Issue with custom WorldQuery with Changed and mutable

[mattzque]

Bevy version

0.11.3

What you did

I tried to cleanup a system query, I made a full example here:

use bevy::{prelude::*, ecs::query::WorldQuery};

#[derive(Component)]
struct MyComponent;

fn my_system0(mut query: Query<&mut MyComponent, Changed<MyComponent>>) {
    for mut component in query.iter_mut() {
        // do something with component
    }
}

// so to clean it up I extract the query into a WorldQuery:

#[derive(WorldQuery)]
#[world_query(mutable)]
struct MyQuery1 {
    entity: Entity,
    component: &'static MyComponent,
    _a: Changed<MyComponent>,
}

fn my_system1(mut query: Query<MyQuery1>) {
    for mut component in query.iter_mut() {
        // do something with component
    }
}

// which works, however, this (with &mut) crashes with a runtime error:

#[derive(WorldQuery)]
#[world_query(mutable)]
struct MyQuery2 {
    entity: Entity,
    component: &'static mut MyComponent, // <- added mut
    _a: Changed<MyComponent>,
}

fn my_system2(mut query: Query<MyQuery2>) {
    for mut component in query.iter_mut() {
        // do something with component
    }
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, (
            my_system0,
            my_system1,
            my_system2, // <- runtime crash
        ))
        .run();
}

What went wrong

The error on my_system2 / MyQuery2 was '$state_name<bevy_minimal::MyComponent> conflicts with a previous access in this query. Shared access cannot coincide with exclusive access.'.

Backtrace:

thread 'main' panicked at '$state_name<bevy_minimal::MyComponent> conflicts with a previous access in this query. Shared access cannot coincide with exclusive access.', /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/query/filter.rs:600:1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d5c2e9c342b358556da91d61ed4133f6f50fc0c3/library/std/src/panicking.rs:593:5
   1: core::panicking::panic_fmt
             at /rustc/d5c2e9c342b358556da91d61ed4133f6f50fc0c3/library/core/src/panicking.rs:67:14
   2: <bevy_ecs::query::filter::Changed<T> as bevy_ecs::query::fetch::WorldQuery>::update_component_access
             at /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/query/filter.rs:536:21
   3: bevy_minimal::_::<impl bevy_ecs::query::fetch::WorldQuery for bevy_minimal::MyQuery2>::update_component_access
             at ./src/main.rs:26:10
   4: bevy_ecs::query::state::QueryState<Q,F>::new
             at /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/query/state.rs:102:9
   5: <bevy_ecs::system::query::Query<Q,F> as bevy_ecs::system::system_param::SystemParam>::init_state
             at /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/system/system_param.rs:169:21
   6: <(P0,) as bevy_ecs::system::system_param::SystemParam>::init_state
             at /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/system/system_param.rs:1384:21
   7: <bevy_ecs::system::function_system::FunctionSystem<Marker,F> as bevy_ecs::system::system::System>::initialize
             at /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/system/function_system.rs:483:33
   8: bevy_ecs::schedule::schedule::ScheduleGraph::initialize
             at /home/apoc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.3/src/schedule/schedule.rs:849:21
   9: bevy_ecs::schedule::schedule::Schedule::initialize
[...]

This only happens when I use "&mut MyComponent" with a "Changed", otherwise WorldQuery works as expected.

This specific case is not mentioned in the examples: https://github.com/bevyengine/bevy/blob/main/examples/ecs/custom_query_param.rs

As an aside, I found those examples in custom_query_param.rs really difficult to understand but perhaps that's just me, at first I thought I was doing something wrong, or perhaps I am?

[rparrett]

That seems like a super confusing error.

But I think the root of your problem is that there are two generic arguments in Query. Query<Q, F> where Q is the "query fetch" and F is the "filter."

Changed is not valid in the fetch position of the query (does not implement ReadOnlyWorldQuery.)

I think the best you can do here is derive a second

#[derive(WorldQuery)]
struct MyQueryFilter {
    changed: Changed<MyComponent>
}

and do

query: Query<MyQuery, MyQueryFilter>

It also seems like you might want SystemParam instead.

[alice-i-cecile]

This should be fixed with #9918 as a result.

[mattzque]

thank you, also thanks for the tip with SystemParam that is great to cleaning up my code even more

[james7132]

Changed directly reads the change ticks of each component. &mut T will mutate them. This results in aliased access within a single query. Even in a filter position, this should panic due to the undefined behavior it would otherwise introduce, and it correctly does that. Even with #9918, this will still happen, and it is working as intended.