X11Clipboard is a data race and should not be `Sync`

[meithecatte]

claim_clipboard_ownership, as well as respond_to_clipboard_request perform unsynchronized accesses on mutable statics:

miniquad/src/native/linux_x11/clipboard.rs

Lines 130 to 149 in cccea68

	// Next message for clipboard request
	static mut MESSAGE: Option<String> = None;
	/// Claim that our app is X11 clipboard owner
	/// Now when some other linux app will ask X11 for clipboard content - it will be redirected to our app
	unsafe fn claim_clipboard_ownership(
	libx11: &mut LibX11,
	display: *mut Display,
	window: Window,
	message: String,
	) {
	(libx11.XSetSelectionOwner)(
	display,
	libx11.extensions.clipboard,
	window,
	CurrentTime as Time,
	);
	MESSAGE = Some(message);
	}

This means that X11Clipboard::set is not threadsafe:

miniquad/src/native/linux_x11/clipboard.rs

Lines 229 to 242 in cccea68

	impl crate::native::Clipboard for X11Clipboard {
	fn get(&mut self) -> Option<String> {
	let utf8_string = self.libx11.extensions.utf8_string;
	let bytes =
	unsafe { get_clipboard(&mut self.libx11, self.display, self.window, utf8_string)? };
	Some(std::str::from_utf8(&bytes).ok()?.to_string())
	}
	fn set(&mut self, data: &str) {
	unsafe {
	claim_clipboard_ownership(&mut self.libx11, self.display, self.window, data.to_owned());
	};
	}
	}

Nevertheless, X11Clipboard explicitly implements Send and Sync:

miniquad/src/native/linux_x11/clipboard.rs

Lines 217 to 218 in cccea68

	unsafe impl Send for X11Clipboard {}
	unsafe impl Sync for X11Clipboard {}

Moreover, removing the Sync impl would not be enough – creating two separate X11Clipboard instances can cause UB even without this impl.

[meithecatte]

Looks like the same can be said about WaylandClipboard.

[bolphen]

In practice this is ok since globally there is only one instance of X11Clipboard guarded behind a Mutex. Although I agree that I don't like the static mut either -- imo the clipboard object should own the string.

[meithecatte]

If it's behind a Mutex, it definitely doesn't need the Sync impl – Send should be enough.