Bevy version

0.16.1

What you did

#[derive(Component)]
struct A;

loop {
    let w = World::new();

    std::thread::scope(|s| {
        let c1 = s.spawn(|| w.components_queue().queue_register_component::<A>());
        let c2 = s.spawn(|| w.components_queue().queue_register_component::<A>());

        let (c1, c2) = (c1.join().unwrap(), c2.join().unwrap());
        assert_eq!(c1, c2);
    });
}

What went wrong

After some iterations the assert_eq fails.

Additional information

The issue is a TOCTOU bug in queue_register_component: it first acquires and releases a read lock to check if the component is registered, then forces a registration of the component if it wasn't already registered. However this ignored the possibility of another thread registering the component inbetween the check and the new registration.