paths.is_hidden() Reports invalid input as hidden

[matyanwek]

A bug in a previous version of Jupyter led to errors when trying to rename a file. After a bit of digging, I think I've tracked down the root cause to line 497 of jupyter_core/paths.py, where the assignment inside_root = abs_path[len(abs_root) :] assumes that the abs_path will have abs_root as a prefix. In some cases, the abs_path could get trimmed down to exactly the file extension, like in this case:

is_hidden("filename.ipynb", "/home/aa")

where filename is exactly the same length as /home/aa. The resulting trimmed file extension .ipynb looks like a hidden file when checked.

A simple way to fix this would be to check that abs_root is a prefix of abs_path before trimming:

if abs_path.startswith(abs_root):
    inside_root = abs_path[len(abs_root):]
else:
    inside_root = abs_path

I'd also propose to fix the check on abs_root on line 495, since the call to os.path.normpath() on line 487 ensures it will never be an empty string:

    abs_path = os.path.normpath(abs_path)
    if abs_root == "":
        abs_root = abs_path.split(os.sep, 1)[0] + os.sep
    else:
        abs_root = os.path.normpath(abs_root)

[minrk]

Perhaps this is a bug in argument validation, because the code assumes (and specifies in the docstring) rather than asserts that abs_path is a sub-path of abs_root and both are absolute. It would also be correct to add the following argument validation checks:

abs_path_p = Path(abs_path)
abs_root_p = Path(abs_root)
if not abs_path_p.is_absolute():
    raise ValueError(f"{abs_path=} is not absolute")
if not abs_root_p.is_absolute():
    raise ValueError(f"{abs_root=} is not absolute")
if not abs_path_p.is_relative_to(abs_root_p):
    raise ValueError(f"{abs_path=} is not a subpath of {abs_root=}")

which would appropriately raise an error for the given arguments.

ensures it will never be an empty string:

A valid abs_root can never be an empty string, since an empty string is not an absolute path. This would also be solved by the above argument validation.