tarfile.is_tarfile() and tarfile.open() when used with file object may cause tarfile operations to fail

[amateja]

BPO	44289
Nosy	@gvanrossum, @gustaebel, @Fidget-Spinner, @akulakov, @amateja
PRs	bpo-44289: Keep argument file object's current position in tarfile.is_tarfile #26488

^{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 2021-06-02.15:09:05.223>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'tarfile.is_tarfile() and tarfile.open() when used with file object may cause tarfile operations to fail'
updated_at = <Date 2022-02-09.16:19:26.161>
user = 'https://github.com/amateja'

bugs.python.org fields:

activity = <Date 2022-02-09.16:19:26.161>
actor = 'gvanrossum'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-06-02.15:09:05.223>
creator = 'mateja.and'
dependencies = []
files = []
hgrepos = []
issue_num = 44289
keywords = ['patch']
message_count = 3.0
messages = ['394922', '408062', '412919']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'lars.gustaebel', 'python-dev', 'kj', 'andrei.avk', 'mateja.and']
pr_nums = ['26488']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44289'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[amateja]

Since Python 3.9 tarfile.is_tarfile accepts not only paths but also files and file-like objects (bpo-29435).

Verification if a file or file-like object is a tar file modifies file object's current position.

Imagine a function listing names of all tar archive members but checking first if this is a valid tar archive. When its argument is a str or pathlib.Path this is quite straightforward. If the argument is a file of file-like object then current position must be reset or TarFile.getmembers() returns empty list.

import tarfile


def list_tar(archive):
    if tarfile.is_tarfile(archive):
        kwargs = {'fileobj' if hasattr(archive, 'read') else 'name': archive}
        t = tarfile.open(**kwargs)
        return [member.name for member in t.getmembers()]
    return []


if __name__ == '__main__':
    path = 'archive.tar.gz'
    print(list_tar(path))
    print(list_tar(open(path, 'rb')))

['spam.py', 'ham.py', 'bacon.py', 'eggs.py']
[]

[akulakov]

This affects more use cases than just is_tarfile() and getmembers() results.

is_tarfile() calls open() which is the root cause of the issue. Calling open() 2+ times will also cause the same issue.

In addition to getmembers(), extracting the tar will also silently fail. (and possibly other operations).

I've suggested a different fix in the comment on the PR:
#26488 (comment)

[gvanrossum]

New changeset 128ab09 by Andrzej Mateja in branch 'main':
bpo-44289: Keep argument file object's current position in tarfile.is_tarfile (GH-26488)
128ab09