We currently test whether path_t.fd is -1 to decide if the argument should be treated as an fd or as a path, and then call fpathconf() or pathconf() according to it.

In the original issue, we passing -1 caused the code incorrectly treats the argument as a real path. But, path->narrow is NULL, so calling pathconf() on it will crash the interpreter.

This change adds a helper that checks whether the original argument is index-like to determine if it's an fd, so we can correctly the behavior and avoid the crash.

There are other os functions that accept path_t and may have the same issue. Since that is outside the scope of the original issue, I think we should fix those in a separate PR.

• Issue: Segfault from running os.pathconf(-1, 1) #145335