`GpuComponentArrayBuffer` panics when two instances leave the array in one frame, or one comes and goes in a frame

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
70/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
rust
Ambito
game-dev

Direzione di ricerca

Start with crates/bevy_render/src/gpu_component_array_buffer.rs, especially update_components and GpuComponentArray::remove; the issue gives two reproductions and the failing assertions or command error. Trace how removals and swap-removal handle multiple entities and components added then removed before the system runs. Done means both reproductions and arbitrary same-frame removal orders complete without panicking.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

A-Rendering C-Bug D-Straightforward S-Ready-For-Implementation

Bevy version

0.20.0 (crates.io). crates/bevy_render/src/gpu_component_array_buffer.rs on main at
fd9e7609109758a2d9af401345f100be9442e9f6 (2026-10-24) is byte-identical, so main has both bugs.

What went wrong

update_components::<C> (gpu_component_array_buffer.rs:130-220) has two reachable panics.

1. A command to a despawned entity. A removed entity's entry is swap-removed: the last entry moves into the gap,
and a command inserts the moved entity's new MeshTag:

if let Some((displaced_entity, displaced_entity_new_tag)) = component_array.remove(&mut buffer, entity) {
    commands.entity(displaced_entity).insert(MeshTag::with_type::<GpuComponentArray<C>>(displaced_entity_new_tag));
}

Sometimes the displaced entity was despawned in the same frame, and its own removal comes later in
RemovedComponents. Then the command fails, and the default error handler panics:

Encountered an error in command `insert<MeshTag>`: Entity despawned: The entity with ID 544v0 is invalid; its index now has generation 1.

Common ways to remove two instances in one frame:

  • despawn a parent whose children carry the component (Children is linked_spawn);
  • despawn a batch of instances, for example on a scene or level change.

Unless the batch happens to go from the highest tag down, this panics.

2. An instance added and removed before the system ran. The component is inserted and then removed (or the entity
despawned) in the same frame, before update_components runs. Its removal is then read from RemovedComponents, but
the entity was never pushed. remove panics:

  • in a debug build: debug_assert!(!self.tag_to_entity.is_empty()) (:305) when the array is empty;
  • otherwise: .expect("Entity must have been present") (:314).

How to reproduce

use bevy::ecs::query::QueryItem;
use bevy::ecs::system::lifetimeless::Read;
use bevy::prelude::*;
use bevy::render::gpu_component_array_buffer::{GpuComponentArrayBuffer, GpuComponentArrayBufferPlugin};
use bevy::render::storage::ShaderBuffer;

#[derive(Component, Clone, Copy)]
struct Data(u32);

impl GpuComponentArrayBuffer for Data {
    type QueryData = Read<Self>;
    type QueryFilter = Changed<Self>;
    type Out = u32;

    fn extract_component(data: QueryItem<'_, '_, Self::QueryData>) -> Option<u32> {
        Some(data.0)
    }
}

fn app() -> App {
    let mut app = App::new();
    app.add_plugins((MinimalPlugins, AssetPlugin::default()))
        .init_asset::<ShaderBuffer>()
        .add_plugins(GpuComponentArrayBufferPlugin::<Data>::default());
    app
}

#[test]
fn two_instances_leaving_in_one_frame() {
    let mut app = app();
    let e: Vec<Entity> = (0..3).map(|i| app.world_mut().spawn(Data(i)).id()).collect();
    app.update(); // tags 0, 1, 2
    app.world_mut().despawn(e[0]); // its slot is filled with e[2]'s entry, and e[2] is re-tagged by a command...
    app.world_mut().despawn(e[2]); // ...but e[2] is gone too
    app.update(); // panics: `insert<MeshTag>`: Entity despawned
}

#[test]
fn an_instance_coming_and_going_in_one_frame() {
    let mut app = app();
    let e = app.world_mut().spawn(Data(0)).id();
    app.world_mut().entity_mut(e).remove::<Data>();
    app.update(); // panics in `GpuComponentArray::remove`
}

Both tests fail on bevy 0.20.0 (run 2026-10-25, a debug build without Bevy's debug feature, which names the command):

two_instances_leaving_in_one_frame: Encountered an error in command `<Enable the debug feature to see the name>`: Entity despawned: The entity with ID 33v0 is invalid; its index now has generation 1.
an_instance_coming_and_going_in_one_frame: gpu_component_array_buffer.rs:305:9: assertion failed: !self.tag_to_entity.is_empty()

Expected

Any set of entities can lose the component in one frame, in any order, and nothing panics.

Lingua principale
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