zipimport should not check pyc timestamps against zipped py files

[gpshead]

BPO	23734
Nosy	@brettcannon, @gpshead, @ncoghlan, @elprans, @ericsnowcurrently, @serhiy-storchaka
PRs	bpo-34726: Fix handling of hash-based pycs in zipimport #10327

^{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 2018-11-14.00:54:49.285>
created_at = <Date 2015-03-21.18:59:07.067>
labels = ['extension-modules', '3.8', 'type-feature', '3.7']
title = 'zipimport should not check pyc timestamps against zipped py files'
updated_at = <Date 2018-11-14.00:54:49.283>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2018-11-14.00:54:49.283>
actor = 'gregory.p.smith'
assignee = 'none'
closed = True
closed_date = <Date 2018-11-14.00:54:49.285>
closer = 'gregory.p.smith'
components = ['Extension Modules']
creation = <Date 2015-03-21.18:59:07.067>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 23734
keywords = ['patch']
message_count = 4.0
messages = ['238824', '325730', '329316', '329877']
nosy_count = 7.0
nosy_names = ['brett.cannon', 'gregory.p.smith', 'ncoghlan', 'Elvis.Pranskevichus', 'eric.snow', 'serhiy.storchaka', 'superluser']
pr_nums = ['10327']
priority = 'low'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23734'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[gpshead]

The zipimport module checks the timestamp of a pyc file loaded from the zip file against the timestamp of a corresponding py file in the zip if any. This seems pointless. By the time someone has created a zip file for zipimport they should have guaranteed that the pyc's are fresh or not have put them into the zip file at all (wasteful).

https://hg.python.org/cpython/file/e8878579eb68/Modules/zipimport.c#l1187

There is a comment in the code alluding to this, but the mtime check is still done right above.

[serhiy-storchaka]

zipimport has been rewritten in pure Python (bpo-25711).

[elprans]

I don't think that breaking the .pyc contract by assumption is a good thing. The desired behavior of never checking the .pyc freshness can be achieved by using unchecked hash-based compilation, or using hash-based compilation and --check-hash-based-pycs=never. Perhaps we can go further and add a --check-timestamp-based-pycs=never, but IMO, this should be explicit in all cases, including zipimport.

[gpshead]

Correct, this issue was filed before we had the new pyc options (which came out of our September core dev. sprint in 2016 iirc).

I'll just close it, the way forward for people using zip files on modern CPython interpreters is to explicitly use the new unchecked feature.