'%Z' strftime specifier never works with musl

[benjaminp]

BPO	34672
Nosy	@abalkin, @benjaminp, @zhangyangyu, @pganssle
PRs	bpo-34672: Try to pass the C library's own timezone strings back to it. #9288 bpo-34672: Don't pass NULL to gmtime_r. #9312 bpo-34672: fix a compiler warning in timemodule.c #10176

^{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 = <Date 2022-01-15.20:08:32.877>
created_at = <Date 2018-09-14.04:01:29.001>
labels = ['3.8', '3.7', 'library']
title = "'%Z' strftime specifier never works with musl"
updated_at = <Date 2022-01-15.20:08:32.876>
user = 'https://github.com/benjaminp'

bugs.python.org fields:

activity = <Date 2022-01-15.20:08:32.876>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-15.20:08:32.877>
closer = 'iritkatriel'
components = ['Library (Lib)']
creation = <Date 2018-09-14.04:01:29.001>
creator = 'benjamin.peterson'
dependencies = []
files = []
hgrepos = []
issue_num = 34672
keywords = ['patch']
message_count = 5.0
messages = ['325319', '325357', '325368', '328695', '328841']
nosy_count = 4.0
nosy_names = ['belopolsky', 'benjamin.peterson', 'xiang.zhang', 'p-ganssle']
pr_nums = ['9288', '9312', '10176']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34672'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[benjaminp]

With the musl C library, one has failures like this:

======================================================================
FAIL: test_strptime (test.test_time.TimeTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-alpine/build/Lib/test/test_time.py", line 273, in test_strptime
    time.strptime(strf_output, format)
ValueError: time data '' does not match format '%Z'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-alpine/build/Lib/test/test_time.py", line 276, in test_strptime
    (format, strf_output))
AssertionError: conversion specifier '%Z' failed with '' input.

---

The root problem is that time.strptime('%Z', time.gmtime()) won't work—it returns an empty string—even though time.gmtime() will probably have a valid tm_zone field. This is because musl has a safety mechanism where it refuses to process a tm_zone entry unless it produced the string. (See https://git.musl-libc.org/cgit/musl/commit/?id=d78be392e144c338f58ce6a51d82c859126c137d.)

As a partial workaround, I propose we check whether an incoming time.struct_time instance has an tm_zone member equal to tzname[0] or tzname[1], and place one of those global strings into the C-level tm struct if they do match.

[benjaminp]

New changeset 5633c4f by Benjamin Peterson in branch 'master':
bpo-34672: Try to pass the C library's own timezone strings back to it. (GH-9288)
5633c4f

[benjaminp]

New changeset b93062b by Benjamin Peterson in branch 'master':
bpo-34672: Don't pass NULL to gmtime_r. (GH-9312)
b93062b

[zhangyangyu]

New changeset 163eca3 by Xiang Zhang in branch 'master':
bpo-34672: fix a compiler warning in timemodule.c (GH-10176)
163eca3

[abalkin]

It looks like this issue has been fixed by #53534. Can this be closed? Should we backport?