reference counter issue in signal module

[animalize]

BPO	38037
Nosy	@vstinner, @ambv, @berkerpeksag, @animalize, @miss-islington, @nanjekyejoannah
PRs	bpo-38037: Fix reference counters in signal module #15701 bpo-38037: Fix reference counters in signal module #15753 [3.8] bpo-38037: Fix reference counters in signal module (GH-15753) #15779
Files	reproduce.py

^{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 2019-09-09.14:44:15.832>
created_at = <Date 2019-09-05.11:14:08.483>
labels = ['3.8', '3.9', 'release-blocker']
title = 'reference counter issue in signal module'
updated_at = <Date 2019-09-09.14:44:15.832>
user = 'https://github.com/animalize'

bugs.python.org fields:

activity = <Date 2019-09-09.14:44:15.832>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-09.14:44:15.832>
closer = 'vstinner'
components = []
creation = <Date 2019-09-05.11:14:08.483>
creator = 'malin'
dependencies = []
files = ['48594']
hgrepos = []
issue_num = 38037
keywords = ['patch']
message_count = 7.0
messages = ['351196', '351200', '351249', '351250', '351466', '351483', '351485']
nosy_count = 6.0
nosy_names = ['vstinner', 'lukasz.langa', 'berker.peksag', 'malin', 'miss-islington', 'nanjekyejoannah']
pr_nums = ['15701', '15753', '15779']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue38037'
versions = ['Python 3.8', 'Python 3.9']

[animalize]

Adding these two lines to /Objects/longobject.c will disable the "preallocated small integer pool":

    #define NSMALLPOSINTS  0
    #define NSMALLNEGINTS  0

Then run this reproduce code (attached):

    from enum import IntEnum
    import _signal

    class Handlers(IntEnum):
        A = _signal.SIG_DFL
        B = _signal.SIG_IGN

When the interpreter exits, will get this error:

d:\dev\cpython\PCbuild\win32>python_d.exe d:\a.py
d:\dev\cpython\include\object.h:541: _Py_NegativeRefcount: Assertion failed: object has negative ref count
<object: freed>
Fatal Python error: _PyObject_AssertFailed

Current thread 0x0000200c (most recent call first):

3.8 and 3.9 branches are affected.
I'm sorry, this issue is beyond my ability.

[animalize]

I did a Git bisect, this is the first bad commit:
9541bd3

nosy involved mates.

[vstinner]

This issue is a Python 3.8 regression.

Joannah: Would you mind to have a look?

    x = DefaultHandler = PyLong_FromVoidPtr((void *)SIG_DFL);
    if (PyModule_AddObject(m, "SIG_DFL", x))
        goto finally;

This change is not easy to read.

DefaultHandler must be a strong reference: finisignal() calls Py_CLEAR(DefaultHandler);

Previously, the code uses PyDict_SetItemString(d, "SIG_DFL", x): PyDict_SetItemString increases the reference counter, whereas PyModule_AddObject leaves the reference counter unchanged (yeah, it's a strange/bad C API).

I guess than an INCREF() is needed somewhere.

Compare it to:

int
PyModule_AddIntConstant(PyObject *m, const char *name, long value)
{
    PyObject *o = PyLong_FromLong(value);
    if (!o)
        return -1;
    if (PyModule_AddObject(m, name, o) == 0)
        return 0;
    Py_DECREF(o);
    return -1;
}

[nanjekyejoannah]

I will look after my next meeting which is in 10 minutes. In the meantime have you perused this PR : #15701 If it can solve this?

[vstinner]

New changeset 77643c4 by Victor Stinner (animalize) in branch 'master':
bpo-38037: Fix reference counters in signal module (GH-15753)
77643c4

[miss-islington]

New changeset b150d0b by Miss Islington (bot) in branch '3.8':
bpo-38037: Fix reference counters in signal module (GH-15753)
b150d0b

[vstinner]

Thanks for the fix animalize.