is_loaded does not work on X11 (Linux) platforms

[khyperia]

The world is a ball of sadness, and glXGetProcAddress never returns null. See point 3 in this page: https://dri.freedesktop.org/wiki/glXGetProcAddressNeverReturnsNULL/

As far as I can tell, gl::__::is_loaded() checks if the loading function returned null or not. As the X11 impl never returns null... is_loaded doesn't work as intended :'(

To be honest, I don't really care if this is somehow fixed or not (who actually uses is_loaded anyway), I'd be okay with a note on the readme or docs.rs front page saying "be careful with X11 and is_loaded, it probably doesn't do what you want".

---

Loading every function with the string "wow, this isn't the right string" returns some valid pointers 🌈

extern crate gl;
extern crate sdl2;

fn main() {
    let sdl = sdl2::init().expect("SDL failed to init");
    let video = sdl.video().expect("SDL does not have video");

    let window = video.window("test", 500, 500).opengl().build().expect("SDL window didn't build");
    let _gl_context = window
        .gl_create_context()
        .expect("Failed to create OpenGL context");

    gl::load_with(|_| video.gl_get_proc_address("wow, this isn't the right string") as *const _);

    if gl::GetError::is_loaded() {
        // Prints "Loaded." :sparkles:
        println!("Loaded.");
    } else {
        println!("Not loaded.");
    }

    unsafe {
        // Prints some random hecking number that I assume is uninitialized memory.
        println!("{}", gl::GetError());
    }
}

---

Non-minimized context that this bug report is from: https://twitter.com/khyperia/status/1133584256834215937

[khyperia]

After rebooting into windows, that example outputs:

Not loaded.
thread 'main' panicked at 'gl function was not loaded', [blah blah]

as expected, because wglGetProcAddress returns null when passed an invalid string.

Well. On my system, it does. https://www.khronos.org/opengl/wiki/Load_OpenGL_Functions states "While the MSDN documentation says that wglGetProcAddress returns NULL on failure, some implementations will return other values. 1, 2, and 3 are used, as well as -1."

✨🌈 Thanks OpenGL! 🌈✨

[brendanzab]

arrrgh - any idea how we might be able to get around this?

[brendanzab]

Or are we doomed? 😭

[khyperia]

My only thought is if the function data file (that I assume everything is generated from) includes extensions required - then it can check against glGetStringi(GL_EXTENSIONS, i). Alternatively/in addition to that, if the document specifies min version, get the version and do a comparison there.

That's a lot of work, though, so shrug