PyStructSequence_NewType broken in 3.8

[wdi2]

BPO	42083
Nosy	@encukou, @ambv, @miss-islington, @stestagg, @Fidget-Spinner
PRs	[3.9] bpo-41832, bpo-42083: PyType_FromModuleAndSpec() can accept the NULL tp_doc slot (GH-25687) #25852 bpo-42083: Add C-API tests for PyStructSequence_NewType doc=NULL #25886 [3.10] bpo-42083: Add C-API tests for PyStructSequence_NewType doc=NULL #25887 [3.9] bpo-42083: Allow NULL doc in PyStructSequence_NewType #25896

^{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 2021-05-04.14:23:08.466>
created_at = <Date 2020-10-19.18:27:05.410>
labels = ['expert-C-API', '3.10', '3.9', 'type-crash']
title = 'PyStructSequence_NewType broken in 3.8'
updated_at = <Date 2021-05-04.14:23:08.466>
user = 'https://bugs.python.org/wdi2'

bugs.python.org fields:

activity = <Date 2021-05-04.14:23:08.466>
actor = 'kj'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-04.14:23:08.466>
closer = 'kj'
components = ['C API']
creation = <Date 2020-10-19.18:27:05.410>
creator = 'wdi2'
dependencies = []
files = []
hgrepos = []
issue_num = 42083
keywords = ['patch']
message_count = 6.0
messages = ['378978', '382853', '392686', '392893', '392899', '392901']
nosy_count = 6.0
nosy_names = ['petr.viktorin', 'lukasz.langa', 'miss-islington', 'stestagg', 'kj', 'wdi2']
pr_nums = ['25852', '25886', '25887', '25896']
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue42083'
versions = ['Python 3.9', 'Python 3.10']

[wdi2]

Calling PyStructSequence_NewType() with a NULL field in the desc.doc parameter (explicitly allowed as per docs) leads to a crash in

Objects/typeobject.c:2956
2956 size_t len = strlen(old_doc)+1;

where old_doc is NULL.
If the doc string is set, the call succeeds, but with a warning

(stdin):1: DeprecationWarning: builtin type G_SGROUP has no __module__ attribute

(where G_SGROUP is my new type), which did not happen in 3.6, and which I do not think can be suppressed by function call arguments.

[stestagg]

It looks like the segfault was fixed in 88c2cfd

as part of https://bugs.python.org/issue41832.

The code in this area of typeobject.c looks a bit different, now, but the backport seems simple?

Simple testcase:

#include <stdio.h>
#include <Python.h>
int main() {
    Py_Initialize();
    PyStructSequence_Field fields[2] = {
        {NULL, NULL}
    };
    PyStructSequence_Desc d = {"test", NULL, &fields[0], 0};
    PyStructSequence_NewType(&d);
    Py_Finalize();
}

Segfault reproducible on 3.8 and 3.9

[ambv]

This missed the train for inclusion in 3.8. There's still time for a backport for 3.9.