os.path.realpath can't handle symlink loops

[akuchling]

BPO	930024
Nosy	@loewis, @akuchling, @brettcannon, @barneygale
PRs	bpo-43757: make pathlib use os.path.realpath() to resolve all symlinks in a path #25264
Files	posixpath.patch: Patch fixing os.path.realpath

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/akuchling'
closed_at = <Date 2021-04-08.02:12:01.014>
created_at = <Date 2004-04-05.20:59:41.000>
labels = ['library']
title = "os.path.realpath can't handle symlink loops"
updated_at = <Date 2021-04-08.02:12:01.014>
user = 'https://github.com/akuchling'

bugs.python.org fields:

activity = <Date 2021-04-08.02:12:01.014>
actor = 'barneygale'
assignee = 'akuchling'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2004-04-05.20:59:41.000>
creator = 'akuchling'
dependencies = []
files = ['1247']
hgrepos = []
issue_num = 930024
keywords = []
message_count = 5.0
messages = ['20433', '20434', '20435', '20436', '20437']
nosy_count = 4.0
nosy_names = ['loewis', 'akuchling', 'brett.cannon', 'barneygale']
pr_nums = ['25264']
priority = 'high'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue930024'
versions = ['Python 2.4']

[akuchling]

Create a symlink pointing to itself:

ln -s infinite infinite

Run os.path.realpath() on it, and it recurses
infinitely (until the stack limit is hit):

>>> import os
>>> os.path.realpath('/home/amk/infinite')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.3/posixpath.py", line 416, in
realpath
    return realpath(newpath)

os.path.realpath() should be fixed; /home/amk/infinite
is a perfectly good path, though it can't be followed.

[akuchling]

Logged In: YES
user_id=11375

Patch attached; review requested.

[brettcannon]

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user_id=357491

Applied as rev. 1.67 in 2.4a2 and rev. 1.62.6.2 in 2.3.5 with only minor
cleanup.

[loewis]

Logged In: YES
user_id=21627

The code that has been committed is incorrect if the link to
be resolved is relative. _resolve_link uses abspath to
determine whether the link is absolute. This function
returns an absolute path (possibly using getcwd); the
function that should have been used is isabs().

[akuchling]

Logged In: YES
user_id=11375

Fixed in 2.3-maint and on the trunk.