assertion failures and a crash when using an uninitialized struct.Struct object

[orenmn]

BPO	31779
Nosy	@mdickinson, @meadori, @berkerpeksag, @serhiy-storchaka, @orenmn, @izbyshev, @iritkatriel
PRs	bpo-31779: Prevent assertion failures and a crash when using an uninitialized struct.Struct object #3984

^{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 2017-10-13.10:54:20.321>
labels = ['extension-modules', '3.10', '3.9', 'type-crash', '3.11']
title = 'assertion failures and a crash when using an uninitialized struct.Struct object'
updated_at = <Date 2021-10-19.10:22:17.196>
user = 'https://github.com/orenmn'

bugs.python.org fields:

activity = <Date 2021-10-19.10:22:17.196>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2017-10-13.10:54:20.321>
creator = 'Oren Milman'
dependencies = []
files = []
hgrepos = []
issue_num = 31779
keywords = ['patch']
message_count = 3.0
messages = ['304328', '325462', '404285']
nosy_count = 7.0
nosy_names = ['mark.dickinson', 'meador.inge', 'berker.peksag', 'serhiy.storchaka', 'Oren Milman', 'izbyshev', 'iritkatriel']
pr_nums = ['3984']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue31779'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[orenmn]

The following code causes an assertion failure:
import _struct
struct_obj = _struct.Struct.__new__(_struct.Struct)
struct_obj.iter_unpack(b'foo')

This is because Struct_iter_unpack() (in Modules/_struct.c) assumes that
Struct.init() was called, and so it does assert(self->s_codes != NULL);.

The same happens in (almost) every method of Struct, and in s_get_format(), so
in all them, too, we would get an assertion failure in case of an uninitialized
Struct object.
The exception is sizeof(), which doesn't have an assert, and simply
crashes while trying to iterate over self->s_codes.

I would open a PR to fix this soon.

[izbyshev]

What are the drawbacks of moving all code from __init__ to __new__, at least in master branch (3.8)? IMO it's much more robust than playing whack-a-mole both with ref/memleaks caused by repeated calls to __init__ and absence of initialization checks in all methods.

[iritkatriel]

Reproduced on 3.11.

[mdickinson]

Re-opening: the original fix for this issue has been reverted, since it broke code for users in the wild who were subclassing struct.Struct.