bevy/flush.md at 43b8fbda93209a681d15d4ac00763401802dc6b1

SpecificProtagonist 13e89a1678

Fix EntityMeta.spawned_or_despawned unsoundness (#19350 )

# Objective

#19047 added an `MaybeUninit` field to `EntityMeta`, but did not
guarantee that it will be initialized before access:

```rust
let mut world = World::new();
let id = world.entities().reserve_entity();
world.flush();
world.entity(id);
```

<details>
<summary>Miri Error</summary>

```
error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
    --> /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1121:26
     |
1121 |                 unsafe { meta.spawned_or_despawned.assume_init() }
     |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
     = note: BACKTRACE:
     = note: inside closure at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1121:26: 1121:65
     = note: inside `std::option::Option::<&bevy_ecs::entity::EntityMeta>::map::<bevy_ecs::entity::SpawnedOrDespawned, {closure@bevy_ecs::entity::Entities::entity_get_spawned_or_despawned::{closure#1}}>` at /home/vj/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/option.rs:1144:29: 1144:33
     = note: inside `bevy_ecs::entity::Entities::entity_get_spawned_or_despawned` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1112:9: 1122:15
     = note: inside closure at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1094:13: 1094:57
     = note: inside `bevy_ecs::change_detection::MaybeLocation::<std::option::Option<&std::panic::Location<'_>>>::new_with_flattened::<{closure@bevy_ecs::entity::Entities::entity_get_spawned_or_despawned_by::{closure#0}}>` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/change_detection.rs:1371:20: 1371:24
     = note: inside `bevy_ecs::entity::Entities::entity_get_spawned_or_despawned_by` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1093:9: 1096:11
     = note: inside `bevy_ecs::entity::Entities::entity_does_not_exist_error_details` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1163:23: 1163:70
     = note: inside `bevy_ecs::entity::EntityDoesNotExistError::new` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/entity/mod.rs:1182:22: 1182:74
     = note: inside `bevy_ecs::world::unsafe_world_cell::UnsafeWorldCell::<'_>::get_entity` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/world/unsafe_world_cell.rs:368:20: 368:73
     = note: inside `<bevy_ecs::entity::Entity as bevy_ecs::world::WorldEntityFetch>::fetch_ref` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/world/entity_fetch.rs:207:21: 207:42
     = note: inside `bevy_ecs::world::World::get_entity::<bevy_ecs::entity::Entity>` at /home/vj/workspace/rust/bevy/crates/bevy_ecs/src/world/mod.rs:911:18: 911:42
note: inside `main`
    --> src/main.rs:12:15
     |
12   |     world.entity(id);
     |
```

</details>

## Solution

- remove the existing `MaybeUninit` in `EntityMeta.spawned_or_despawned`
- initialize during flush. This is not needed for soundness, but not
doing this means we can't return a sensible location/tick for flushed
entities.

## Testing

Test via the snippet above (also added equivalent test).

---------

Co-authored-by: urben1680 <55257931+urben1680@users.noreply.github.com>

2025-05-27 22:45:07 +00:00

274 B

Raw Blame History

title

pull_requests

Flushing

19350

Entities::flush now also asks for metadata about the flush operation that will be stored for the flushed entities. For the source location, MaybeLocation::caller() can be used; the tick should be retrieved from the world.

274 B Raw Blame History

274 B

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