Pipelined rendering thread uses panic for control flow

[bonsairobo]

Bevy version

0.10

What you did

This just happens sometimes when my app exits.

What went wrong

The app prints a panic message (non-deterministically) when you close the window.

2023-06-23T06:03:07.402509Z  INFO bevy_app:winit event_handler:main app:schedule{name=Outer}:system{name="bevy_app::CoreSchedule::outer_loop"}:schedule{name=Main}:multithreaded executor:system{name="bevy_winit::system::despawn_window"}: bevy_winit::system: Closing window 0v0

thread 'Compute Task Pool (6)' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', /home/duncan/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_render-0.10.1/src/pipelined_rendering.rs:111:77
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
   0: bevy_render::pipelined_rendering::render thread
             at /home/duncan/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_render-0.10.1/src/pipelined_rendering.rs:105
thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', /home/duncan/.cargo/registry/src/github.com-1ecc6299db9ec823/bevy_tasks-0.10.1/src/task_pool.rs:376:49

Additional information

I'm guessing the error is non-deterministic because there is a race between the render thread panicking and the app shutting down (killing the thread). But it's clear from reading this code that the render thread can only exit via panicking:

bevy/crates/bevy_render/src/pipelined_rendering.rs

Lines 102 to 121 in 29f7293

	std::thread::spawn(move || {
	#[cfg(feature = "trace")]
	let _span = bevy_utils::tracing::info_span!("render thread").entered();
	loop {
	// run a scope here to allow main world to use this thread while it's waiting for the render app
	let mut render_app = ComputeTaskPool::get()
	.scope(|s| {
	s.spawn(async { app_to_render_receiver.recv().await.unwrap() });
	})
	.pop()
	.unwrap();
	#[cfg(feature = "trace")]
	let _sub_app_span =
	bevy_utils::tracing::info_span!("sub app", name = ?RenderApp).entered();
	render_app.run();
	render_to_app_sender.send_blocking(render_app).unwrap();
	}
	});