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Created attachment 3134 [details]
Patch to allow GetInstanceExtensions output array larger than required (UNTESTED but simple)

The doc comment for SDL_Vulkan_GetInstanceExtensions says:

 *  If \a pNames is \c NULL, then the number of required Vulkan instance
 *  extensions is returned in pCount. Otherwise, \a pCount must point to a
 *  variable set to the number of elements in the \a pNames array, and on
 *  return the variable is overwritten with the number of names actually
 *  written to \a pNames. If \a pCount is less than the number of required
 *  extensions, at most \a pCount structures will be written. If \a pCount
 *  is smaller than the number of required extensions, \c SDL_FALSE will be
 *  returned instead of \c SDL_TRUE, to indicate that not all the required
 *  extensions were returned.

But the code actually returns false not just if *pCount is smaller than required, but also if it is *larger* than required. In particular, this call pattern fails:

    const char *names[16] = {};  // Big enough for most purposes.
    unsigned int count = 16;
    if (!SDL_Vulkan_GetInstanceExtensions(window, &count, names)) {
      abort();
    }
    // ...

This fails even though 16 is far more than the number of required extensions because the code (SDL_Vulkan_GetInstanceExtensions_Helper) does:

    if(*userCount != nameCount)
    {
        SDL_SetError(
            "Count doesn't match count from previous call of SDL_Vulkan_GetInstanceExtensions");
        return SDL_FALSE;
    }

(userCount is pCount, nameCount is the number of extensions that need to be stored in the array)