Behavior of SDL_OpenFileStorage, SDL_GlobStorageDirectory and empty paths

[SimonMaracine]

Hello.

TLDR I think I discovered a bug in SDL_genericstorage.c and an issue with opendir in SDL_sysfsops.c (on Linux). Passing an empty string to SDL_OpenFileStorage comes with bad surprises.

For a while in my application I've been accessing files on the disk using SDL_OpenFileStorage, SDL_ReadStorageFile and SDL_WriteStorageFile. I've been passing an empty string to SDL_OpenFileStorage to indicate the current (relative) working directory and it worked perfectly:

m_storage = SDL_OpenFileStorage("");

However, I realized now that SDL_GlobStorageDirectory doesn't work with an empty string. It crashes in SDL_genericstorage.c in GENERIC_EnumerateStorageDirectory, because userdata is NULL and it is passed to SDL_strlen. After reading the documentation, it looks like often an empty string is analogous to a null pointer, at least to the API. userdata, as far as I understand, can be null (no base path). So that looks like a bug that might be fixed like so:

if (fullpath) {
    wrap_data.base_len = userdata ? SDL_strlen((char *)userdata) : 0;
    wrap_data.real_callback = callback;

Even after that, SDL_GlobStorageDirectory was failing to open a directory (no such file or directory) in SDL_sysfsops.c in SDL_SYS_EnumerateDirectory, because opendir fails when it receives empty strings. It was trying to open the root directory of the file storage:

int count {};
char** files {SDL_GlobStorageDirectory(m_storage, nullptr, pattern, 0, &count)};

For now, I fixed all of my problems by passing the current full path working directory to SDL_OpenFileStorage, instead of an empty string. However, if SDL_OpenFileStorage indeed supports relative paths (which it looks like it sometimes does), then that also looks like an issue. If I misunderstood and misused SDL_OpenFileStorage all this time, then I suggest maybe updating the documentation, to warn against bad paths.