[Big endian] Linux - select_xinput_events is not endian agnostic

[msirringhaus]

Trying the examples on a Linux machine s390x (big endian), I got errors upon cargo run --example window:

2021-03-11 16:14:17,387 ERROR [winit::platform_impl::platform] X11 error: XError {
    description: "BadValue (integer parameter out of range for operation)",
    error_code: 2,
    request_code: 132,
    minor_code: 46,
}

This is, I think, because select_xinput_events() deconstructs mask: i32 to an array of bytes (see here). This leads to wrong results on a big endian machine.
Putting mask: &mask.to_le_bytes() as *const _ as *mut c_uchar, the examples seem to work fine. But this is probably a very hacky solution, as I haven't thought very hard about possible consequences.

[j-piecuch]

It appears (here and here) that in the case of mask the X server assumes a little-endian representation (i.e. bits 0-7 are in the first byte, bits 8-15 in the second byte, and so on). Your fix seems correct to me.

[notgull]

As of #2825 this code has been replaced, I believe that x11rb automatically translates the endianness into its proper form. If you have the opportunity can you test this again?

winit/src/platform_impl/linux/x11/util/input.rs

Lines 39 to 55 in f975852

	impl XConnection {
	pub fn select_xinput_events(
	&self,
	window: xproto::Window,
	device_id: u16,
	mask: xinput::XIEventMask,
	) -> Result<VoidCookie<'_>, X11Error> {
	self.xcb_connection()
	.xinput_xi_select_events(
	window,
	&[xinput::EventMask {
	deviceid: device_id,
	mask: vec![mask],
	}],
	)
	.map_err(Into::into)
	}

[msirringhaus]

cargo run --example window doesn't crash anymore, when run from main!

I'm not 100% sure, if key events work as expected, though.
E.g. running cargo run --example theme, I see no output when pressing keys. After added a few println!() for the default-cases, I only see NewEvents(WaitCancelled {..}), followed by AboutToWait, whenever I hit a key or move the mouse, etc.

But this might very well be a problem with VNC. I think, we can close this.