glob.glob with ** does not always detect symlink loops

[rrt]

BPO	43875

^{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 = None
closed_at = None
created_at = <Date 2021-04-16.20:08:18.994>
labels = ['library', '3.9']
title = 'glob.glob with ** does not always detect symlink loops'
updated_at = <Date 2021-04-16.20:10:06.645>
user = 'https://bugs.python.org/rrt'

bugs.python.org fields:

activity = <Date 2021-04-16.20:10:06.645>
actor = 'rrt'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-04-16.20:08:18.994>
creator = 'rrt'
dependencies = []
files = []
hgrepos = []
issue_num = 43875
keywords = []
message_count = 1.0
messages = ['391253']
nosy_count = 1.0
nosy_names = ['rrt']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43875'
versions = ['Python 3.9']

[rrt]

Example session:

$ mkdir foo
$ cd foo
$ ln -s .. bar
$ ln -s .. baz
$ python3.9
Python 3.9.0+ (default, Oct 20 2020, 08:43:38) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from glob import glob
>>> glob('**/README', recursive=True)
[HANG]

Removing either of the symlinks fixes the hang.

[calestyo]

It seems to blow up already when using a **/* pattern on a directory containing a symlink to ..

[calestyo]

Sidenote: pathlib’s Path.glob()-function does not seem to suffer from that problem, however pathlib does not seem to be able to handle all possible pathnames (in POSIX pathnames allow any characters except NUL, while the module seems to handle only UTF8 strings), so may not be an alternative.

[barneygale]

Still reproducible in main as of 68b8fff

[aknrdureegaesr]

In a variant of apparently the same problem, I found out that the looping does not happen in an unbounded fashion.

docker run --rm -ti --entrypoint /usr/bin/bash python:3.13-slim-bookworm
root@c65eb74a1bfd:/# cd
root@c65eb74a1bfd:~# mkdir upper upper/lower                                                                   
root@c65eb74a1bfd:~# (cd upper/lower && touch marker && ln -s ../../upper trap)                                
root@c65eb74a1bfd:~# python3                                                                                   
Python 3.13.3 (main, May  9 2025, 23:48:58) [GCC 12.2.0] on linux                                              
Type "help", "copyright", "credits" or "license" for more information.                                         
>>> from glob import iglob                                                                                     
>>> for p in iglob("upper/**/marker", recursive=True):                                                         
...     print(p)                                                                                               
...                                                                                                            
upper/lower/marker                                                                                             
upper/lower/trap/lower/marker                                                                                  
upper/lower/trap/lower/trap/lower/marker   
[and so on]
upper/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/
trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/t
rap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/tr
ap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/trap/lower/tra
p/lower/marker
>>>

[aknrdureegaesr]

Personally, I'd be happy if I could ask glob / iglob to simply not follow symlinks to directories.