tarfile recognizes .gz file as tar

[spetrunin]

BPO	32435
Nosy	@serhiy-storchaka
Files	big_0_file_1.gz: 10 GB of '0' compressed with GZip

^{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 2017-12-27.21:56:20.522>
labels = ['library']
title = 'tarfile recognizes .gz file as tar'
updated_at = <Date 2017-12-27.22:24:14.191>
user = 'https://bugs.python.org/spetrunin'

bugs.python.org fields:

activity = <Date 2017-12-27.22:24:14.191>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-12-27.21:56:20.522>
creator = 'spetrunin'
dependencies = []
files = ['47353']
hgrepos = []
issue_num = 32435
keywords = []
message_count = 2.0
messages = ['309106', '309108']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'spetrunin']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue32435'
versions = ['Python 2.7', 'Python 3.6']

[spetrunin]

Library's function 'tarfile.is_tarfile' returns 'True' on GZip file (9.7 megabytes see attachment), that is a file with 10 gigabytes of '0' character compressed with 'gzip' utility (no 'tar' whatsoever).

Desired result: this function should return False in this case, b.c. this file is not a tar file.

[serhiy-storchaka]

This file is a correct empty tar file. The tar utility accepts it too.

$ tar tf big_0_file_1.gz; echo $?
0