Is There a Way to Create a Thread Local Exclusive System?

[zicklag]

I need to create an exclusive system that is local to the main thread. The issue is that the system I have is !Send + !Sync. At the same time, I only need it to be thread local on WASM, but there aren't threads on WASM anyway right? Maybe I can just unsafe impl Send {} for the !Send because WASM doesn't have threads so it won't be a problem?

[Ratysz]

Exclusive systems always run on the main thread. Or are there API discrepancies that don't let it work as expected? Can you provide an example?

[zicklag]

Oh, OK. Here's an example that doesn't work:

use bevy::prelude::*;

struct NotSend {
    not_send: *const u8,
}

fn main() {
    let system = make_system();
    App::build().add_system(system.exclusive_system()).run();
}

fn make_system() -> impl Fn(&mut World) {
    let not_send = NotSend {
        not_send: std::ptr::null(),
    }

    move |_| {
        let _ = not_send;
    }
}

It complains that exclusive_system is not implemented for system in the current scope because the IntoExclusiveSystem trait requires that the Fn be Sync + Send:

bevy/crates/bevy_ecs/src/system/exclusive_system.rs

Lines 43 to 45 in 3a2a688

	impl<F> IntoExclusiveSystem<&mut World, ExclusiveSystemFn> for F
	where
	F: FnMut(&mut World) + Send + Sync + 'static,

---

If exclusive systems only run on the current thread, then Send + Sync shouldn't be required for that impl should it?

[Ratysz]

Oh, I may have forgotten that closures actually work for exclusive systems...

Naively removing those bounds from exclusive systems and Stage seems to work, but I'm not clear on all the implications yet. Ideally, we'd like to allow things like Local<(World, SystemStage)> in parallel (and/or async) systems - it doesn't fully work or even make sense right now, but removing the bounds would be a step further away from it in either case.

Gonna need more thinking time to get a good solution.

[zicklag]

Yeah, for now, in my particular use-case, I was able to just unsafe-ly implement Sync + Send for struct which I only use in the WASM build anyway and I think that seems to work. So at least I'm not waiting on anything. 🙂