const DURATION: Duration = Duration::from_micros(100);

#[tokio::main(flavor = "multi_thread")]
async fn main() {
    let local_set = LocalSet::new();
    local_set
        .run_until(async {
            tokio::task::block_in_place(|| std::thread::sleep(DURATION));
        })
        .await;
}

results in

thread 'main' panicked at src/main.rs:12:13:
can call blocking only when running on the multi-threaded runtime

but

#[tokio::main(flavor = "multi_thread")]
async fn main() {
    let local_set = LocalSet::new();
    local_set.spawn_local(async {
        tokio::task::block_in_place(|| std::thread::sleep(DURATION));
    });
    local_set.await
}

doesn't panic.

[Darksonn]

That looks like it should panic. Can you file a bug?

[dspyz-matician]

Created #7368