Add method to hook xlib error handler

This should help glutin to handle errors coming from GLX
and offer multithreading support in a safe way.

Fixes #2378.

CHANGELOG.md

@@ -73,6 +73,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - On Web, add `with_prevent_default` and `with_focusable` to `WindowBuilderExtWebSys` to control whether events should be propagated.
 - On Windows, fix focus events being sent to inactive windows.
 - **Breaking**, update `raw-window-handle` to `v0.5` and implement `HasRawDisplayHandle` for `Window` and `EventLoopWindowTarget`.
+- On X11, add function `register_xlib_error_hook` into `winit::platform::unix` to subscribe for errors comming from Xlib.
 
 # 0.26.1 (2022-01-05)
 

src/platform/unix.rs

@@ -19,7 +19,7 @@ use crate::{
 #[cfg(feature = "x11")]
 use crate::dpi::Size;
 #[cfg(feature = "x11")]
-use crate::platform_impl::x11::{ffi::XVisualInfo, XConnection};
+use crate::platform_impl::{x11::ffi::XVisualInfo, x11::XConnection, XLIB_ERROR_HOOKS};
 use crate::platform_impl::{
     ApplicationName, Backend, EventLoopWindowTarget as LinuxEventLoopWindowTarget,
     Window as LinuxWindow,
@@ -35,6 +35,29 @@ pub use crate::platform_impl::{x11::util::WindowType as XWindowType, XNotSupport
 #[cfg(feature = "wayland")]
 pub use crate::window::Theme;
 
+/// The first argument in the provided hook will be the pointer to XDisplay
+/// and the second one the pointer to XError. The return `bool` is an indicator
+/// whether error was handled by the callback.
+#[cfg(feature = "x11")]
+pub type XlibErrorHook =
+    Box<dyn Fn(*mut std::ffi::c_void, *mut std::ffi::c_void) -> bool + Send + Sync>;
+
+/// Hook to winit's xlib error handling callback.
+///
+/// This method is provided as a safe way to handle the errors comming from X11 when using xlib
+/// in external crates, like glutin for GLX access. Trying to handle errors by speculating with
+/// `XSetErrorHandler` is [`unsafe`].
+///
+/// [`unsafe`]: https://www.remlab.net/op/xlib.shtml
+#[inline]
+#[cfg(feature = "x11")]
+pub fn register_xlib_error_hook(hook: XlibErrorHook) {
+    // Append new hook.
+    unsafe {
+        XLIB_ERROR_HOOKS.lock().push(hook);
+    }
+}
+
 /// Additional methods on [`EventLoopWindowTarget`] that are specific to Unix.
 pub trait EventLoopWindowTargetExtUnix {
     /// True if the [`EventLoopWindowTarget`] uses Wayland.

src/platform_impl/linux/mod.rs

@@ -26,6 +26,8 @@ use raw_window_handle::{RawDisplayHandle, RawWindowHandle};
 pub use self::x11::XNotSupported;
 #[cfg(feature = "x11")]
 use self::x11::{ffi::XVisualInfo, util::WindowType as XWindowType, XConnection, XError};
+#[cfg(feature = "x11")]
+use crate::platform::unix::XlibErrorHook;
 #[cfg(feature = "wayland")]
 use crate::window::Theme;
 use crate::{
@@ -583,13 +585,23 @@ impl Window {
     }
 }
 
+/// Hooks for X11 errors.
+#[cfg(feature = "x11")]
+pub(crate) static mut XLIB_ERROR_HOOKS: Mutex<Vec<XlibErrorHook>> = Mutex::new(Vec::new());
+
 #[cfg(feature = "x11")]
 unsafe extern "C" fn x_error_callback(
     display: *mut x11::ffi::Display,
     event: *mut x11::ffi::XErrorEvent,
 ) -> c_int {
     let xconn_lock = X11_BACKEND.lock();
     if let Ok(ref xconn) = *xconn_lock {
+        // Call all the hooks.
+        let mut error_handled = false;
+        for hook in XLIB_ERROR_HOOKS.lock().iter() {
+            error_handled |= hook(display as *mut _, event as *mut _);
+        }
+
         // `assume_init` is safe here because the array consists of `MaybeUninit` values,
         // which do not require initialization.
         let mut buf: [MaybeUninit<c_char>; 1024] = MaybeUninit::uninit().assume_init();
@@ -608,7 +620,10 @@ unsafe extern "C" fn x_error_callback(
             minor_code: (*event).minor_code,
         };
 
-        error!("X11 error: {:#?}", error);
+        // Don't log error.
+        if !error_handled {
+            error!("X11 error: {:#?}", error);
+        }
 
         *xconn.latest_error.lock() = Some(error);
     }