Repaint signal cannot be shared between threads

[DRvader]

Hey,

I've been having problems sharing the repaint signal between threads. It seems the problem is that Arc requires that it wraps types that implement send and sync. Though I am new enough to rust that it could also be my fault.

The following code (in the ui function) will cause the error.

let repaint_signal = integration_context.repaint_signal.clone();
let thread = std::thread::spawn(|| {
  repaint_signal.request_repaint();
});
thread.join();

I'm happy to fix this issue if the problem wasn't caused by my own lack of understanding.

[emilk]

Hi!

RepaintSignal is not meant to be shared between threads (which would require Sync), instead it is meant to be cloned (as you do) and then moved to the other thread (requiring just Send). You are almost there with your code, you just lack a move:

let thread = std::thread::spawn(move || {
  repaint_signal.request_repaint();
});

That should work!

[DRvader]

Thanks for getting back to me 😄

I tried adding the move as you suggested but that didn't seem to fix the problem.

The error message that cargo spits out seemed to say that Arc requires both Send and Sync to be implemented for the type it contains.

This seems to agree with the source for Arc that I found in the rust-doc

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}

I don't understand enough about the internals of rust to be even remotely confident that I didn't make an unrelated mistake or messed up my build environment somehow so please let me know if I've gone wrong somewhere.

I've also attached the error message and the file I used to generate it so you can let me know if I'm interpreting the output correctly.

error[E0277]: `dyn RepaintSignal` cannot be shared between threads safely
   --> src\main.rs:12:22
    |
12  |         let thread = std::thread::spawn(move || {
    |                      ^^^^^^^^^^^^^^^^^^ `dyn RepaintSignal` cannot be shared between threads safely
    | 
   ::: C:\Users\micro\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\std\src\thread\mod.rs:607:8
    |
607 |     F: Send + 'static,
    |        ---- required by this bound in `spawn`
    |
    = help: the trait `Sync` is not implemented for `dyn RepaintSignal`
    = note: required because of the requirements on the impl of `Send` for `Arc<dyn RepaintSignal>`
    = note: required because it appears within the type `[closure@src\main.rs:12:41: 14:10]`

error: aborting due to previous error

use egui::{app::IntegrationContext, CtxRef};

struct App {}

impl egui::app::App for App {
    fn name(&self) -> &str {
        "Name"
    }

    fn ui(&mut self, ctx: &CtxRef, integration_context: &mut IntegrationContext) {
        let repaint_signal = integration_context.repaint_signal.clone();
        let thread = std::thread::spawn(move || {
            repaint_signal.request_repaint();
        });
        thread.join();
    }
}

fn main() {
    egui_glium::run(Box::new(App {}))
}

[emilk]

You are of course correct! I will come with a fix shortly. And thanks for your report!

[emilk]

You should now be able to continue if you add the following patch to your Cargo.toml:

[patch.crates-io]
egui = { git = "https://github.com/emilk/egui", rev = "958fc2753aef7e35b615a13c54324860ee7f1427" } # 2020-12-27
egui_glium = { git = "https://github.com/emilk/egui", rev = "958fc2753aef7e35b615a13c54324860ee7f1427" } # 2020-12-27
egui_web = { git = "https://github.com/emilk/egui", rev = "958fc2753aef7e35b615a13c54324860ee7f1427" } # 2020-12-27

Good luck!

[DRvader]

Great! Thanks for the help 😁