tarfile.TarFile.next() crashes on empty tar files

[progval]

BPO	46922
Nosy	@ethanfurman

^{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 2022-03-04.16:43:29.909>
labels = ['library', '3.9', '3.10']
title = 'tarfile.TarFile.next() crashes on empty tar files'
updated_at = <Date 2022-03-04.18:05:20.357>
user = 'https://bugs.python.org/progval'

bugs.python.org fields:

activity = <Date 2022-03-04.18:05:20.357>
actor = 'ethan.furman'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-03-04.16:43:29.909>
creator = 'progval'
dependencies = []
files = []
hgrepos = []
issue_num = 46922
keywords = []
message_count = 1.0
messages = ['414536']
nosy_count = 2.0
nosy_names = ['progval', 'ethan.furman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue46922'
versions = ['Python 3.9', 'Python 3.10']

Linked PRs

• gh-91078: Return None from TarFile.next when the tarfile is empty #91850
• [3.11] gh-91078: Return None from TarFile.next when the tarfile is empty (GH-91850) #99798

[progval]

Hi,

I found two related bugs when calling tarfile.TarFile.next() on an empty tar file, both when trying to seek when it should not:

import io
import tarfile

# Create a tarball with no member:
fd = io.BytesIO()
with tarfile.open(fileobj=fd, mode="w") as tf:
    pass

# read in stream mode:
fd.seek(0)
with tarfile.open(fileobj=fd, mode="r|") as tf:
    print(tf.next())   # tarfile.StreamError: seeking backwards is not allowed

# read in normal mode:
fd.seek(0)
with tarfile.open(fileobj=fd, mode="r") as tf:
    print(tf.next())  # ValueError: negative seek value -1

[dignissimus]

It looks like raising ReadError instead might be better.

Edit: After reading the documentation for the next function, merely returning None seems to be the best option here.