x11: Use XCheckIfEvent() instead of XNextEvent() for thread-safety#4934
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A racing reader could read from our fd between SDL_IOReady()/X11_Pending() and our call to XNextEvent() which will cause XNextEvent() to block for more data. Avoid this by using XCheckIfEvent() which will never block. This also fixes a bug where we could poll() for data, even when events were already read and pending in the queue. Unlike the Wayland implementation, this isn't totally thread-safe because nothing prevents a racing reader from reading events into the queue between our XCheckIfEvent() and SDL_IOReady() calls, but I think this is the best we can do with Xlib.
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Description
A racing reader could read from our fd between SDL_IOReady()/X11_Pending() and our call to XNextEvent() which will cause XNextEvent() to block for more data. Avoid this by using XCheckIfEvent() which will never block.
This approach is based on that from rust-windowing/winit#782 and glfw/glfw@3d6221c
This also fixes a bug where we could poll() for data, even when events were already read and pending in the queue. Unlike the Wayland implementation, this isn't totally thread-safe because nothing prevents a racing reader from reading events into the queue between our XCheckIfEvent() and SDL_IOReady() calls, but I think this is the best we can do with Xlib.
Existing Issue(s)
#4928