Hooks and observers that run on spawn and immediately despawn their entity are unsound

[grind086]

Bevy version

0.16.1 and main branch

What you did

Attempted to despawn an entity from an insert hook, which triggered a debug panic in EntityWorldMut::new. For example:

use bevy::ecs::{
    component::{Component, HookContext},
    world::{DeferredWorld, World},
};

#[derive(Component)]
#[component(on_insert = on_insert_a)]
struct A;

fn on_insert_a(mut world: DeferredWorld, HookContext { entity, .. }: HookContext) {
    // This causes a panic in debug builds, but not in release builds.
    world.commands().entity(entity).despawn();
}

fn main() {
    let mut world = World::new();
    world.spawn(A);
}

What went wrong

When an entity is immediately despawned by a hook or observer, this flush in World::spawn_with_caller:

bevy/crates/bevy_ecs/src/world/mod.rs

Lines 1182 to 1185 in 479c93d

	if !unsafe { self.command_queue.is_empty() } {
	self.flush();
	entity_location = self.entities().get(entity);
	}

Invalidates this safety guarantee a few lines later:

bevy/crates/bevy_ecs/src/world/mod.rs

Lines 1187 to 1188 in 479c93d

	// SAFETY: entity and location are valid, as they were just created above
	let mut entity = unsafe { EntityWorldMut::new(self, entity, entity_location) };

[alice-i-cecile]

Thanks for the report. I agree with your analysis of the problem.

[Wuketuke]

it is very easy to detect this.

if !unsafe { self.command_queue.is_empty() } {
    self.flush();
    entity_location = self.entities().get(entity);
    if entity_location.is_none() {
        // entity has already been removed
    }
}

however, i dont know how to proceed.
do we panic here, or return None?

since this is on Alice's Work Planning, i assume she already has some ideas?