Tracked one-shot systems

[ItsDoot]

Objective

#24087 introduces scene templating for SystemIds, however it can result in a memory leak if a scene is re-constructed multiple times:

#24087 (comment)

This was proposed basically 1:1 in #24026 (this was later changed though). The issue is that it's unclear who owns these systems, that is who is responsible for unregistering them once they are no longer needed. Given that recreating the template will spawn the system again this basically becomes a memory leak.

#24087 (comment)

Hm, are you sure this is the case even tho in build_template it only registers the system the first time its called, switching over to storing the SystemId after the first call?

If you recreate the template (e.g. you call my_scene() again) then you will create a new instance of the system. And since the system is not scoped to the scene once the scene is despawned the system entity will be leaked.

Essentially, we need a way to connect the lifetime of the registered system to the lifetime of the scene.

Solution

This is a purely additive / opt-in / backwards-compatible version of #24114

Introducing: SystemHandles

pub enum SystemHandle<I: SystemInput = (), O = ()> {
    /// A strong handle keeps the system entity alive as long as the handle
    /// (and any clones of it) exist.
    Strong(Arc<StrongSystemHandle>),
    /// A weak handle does not keep the system entity alive.
    Weak(SystemId<I, O>),
}

pub struct StrongSystemHandle {
    entity: Entity,
    drop_queue: Arc<ConcurrentQueue<<Entity>>,
}

Similar to bevy_asset::Handles,SystemHandle's custom Drop implementation enqueues the registered system entity into a concurrent queue. The system despawn_unused_registered_systems pulls from the other end of this queue and despawns the registered system entities.

World::register_tracked_system and World::register_tracked_boxed_system are the only functions that return SystemHandles.

Testing

• Added a test to ensure that despawn_unused_registered_systems does its job
• Added a test to ensure that the default app will automatically call despawn_unused_registered_systems

Future work

• SystemId scene templating #24087 will use this PR as a base

[chescock]

Looks good! I left some comments, but they're just style nits.

[chescock]

Do we need to worry about this monomorphizing two versions of this function?

[ItsDoot]

Maybe! Would be good to check how much of a difference the codegen is