Panic "RefCell already borrowed" when closing window immediately after opening

[kumikaya]

Bevy version and features

0.17.1

What you did

1. Ran cargo run --example mesh2d
2. Closed the window immediately after it appeared

What went wrong

thread 'main' panicked at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\std\src\thread\local.rs:559:38:
RefCell already borrowed
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/1159e78c4747b02ef996e55082b704c09b970588/library\std\src\panicking.rs:697
   1: core::panicking::panic_fmt
             at /rustc/1159e78c4747b02ef996e55082b704c09b970588/library\core\src\panicking.rs:75
   2: core::cell::panic_already_borrowed::do_panic::runtime
             at /rustc/1159e78c4747b02ef996e55082b704c09b970588/library\core\src\panic.rs:218
   3: core::cell::panic_already_borrowed
             at /rustc/1159e78c4747b02ef996e55082b704c09b970588/library\core\src\panic.rs:223
   4: core::cell::RefCell<bevy_winit::winit_windows::WinitWindows>::borrow_mut
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\cell.rs:1089
   5: std::thread::local::impl$6::with_borrow_mut::closure$0<bevy_winit::winit_windows::WinitWindows,bevy_winit::system::despawn_windows::closure_env$0,tuple$<> >
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\thread\local.rs:559
   6: std::thread::local::LocalKey<core::cell::RefCell<bevy_winit::winit_windows::WinitWindows> >::try_with<core::cell::RefCell<bevy_winit::winit_windows::WinitWindows>,std::thread::local::impl$6::with_borrow_mut::closure_env$0<bevy_winit::winit_windows::WinitWi
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\thread\local.rs:315
   7: std::thread::local::LocalKey<core::cell::RefCell<bevy_winit::winit_windows::WinitWindows> >::with<core::cell::RefCell<bevy_winit::winit_windows::WinitWindows>,std::thread::local::impl$6::with_borrow_mut::closure_env$0<bevy_winit::winit_windows::WinitWindow
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\thread\local.rs:279
   8: std::thread::local::LocalKey<core::cell::RefCell<bevy_winit::winit_windows::WinitWindows> >::with_borrow_mut<bevy_winit::winit_windows::WinitWindows,bevy_winit::system::despawn_windows::closure_env$0,tuple$<> >
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std\src\thread\local.rs:559
   9: bevy_winit::system::despawn_windows
             at .\crates\bevy_winit\src\system.rs:262
  10: core::ops::function::FnMut::call_mut<void (*)(bevy_ecs::system::query::Query<bevy_ecs::entity::Entity,bevy_ecs::query::filter::With<bevy_window::window::ClosingWindow> >,bevy_ecs::lifecycle::RemovedComponents<bevy_window::window::Window>,bevy_ecs::system::
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:168
  11: core::ops::function::impls::impl$3::call_mut<tuple$<bevy_ecs::system::query::Query<bevy_ecs::entity::Entity,bevy_ecs::query::filter::With<bevy_window::window::ClosingWindow> >,bevy_ecs::lifecycle::RemovedComponents<bevy_window::window::Window>,bevy_ecs::sy
             at C:\Users\kumikaya\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:301
  12: bevy_ecs::system::function_system::impl$48::run::call_inner<tuple$<>,bevy_ecs::system::query::Query<bevy_ecs::entity::Entity,bevy_ecs::query::filter::With<bevy_window::window::ClosingWindow> >,bevy_ecs::lifecycle::RemovedComponents<bevy_window::window::Win
             at .\crates\bevy_ecs\src\system\function_system.rs:926
  13: bevy_ecs::system::function_system::impl$48::run<tuple$<>,void (*)(bevy_ecs::system::query::Query<bevy_ecs::entity::Entity,bevy_ecs::query::filter::With<bevy_window::window::ClosingWindow> >,bevy_ecs::lifecycle::RemovedComponents<bevy_window::window::Window
             at .\crates\bevy_ecs\src\system\function_system.rs:929
  14: bevy_ecs::system::function_system::impl$12::run_unsafe<void (*)(bevy_ecs::system::query::Query<bevy_ecs::entity::Entity,bevy_ecs::query::filter::With<bevy_window::window::ClosingWindow> >,bevy_ecs::lifecycle::RemovedComponents<bevy_window::window::Window>,
             at .\crates\bevy_ecs\src\system\function_system.rs:711
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
Encountered a panic in system `bevy_winit::system::despawn_windows`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
error: process didn't exit successfully: `D:\build\debug\examples\mesh2d.exe` (exit code: 101)

[alice-i-cecile]

What OS are you on?

[kumikaya]

Windows 11

[ejrh]

I get this as well (Windows 11, too). After updating to Bevy 0.17 (and updating a few other related dependencies like bevy_egui_inspector to 0.34). Running on the multithreaded executor.

I have seen it alternately in:

• bevy::winit::system::despawn_windows (which runs in Last schedule)
• bevy_egui::input::process_ime_system (which runs in PostUpdate)

In both places, at the line like:

WINIT_WINDOWS.with_borrow_mut(|winit_windows| { ... })

Maybe something to do with the change to using thread locals?

Both those places are using the new NonSendMarker system param though. Not sure if there are any other systems that use the variable in the same schedules as those systems. But it is also used quite extensively in the winit event loop.

If I comment out my DebugPlugin (which is where the egui plugins are used), the application starts normally. Otherwise, I always get this error immediately when starting.

[ejrh]

Looking at one stack trace I noticed that WINIT_WINDOWS is borrowed in about_to_wait, and the closure ends up calling (via redraw_requested) the process_ime_system system that tries to borrow it again?

That's in a Windows-only block, too.

winit_windows_borrow.txt

Maybe the call to redraw_requested could be moved outside the borrow, since it doesn't directly use anything from it except the decision of whether to call it.

I have a pull request which I will submit... my first one 😟 .

[ejrh]

Hello @kumikaya, could you test my fix? It's in my repo fork: https://github.com/ejrh/bevy/tree/winit_windows_borrow

Let me know if you need help.

[ejrh]

@joshua-holmes might be good to get your review of my analysis and PR #21338.

[kumikaya]

@ejrh , I tested your fix on your fork branch and the bug no longer occurs.