tarfile fills devmajor and devminor fields even for non-devices

[NuuttiKotivuori]

BPO	18819
Nosy	@gustaebel, @ethanfurman, @wchargin
PRs	bpo-18819: tarfile: only set device fields for device files #18080
Files	tarfile.py.diff: patch

^{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/ethanfurman'
closed_at = <Date 2020-02-14.16:03:01.387>
created_at = <Date 2013-08-23.07:56:10.137>
labels = ['type-bug', 'library', '3.9']
title = 'tarfile fills devmajor and devminor fields even for non-devices'
updated_at = <Date 2020-02-14.16:03:01.386>
user = 'https://bugs.python.org/NuuttiKotivuori'

bugs.python.org fields:

activity = <Date 2020-02-14.16:03:01.386>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2020-02-14.16:03:01.387>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2013-08-23.07:56:10.137>
creator = 'Nuutti.Kotivuori'
dependencies = []
files = ['31434']
hgrepos = []
issue_num = 18819
keywords = ['patch']
message_count = 5.0
messages = ['195949', '360327', '360441', '361202', '361911']
nosy_count = 4.0
nosy_names = ['lars.gustaebel', 'ethan.furman', 'Nuutti.Kotivuori', 'wchargin']
pr_nums = ['18080']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue18819'
versions = ['Python 3.9']

[NuuttiKotivuori]

when tarfile generates a tar, it uses TarInfo objects which are packed to the binary format. This packing uses "itn" format for filling the "devmajor" and "devminor" fields of the tar file entry, with a default value of zero. The field length is 8 characters, and the formatting will format the values as octal strings, followed by a NUL byte, eg. '0000000\x00'.

However, other common tar tools do not exhibit this behavior, instead leaving the field filled with NUL bytes in case the earlier type field does not indicate a device type. Explicitly, the value generated by other tools is then '\x00\x00\x00\x00\x00\x00\x00\x00'.

This causes no interoperability issues (that I am aware of), but makes it hard to attempt to replicate the tar files generated by other tools exactly.

I am attaching a simple diff fixing this, but there are probably better ways to fix this.

[wchargin]

I've just independently run into this and sent a patch as a pull
request. Happily, once this is fixed, the output of tarfile is
bit-for-bit compatible with the output of GNU tar(1).

PR: <https://github.com/python/cpython/pull/18080\>

[ethanfurman]

Thanks for moving this issue forward, William!

[wchargin]

My pleasure. Is there anything else that you need from me to close this
out? It looks like the PR is approved and in an “awaiting merge” state,
but I don’t have access to merge it.

[ethanfurman]

New changeset 674935b by William Chargin in branch 'master':
bpo-18819: tarfile: only set device fields for device files (GH-18080)
674935b