tests/readlink.py

errors: return more detailed errors when failing to parse or apply patch

This patch adds subclasses of `PatchError` so we can distinguish
between failure to parse a patch from failure to apply it. It updates
the callers to raise either `InputError` or `StateError` depending on
which type of error occurred.

Differential Revision: https://phab.mercurial-scm.org/D11824

#!/usr/bin/env python3

from __future__ import absolute_import, print_function

import errno
import os
import sys

for f in sys.argv[1:]:
    try:
        print(f, '->', os.readlink(f))
    except OSError as err:
        if err.errno != errno.EINVAL:
            raise
        print(f, '->', f, 'not a symlink')

sys.exit(0)