udev: Clear intermediate errors when attempting to load libudev.so

[mgorny]

Description

Call SDL_ClearError() after a successful SDL_LoadObject() call to clear the previous error left over by the previous SDL_UDEV_load_syms() procedure (set by SDL_LoadFunction()). This ensures that no error is set when SDL_UDEV_Init() returns successfully.

SDL_UDEV_LoadLibrary() first attempts to find udev symbols in the libraries that are already loaded, then attempts to find and load libudev.so* using a list of names. Every unsuccessful call to SDL_UDEV_load_syms() leaves the error set. However, if a subsequent call succeeds (i.e. after loading the library), the previous error remains set until SDL_UDEV_Init() returns.

This is a serious problem in SDL2 since the (stale) error leaks to the caller after:

SDL_Init(SDL_INIT_GAMECONTROLLER);

In SDL3, the issue is still present but the error is incidentally cleared by the call to PLATFORM_hid_init().

[slouken]

I don't mind accepting this, but in general you should only call SDL_GetError() when the API level function returns an error. There are many places where internal functions can generate an error that is resolvable by the higher level API, and we only guarantee that the error message is correct when the API returns an error.

After thinking about this, it may be something we want to change for SDL3, as a global pass.

[chewi]

@slouken I worked with @mgorny on figuring this out. In case it wasn't clear from the description, SDL_UDEV_Init() never works under SDL2 without this change unless you LD_PRELOAD libudev first. The first call to SDL_UDEV_load_syms() fails, then the second call succeeds, but the SDL_UDEV_LoadLibrary() still thinks it failed because the error hasn't been cleared.

Never mind, I now realise it does work, it's just the error flag is set. I previously thought the error was fatal.