Adding non-rollbacked children to rollbacked entities break the hierarchy

[johanhelsing]

Describe the bug

I'd like it to be possible to spawn cosmetic entities as children of rolled back entities.

Used to work prior to #31

For instance to:

• compose a character of several sprites
• use 3rd-party plugins for particle effects

pub fn spawn_character_particles(
    commands: &mut Commands,
    character_entity: Entity,
    sprites: &Res<SpriteAssets>,
) {
    info!("Adding particles to character: {character_entity:?}");
    commands
        .entity(character_entity)
        .with_children(|character| {
            character
                .spawn((
                    Name::new("JetPackParticles"),
                    ParticleSystemBundle {
                        // transform: Transform::from_xyz()
                        particle_system: ParticleSystem {
                            max_particles: 10_000,
                            texture: ParticleTexture::Sprite(sprites.particle.clone()),
                            spawn_rate_per_second: 40.0.into(),
                            emitter_shape: CircleSegment {
                                radius: 0.0.into(),
                                opening_angle: 0.5,
                                direction_angle: PI * 3. / 2.,
                            }
                            .into(),

                            initial_speed: JitteredValue::jittered(1.0, -0.5..0.5),
                            // acceleration: todo!(),
                            lifetime: JitteredValue::jittered(0.3, -0.1..0.1),
                            color: ColorOverTime::Gradient(Curve::new(vec![
                                CurvePoint::new(Color::WHITE, 0.0),
                                CurvePoint::new(Color::NONE, 1.0),
                            ])),
                            scale: ValueOverTime::Constant(0.0005),
                            looping: true,
                            system_duration_seconds: 10.0,
                            z_value_override: Some(JitteredValue::new(55.0)),
                            // bursts: todo!(),
                            // space: todo!(),
                            ..default()
                        },
                        ..default()
                    },
                    bevy_particle_systems::Playing,
                    JetPackParticles,
                ))
                .log_entity_id("spawned jetpack_particles");
        });
}

Currently, this results in Children containing "dead" entities after the first time a snapshot restore happens.

To Reproduce
Steps to reproduce the behavior:

1. spawn an entity with .add_rollback(), and a child without .add_rollback()
2. trigger a rollback
3. traverse Children hierarchy, it now points to a "dead" entity

Expected behavior

I would expect the entity ids of the children without Rollback components to be left alone (like they used to).

Describe alternatives you've considered

Adding "cosmetic follower entities" in a separate hierarchy branch that contain a reference to the rolled back entity, and have a book-keeping step where entities with dangling refs are despawned, and positions are synced.

Additional context

The problem is that map_entities for unknown entities returns "dead" entities: https://github.com/bevyengine/bevy/blob/8ace2ff9e361dd7ef1bc620b84674def0cb56454/crates/bevy_hierarchy/src/components/children.rs#L26

bevyengine/bevy#6790

[Illiux]

I'm not sure if it's possible in your context, but can you add identity mappings for the entities in question to the map? That way, they'll be preserved during entity mapping instead of being replaced by dead entities.