Automatic backend selection fails under Ubunty on XOrg

[radoye]

Automatic selection fails when logged in using "Ubuntu on XOrg"; moving to "Wayland" makes this work. Forcing x11 backend in code works, but not through an envvar.

cargo build && WINIT_UNIX_BACKEND=x11 cargo run
warning: unused import: `EventLoopBuilder`
 --> src/main.rs:2:42
  |
2 |     event_loop::{ControlFlow, EventLoop, EventLoopBuilder},
  |                                          ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: `winit_repro` (bin "winit_repro") generated 1 warning (run `cargo fix --bin "winit_repro"` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.05s
warning: unused import: `EventLoopBuilder`
 --> src/main.rs:2:42
  |
2 |     event_loop::{ControlFlow, EventLoop, EventLoopBuilder},
  |                                          ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: `winit_repro` (bin "winit_repro") generated 1 warning (run `cargo fix --bin "winit_repro"` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.05s
     Running `target/debug/winit_repro`
WINIT_UNIX_BACKEND = Ok("x11")

thread 'main' panicked at src/main.rs:11:39:
called `Result::unwrap()` on an `Err` value: Os(OsError { line: 89, file: "/home/rrs/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/winit-0.30.12/src/platform_impl/linux/wayland/event_loop/mod.rs", error: WaylandError(Connection(NoCompositor)) })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
rrs@cypress:~/docks/dev/winit_repro$ 

Here's the code:

use winit::{
    event_loop::{ControlFlow, EventLoop, EventLoopBuilder}, platform::x11::EventLoopBuilderExtX11,
};

fn main() {
    println!(
        "WINIT_UNIX_BACKEND = {:?}",
        std::env::var("WINIT_UNIX_BACKEND")
    );

    //let event_loop = EventLoop::new().unwrap();
    let event_loop = EventLoopBuilder::new()
        .with_x11()
        .build()
        .expect("Failed to build event loop");
    event_loop.set_control_flow(ControlFlow::Poll);

}

And the deps

[package]
name = "winit_repro"
version = "0.1.0"
edition = "2024"

[dependencies]
winit = "0.30.12"

uname -a
Linux cypress 6.14.0-24-generic #24~24.04.3-Ubuntu SMP PREEMPT_DYNAMIC Mon Jul  7 16:39:17 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
rrs@cypress:~/docks/dev/winit_repro$ 
apt list libwayland*
Listing... Done
libwayland-bin-dbgsym/noble 1.22.0-2.1build1 amd64
libwayland-bin/noble,now 1.22.0-2.1build1 amd64 [installed,automatic]
libwayland-client++1-dbgsym/noble 1.0.0-5ubuntu2 amd64
libwayland-client++1/noble 1.0.0-5ubuntu2 amd64
libwayland-client-extra++1-dbgsym/noble 1.0.0-5ubuntu2 amd64
libwayland-client-extra++1/noble 1.0.0-5ubuntu2 amd64
libwayland-client0-dbgsym/noble,now 1.22.0-2.1build1 amd64 [installed]
libwayland-client0/noble,now 1.22.0-2.1build1 amd64 [installed]
libwayland-cursor++1-dbgsym/noble 1.0.0-5ubuntu2 amd64
libwayland-cursor++1/noble 1.0.0-5ubuntu2 amd64
libwayland-cursor0-dbgsym/noble 1.22.0-2.1build1 amd64
libwayland-cursor0/noble,now 1.22.0-2.1build1 amd64 [installed,automatic]
libwayland-dev/noble,now 1.22.0-2.1build1 amd64 [installed]
libwayland-doc/noble 1.22.0-2.1build1 all
libwayland-egl++1-dbgsym/noble 1.0.0-5ubuntu2 amd64
libwayland-egl++1/noble 1.0.0-5ubuntu2 amd64
libwayland-egl-backend-dev/noble 1.22.0-2.1build1 amd64
libwayland-egl1-dbgsym/noble 1.22.0-2.1build1 amd64
libwayland-egl1/noble,now 1.22.0-2.1build1 amd64 [installed,automatic]
libwayland-server++1-dbgsym/noble 1.0.0-5ubuntu2 amd64
libwayland-server++1/noble 1.0.0-5ubuntu2 amd64
libwayland-server0-dbgsym/noble 1.22.0-2.1build1 amd64
libwayland-server0/noble,now 1.22.0-2.1build1 amd64 [installed,automatic]
rrs@cypress:~/docks/dev/winit_repro$ 

[kchibisov]

If you want to force a backend you can pass an option for that. IIRC there could be an issue if you have WAYLAND_* var set, but then it failed to create it with that variable.

[radoye]

Thanks for the quick response, but I don't think I understand. To clarify: I do not want to force a backend. The version of the code with let event_loop = EventLoop::new().unwrap(); fails on XOrg. The only reason I'm mentioning forcing is to provide information that the backend exists and can work.

So, is this a bug? Or am I misunderstanding something? Thanks!

[kchibisov]

It seems like you have WAYLAND_DISPLAY or WAYLAND_SOCKET set when you don't have wayland at all according to your log.

In that case as I set, it won't fallback indeed, because it assumes that you have Wayland.