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https://github.com/bevyengine/bevy
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af20cad830
# Objective
Sometimes, the unwraps in `entity_mut` could fail here, if the entity was despawned *before* this command was applied.
The simplest case involves two command buffers:
```rust
use bevy::prelude::*;
fn b(mut commands1: Commands, mut commands2: Commands) {
let id = commands2.spawn().insert_bundle(()).id();
commands1.entity(id).despawn();
}
fn main() {
App::build().add_system(b.system()).run();
}
```
However, a more complicated version arises in the case of ambiguity:
```rust
use std::time::Duration;
use bevy::{app::ScheduleRunnerPlugin, prelude::*};
use rand::Rng;
fn cleanup(mut e: ResMut<Option<Entity>>) {
*e = None;
}
fn sleep_randomly() {
let mut rng = rand::thread_rng();
std:🧵:sleep(Duration::from_millis(rng.gen_range(0..50)));
}
fn spawn(mut commands: Commands, mut e: ResMut<Option<Entity>>) {
*e = Some(commands.spawn().insert_bundle(()).id());
}
fn despawn(mut commands: Commands, e: Res<Option<Entity>>) {
let mut rng = rand::thread_rng();
std:🧵:sleep(Duration::from_millis(rng.gen_range(0..50)));
if let Some(e) = *e {
commands.entity(e).despawn();
}
}
fn main() {
App::build()
.add_system(cleanup.system().label("cleanup"))
.add_system(sleep_randomly.system().label("before_despawn"))
.add_system(despawn.system().after("cleanup").after("before_despawn"))
.add_system(sleep_randomly.system().label("before_spawn"))
.add_system(spawn.system().after("cleanup").after("before_spawn"))
.insert_resource(None::<Entity>)
.add_plugin(ScheduleRunnerPlugin::default())
.run();
}
```
In the cases where this example crashes, it's because `despawn` was ordered before `spawn` in the topological ordering of systems (which determines when buffers are applied). However, `despawn` actually ran *after* `spawn`, because these systems are ambiguous, so the jiggles in the sleeping time triggered a case where this works.
## Solution
- Give a better error message
|
||
|---|---|---|
| .. | ||
| command_queue.rs | ||
| mod.rs | ||