Component hooks are not applied during exclusive world access

[alice-i-cecile]

Problem

When an entity is spawned (or other structural changes occur) as part of exclusive world access, commands to apply changes via hooks are generated, but are not immediately applied.

In order to apply these commands, we must call world.flush, as demonstrated here.

This allows users to accidentally observe the state of the world before component lifecycle hooks are applied, but only when working with exclusive world access. As a result, we must be extremely careful if we are relying on hooks for soundness.

Bevy version

0.14.1

Additional context

This is related to #14621, but is intended as a clearer demonstration of a specific problem.

#12235 may end up being blocked on this, as otherwise we could get UB by calling unsafe code on an invalid entity hierarchy.

[alice-i-cecile]

Of course, it's not clear that this is fully undesirable behavior! Better control over when hooks are applied in the context of exclusive world access exposes more power for users, particularly around performance optimizations (e.g. #10154).

We could decide that this is intended behavior, and simply clearly document it. Relying on fully valid hierarchies for soundness is very challenging already (see #15575), we should consider making this code robust or marking the system itself as unsafe.

[alice-i-cecile]

I would have preferred to always apply them, but it's very difficult to make work with EntityWorldMut since you can't risk despawning the entity so you have to run them after dropping it. The problem is you can't implement drop for EntityWorldMut without UX regressions due to it borrowing &mut World until end of scope

There are 3 ways to fix it:

1. Implement Drop for EntityWorldMut
2. Provide EntityWorldMut in a closure like world.entity_mut(|e| { ... })
3. Allow EntityWorldMut to point to a non-existent entity
   All come with UX regressions though

From @james-j-obrien on Discord.