Panic when using spawn command within Trigger<OnRemove> observer

[swejk]

Bevy version

0.14.2

What you did

Run this code:

#[derive(Component)]
struct TestRemoveComp;

#[derive(Component)]
struct CoundownTimer {
    pub start: Instant,
    pub duration: Duration,
}

#[derive(Component)]
struct TestSpawnComp;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, |mut commands: Commands| {
            commands.spawn((
                TestRemoveComp,
                CoundownTimer {
                    start: Instant::now(),
                    duration: Duration::from_secs_f32(1.),
                },
            ));
        })
        .add_systems(
            Update,
            |mut commands: Commands, query: Query<(Entity, &CoundownTimer)>| {
                for (entity, timer) in &mut query.iter() {
                    if timer.start.elapsed() > timer.duration {
                        commands.entity(entity).despawn();
                    }
                }
            },
        )
        .observe(
            |_: Trigger<OnRemove, TestRemoveComp>, mut commands: Commands| {
                commands.spawn(TestSpawnComp);
            },
        )
        .run();
    }

What went wrong

App panicked.
I am noticing this behaviour only when Commands::spawn is issued within Trigger<OnRemove> observer. Commands::insert and Commands::add works correctly in this context.

thread 'main' panicked at C:\Users\User\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\entity\mod.rs:582:9:
flush() needs to be called before this operation is legal
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic when applying buffers for system `test::main::{{closure}}`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!

Additional information

As a workaround I am spawning from dedicated Command instead of spawning directly.

...
 .observe(
            |_: Trigger<OnRemove, TestRemoveComp>, mut commands: Commands| {
                commands.add(TriggerOnRemoveSpawnWorkaround {bundle: TestSpawnComp});
            },
        )

pub struct TriggerOnRemoveSpawnWorkaround<B: Bundle> {
    pub bundle: B,
}

impl <B> Command for TriggerOnRemoveSpawnWorkaround<B> 
where B: Bundle {
    fn apply(self, world: &mut bevy::prelude::World) {
        world.spawn(self.bundle);
    }
}

[alice-i-cecile]

Can you reproduce this on main? #15398 and/or #15405 may have fixed this.

[chompaa]

I ran the following code on main and the commit before 3d0f240.

Code

use std::time::{Duration, Instant};

use bevy::prelude::*;

#[derive(Component)]
struct TestRemoveComp;

#[derive(Component)]
struct CountdownTimer {
    pub start: Instant,
    pub duration: Duration,
}

#[derive(Component)]
struct TestSpawnComp;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, |mut commands: Commands| {
            commands.spawn((
                TestRemoveComp,
                CountdownTimer {
                    start: Instant::now(),
                    duration: Duration::from_secs_f32(1.),
                },
            ));
        })
        .add_systems(
            Update,
            |mut commands: Commands, query: Query<(Entity, &CountdownTimer)>| {
                for (entity, timer) in &query {
                    if timer.start.elapsed() > timer.duration {
                        commands.entity(entity).despawn();
                    }
                }
            },
        )
        .observe(
            |_: Trigger<OnRemove, TestRemoveComp>, mut commands: Commands| {
                commands.spawn(TestSpawnComp);
            },
        )
        .observe(|_: Trigger<OnAdd, TestRemoveComp>| {
            info!("It works!");
        })
        .run();
}

The panic does not occur on main, but does before 3d0f240, so I believe this to be solved.