Consider making egui::Context !Send+!Sync

[emilk]

Prompted by #1398

Currently Context and Shape are both Send+Sync. This means Shape::Callback can only capture things that are Send+Sync, even though almost all users will be running their UI code on the the paint thread.

We could consider making Shape: !Send + !Sync, but that would mean egui::Context could not be Send+Sync either (because the egui context stores shapes). This could actually be fine. egui::Context should only be used from a background thread for calling request_repaint and allocating textures. These could be made separate parts of the egui Context, so that one would do:

let repaint_signal = egui_ctx.repaint_signal();
let tex_mngr = egui_ctx.tex_mngr();
std::thread::spawn(move || {
    // We can use repaint_signal and tex_mngr here,
    // but NOT `egui_ctx`.
}):

Making Context: !Sync would also solve deadlocks discussed in #1379 and #1380.

The downside of this is that it would stop users from running their UI code in a separate thread from their main window/paint thread.

Related:

• Changes to the repaint signal causes problems with multi-threaded code #1379
• Fix code that could lead to a possible deadlock. #1380
• Put repaint callback and tex manager behind separate mutexes #1389

[Pjottos]

I'm looking into this, removing locks entirely from the egui crate. It's quite a challenge to both keep the API simple and refactor the way multiple (mutable) references are kept to the context, but I think eliminating the deadlocks is worth it since it's a pretty confusing issue.

[emilk]

@Pjottos thank you so much for working on this!

[Pjottos]

I got the demo to compile and run without a lock on the context. Seems to be working as expected but the performance actually regressed which might be because of extra copying? Should be fixable. Can't reproduce this anymore, frame times seem a bit lower in the demo app with all windows open.

[melody-rs]

With wgpu marking its types !Send + !Sync on wasm this means using egui_wgpu::CallbackTrait is still really, really difficult, even with the API improvments in 0.23.0. Making Context !Send + !Sync (at least on wasm) would go a long way to alleviate that imho.

At the moment, if you want to access any wgpu types in your custom callbacks you can't store them directly in a struct impl'ing egui_wgpu::CallbackTrait, you need to instead manually append stuff to egui_wgpu::Renderer::callback_resources, and getting a hold of an egui_wgpu::Renderer isn't very easy.

By making Context !Send + !Sync egui_wgpu::CallbackTrait would no longer need to be Send + Sync either and it'd be possible to directly store wgpu types in your callback structs. It might even be possible at that point to drop callback_resources entirely?