UNC path handling issues #4911

SCons doesn't always handle UNC paths correctly - _my_splitdrive returns a // as the "drive" - to match what Python's libraries do, it really needs to recognize the server part of the path as a whole, so specifically this will come out wrong:

return '//', p[1:]

_my_splitdrive also doesn't normalize paths, expecting this to have been done - there's a path through FileFinder that does not, which leads to bad results.

The node get_relpath gets into trouble if the target is on a UNC mount and the source is not (or on a different UNC mount) - you can't compute a relative path here, so the result should be an absolute path - but instead a ValueError is raised. rel_path gets into the same trouble, although the outcome is different - an invalid path is produced.

On POSIX systems, SCons.Util.make_path_relative also fails on UNC, since it gets a no-op splitdrive - Windows gets this one right.