RemovedComponents can miss removals in 0.14.0-rc2

[chrisjuchem]

Bevy version

0.14.0-rc.2

Reproduction Example

use bevy::core::{FrameCount, FrameCountPlugin};
use bevy::prelude::*;

#[derive(Component)]
struct Foo;

fn main() {
    let mut app = App::new();
    app.add_plugins(FrameCountPlugin);
    app.add_systems(Startup, |mut commands: Commands| {
        for _ in 0..100 {
            commands.spawn(Foo);
        }
    });

    app.add_systems(First, |counter: Res<FrameCount>| {
        println!("Frame {}:", counter.0)
    });

    fn detector_system(mut removals: RemovedComponents<Foo>, foos: Query<Entity, With<Foo>>) {
        for e in removals.read() {
            println!("  Detected removed Foo component for {e:?}")
        }
        println!("  Total Foos: {}", foos.iter().count())
    }
    fn deleter_system(foos: Query<Entity, With<Foo>>, mut commands: Commands) {
        foos.iter().next().map(|e| {
            commands.entity(e).remove::<Foo>();
        });
    }
    app.add_systems(Update, (detector_system, deleter_system).chain());

    app.update();
    app.update();
    app.update();
    app.update();
}

Output when using bevy 0.13.2

Frame 0:
  Total Foos: 100
Frame 1:
  Detected removed Foo component for 0v1
  Total Foos: 99
Frame 2:
  Detected removed Foo component for 99v1
  Total Foos: 98
Frame 3:
  Detected removed Foo component for 98v1
  Total Foos: 97

Output when using bevy `0.14.0-rc.2

Frame 0:
  Total Foos: 100
Frame 1:
  Total Foos: 99
Frame 2:
  Total Foos: 98
Frame 3:
  Total Foos: 97

Additional information

Swapping the order of detector_system and deleter_system results in the events showing up as expected.

Replacing remove::<Foo> with .despawn() does not change the outcome.

[alice-i-cecile]

I believe this is a genuine bug: we changed the backing of RemovedComponents to use double buffered events at some point. It's the weekend right now but bother me if you can't find the PR.

[chrisjuchem]

Looks like that was back in 0.10 #5680

[alice-i-cecile]

Great! Can you turn your example into a test and try to bisect the problem?