rt: interleaving multiple calls to Handle::enter() does not work

[carllerche]

When calling Handle::enter() multiple times and dropping guards in an unexpected order, the "current runtime" is set incorrectly.

This is because the guard stores the previous state and updates the current runtime on drop.

Repro:

#[test]
fn interleave_enter() {
    let rt1 = rt();
    let rt2 = rt();
    let rt3 = rt();

    let _enter1 = rt1.enter();
    let enter2 = rt2.enter();
    let enter3 = rt3.enter();

    drop(enter2);
    drop(enter3);

    drop(rt2);

    let handle = tokio::spawn(async {});
    rt1.block_on(handle).unwrap();
}

[carllerche]

@Darksonn Why do we check that EnterGuard is Send here? It would not be good for the guard to move threads...

[carllerche]

Unfortunately, we can't deprecate the method. As far as I can see, the options are:

• panic if the guards are dropped out of order.
• Maintain a linked list of "previous" states. This could be intrusive when the API is a closure block_on but enter() will need to allocate.

Either way, EnterGuard should not be Send (Sync is fine).