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

[c103b3da-8d2d-4263-a996-57e557bc5081]

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: ```python assignee = None closed_at = None created_at = labels = ['library', '3.9'] title = 'glob.glob with ** does not always detect symlink loops' updated_at = user = 'https://bugs.python.org/rrt' ``` bugs.python.org fields: ```python activity = actor = 'rrt' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = 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'] ```

[c103b3da-8d2d-4263-a996-57e557bc5081]

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 68b8ffff8c4b20d2f46b708b1a7906377ecc255f