`EguiContexts::ctx_mut` occasionally causes a panic when closing the app.

[JoJoJet]

thread 'Compute Task Pool (2)' panicked at '`EguiContexts::ctx_mut` was called for an uninitialized context (primary window), make sure your system is run after [`EguiSet::InitContexts`] (or [`EguiStartupSet::InitContexts`] for startup systems)', /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_egui-0.21.0/src/lib.rs:337:14

This panic occurs intermittently when closing my app. Based on the panic message, I'm assuming that the EguiContext for the primary window is being deinitialized before the app fully closes, after which my system is allowed to run, which fails since it's trying to access an uninitialized context.

Backtrace:

stack backtrace:
   0: rust_begin_unwind
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:593:5
   1: core::panicking::panic_fmt
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panicking.rs:67:14
   2: core::panicking::panic_display
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panicking.rs:150:5
   3: core::panicking::panic_str
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panicking.rs:134:5
   4: core::option::expect_failed
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/option.rs:1952:5
   5: core::option::Option<T>::expect
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/option.rs:898:21
   6: bevy_egui::EguiContexts::ctx_mut
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_egui-0.21.0/src/lib.rs:333:47
   7: creator::home::show_loading_screen
             at ./src/home.rs:38:15
   8: core::ops::function::FnMut::call_mut
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/ops/function.rs:166:5
   9: core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/ops/function.rs:294:13
  10: <Func as bevy_ecs::system::function_system::SystemParamFunction<fn(F0,F1,F2,F3) .> Out>>::run::call_inner
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.2/src/system/function_system.rs:622:21
  11: <Func as bevy_ecs::system::function_system::SystemParamFunction<fn(F0,F1,F2,F3) .> Out>>::run
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.2/src/system/function_system.rs:625:17
  12: <bevy_ecs::system::function_system::FunctionSystem<Marker,F> as bevy_ecs::system::system::System>::run_unsafe
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.2/src/system/function_system.rs:460:19
  13: bevy_ecs::schedule::executor::multi_threaded::MultiThreadedExecutor::spawn_system_task::{{closure}}::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.2/src/schedule/executor/multi_threaded.rs:505:26
  14: core::ops::function::FnOnce::call_once
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/ops/function.rs:250:5
  15: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panic/unwind_safe.rs:271:9
  16: std::panicking::try::do_call
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:500:40
  17: __rust_try
  18: std::panicking::try
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:464:19
  19: std::panic::catch_unwind
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panic.rs:142:14
  20: bevy_ecs::schedule::executor::multi_threaded::MultiThreadedExecutor::spawn_system_task::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.11.2/src/schedule/executor/multi_threaded.rs:500:23
  21: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::future::future::Future>::poll
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panic/unwind_safe.rs:296:9
  22: <futures_lite::future::CatchUnwind<F> as core::future::future::Future>::poll::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-lite-1.13.0/src/future.rs:626:42
  23: <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panic/unwind_safe.rs:271:9
  24: std::panicking::try::do_call
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:500:40
  25: __rust_try
  26: std::panicking::try
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:464:19
  27: std::panic::catch_unwind
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panic.rs:142:14
  28: <futures_lite::future::CatchUnwind<F> as core::future::future::Future>::poll
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-lite-1.13.0/src/future.rs:626:9
  29: async_executor::Executor::spawn::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-executor-1.5.1/src/lib.rs:145:20
  30: async_task::raw::RawTask<F,T,S,M>::run
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-task-4.4.0/src/raw.rs:563:17
  31: async_task::runnable::Runnable<M>::run
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-task-4.4.0/src/runnable.rs:782:18
  32: async_executor::Executor::run::{{closure}}::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-executor-1.5.1/src/lib.rs:236:21
  33: <futures_lite::future::Or<F1,F2> as core::future::future::Future>::poll
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-lite-1.13.0/src/future.rs:529:33
  34: async_executor::Executor::run::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-executor-1.5.1/src/lib.rs:243:32
  35: futures_lite::future::block_on::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-lite-1.13.0/src/future.rs:89:27
  36: std::thread::local::LocalKey<T>::try_with
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/thread/local.rs:270:16
  37: std::thread::local::LocalKey<T>::with
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/thread/local.rs:246:9
  38: futures_lite::future::block_on
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-lite-1.13.0/src/future.rs:79:5
  39: bevy_tasks::task_pool::TaskPool::new_internal::{{closure}}::{{closure}}::{{closure}}::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_tasks-0.11.2/src/task_pool.rs:171:37
  40: std::panicking::try::do_call
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:500:40
  41: __rust_try
  42: std::panicking::try
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:464:19
  43: std::panic::catch_unwind
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panic.rs:142:14
  44: bevy_tasks::task_pool::TaskPool::new_internal::{{closure}}::{{closure}}::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_tasks-0.11.2/src/task_pool.rs:165:43
  45: std::thread::local::LocalKey<T>::try_with
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/thread/local.rs:270:16
  46: std::thread::local::LocalKey<T>::with
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/thread/local.rs:246:9
  47: bevy_tasks::task_pool::TaskPool::new_internal::{{closure}}::{{closure}}
             at /home/joseph/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_tasks-0.11.2/src/task_pool.rs:158:25

[hammypants]

I am also seeing this. It looks like bevy_window can/will despawn window entities during the default Update schedule?

[AntiBlueQuirk]

My guess is that the Entity with the PrimaryWindow gets despawned when the window closes, taking the context with it. For me at least, the panic only happens for systems in the PostUpdate schedule. So you can probably fix this by moving your system to the Update schedule.

I think this is kind of a flaw in EguiContexts. Calling EguiContexts::ctx_mut implies that your system can only run if there is a context on the PrimaryWindow, but using EguiContexts as system parameter does not encode this condition to Bevy, since it uses Option<PrimaryWindow> internally. I think there should be a separate PrimaryEguiContext struct for running systems only when the primary contexts exists. Then, EguiContexts::try_ctx_mut() could be added that returns an Option, so that EguiContexts represents "contexts that may or may not exist, including the primary context".

An alternative temporary fix is to add a run condition to your system. This is a little silly, since you're running the same query twice, but it's a one line change until this gets resolved.

// In setup:
app.add_systems(PostUpdate, (
  some_system.run_if(egui_has_primary_context),
))

// Elsewhere:
pub fn egui_has_primary_context(
    query: Query<(With<bevy_egui::EguiContext>, With<bevy_window::PrimaryWindow>)>,
) -> bool {
    !query.is_empty()
}

[rlidwka]

Can we have EguiContexts::try_ctx_mut() specifically to work around this issue?

[vladbat00]

Sounds good, I'll add it in the next release. Meanwhile, you can use the workaround suggested by @AntiBlueQuirk or use the following query, as in the two_windows example: https://github.com/mvlabat/bevy_egui/blob/9dfe7076a96e09fc4d232dd5ddba60472f86ac90/examples/two_windows.rs#L69-L74

[extrawurst]

@mvlabat this did not make it in 0.28, right? i ran into this now after upgrading to 0.28.

[vladbat00]

@extrawurst the try_ctx_mut method is available since 0.27. If your system runs during the PostUpdate schedule, you should use it instead of ctx_mut to avoid the panic.

[extrawurst]

If your system runs during the PostUpdate schedule

but it does not 🤔 its regular Update schedule

[vladbat00]

@extrawurst is it consistent? Do you have a reproducible example?

[extrawurst]

nope not reliable reproducible, just like in the original posters case :(

[extrawurst]

its just the first time now that I got it after updating to bevy 0.14 and bevy_egui 0.28.