Hang with LocalSet

[alecmocatta]

Version

0.3.3 (current latest)

Platform

Tested on Linux and macOS

Description

This code hangs, whereas I would expect it to complete. The LocalSet is being awaited upon, therefore I would expect the spawned task to complete.

use futures::{
    future::{pending, ready},
    FutureExt,
};
use tokio::task::LocalSet;

#[tokio::main]
async fn main() {
    let ls = LocalSet::new();
    futures::select! {
        _ = ls.run_until(pending::<()>()).fuse() => unreachable!(),
        ret = async { ls.spawn_local(ready(())).await.unwrap() }.fuse() => ret
    }
}

(Playground)

[Darksonn]

I played a bit around with this. The cause appears to be the following:

1. The select! polls the first branch, resulting in a run_until call that's sleeping.
2. The select! polls the second branch, spawning the future onto the local set.
3. But run_until is sleeping and the spawn does not emit a wake-up to it, so it keeps sleeping.

If you change it to spawn first, or for run_until to have a wake-up after the spawn, then it works.

[carllerche]

Seems like a spawn should result in waking the local set?. cc @hawkw

[hawkw]

Seems like a spawn should result in waking the local set?. cc @hawkw

That seems right, we can do that!