Recursive run_system

[SpecificProtagonist]

Objective

Fixes #18030

Solution

When running a one-shot system, requeue the system's command queue onto the world's command queue, then execute the later.

If running the entire command queue of the world is undesired, I could add a new method to RawCommandQueue to only apply part of it.

Testing

See the new test.

---

Showcase

#[derive(Resource)]
pub struct Test {
    id: SystemId,
    counter: u32,
}

let mut world = World::new();
let id = world.register_system(|mut commands: Commands, mut test: ResMut<Test>| {
    print!("{:?} ", test.counter);
    test.counter -= 1;
    if test.counter > 0 {
        commands.run_system(test.id);
    }
});
world.insert_resource(Test { id, counter: 5 });
world.run_system(id).unwrap();

5 4 3 2 1 

[alice-i-cecile]

Oh hey, I remember being annoyed by this and not being able to figure out how to fix it when I was working on the MVP one-shot systems. Definitely a bug.

[chescock]

This is a useful API to expose! The single-threaded executor has some unsafe code and a special case for exclusive systems because it wants exactly this behavior:

bevy/crates/bevy_ecs/src/schedule/executor/single_threaded.rs

Line 116 in b73811d

if system.is_exclusive() {

It might be worth doing a follow-up to use run_without_applying_deferred there.

[Carter0]

I thought commands were only run at sync points? Not after any system has been run?

[SpecificProtagonist]

We are at a sync point, in the sense that we have an exclusive reference to the world. Before this PR, commands queued by the system were executed here via System::run.

There is a difference in behavior though: Previously, only the commands queued by the system were applied (which might include running other systems and their commands); now, this includes commands that were already queued before the system was run. I've documented it, but if we think the previous behavior is less surprising, I'll add a method to CommandQueue to only apply part of it and use it here. I'd do that in a followup PR though because it also involves unsafe and I want to keep it easy to review.

[Carter0]

All good I mostly asked for my own understanding.

[Carter0]

Minor question, but I think its good