Leak in atexitmodule

[skrah]

BPO	11826
Nosy	@loewis, @amauryfa, @pitrou, @skrah
Files	atexit-leak.patch

^{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 2012-03-27.10:40:16.246>
created_at = <Date 2011-04-11.06:56:19.408>
labels = ['extension-modules', 'performance']
title = 'Leak in atexitmodule'
updated_at = <Date 2012-03-27.10:40:16.245>
user = 'https://github.com/skrah'

bugs.python.org fields:

activity = <Date 2012-03-27.10:40:16.245>
actor = 'skrah'
assignee = 'none'
closed = True
closed_date = <Date 2012-03-27.10:40:16.246>
closer = 'skrah'
components = ['Extension Modules']
creation = <Date 2011-04-11.06:56:19.408>
creator = 'skrah'
dependencies = []
files = ['21610']
hgrepos = []
issue_num = 11826
keywords = ['patch']
message_count = 6.0
messages = ['133501', '133983', '155742', '155883', '156826', '156900']
nosy_count = 5.0
nosy_names = ['loewis', 'amaury.forgeotdarc', 'pitrou', 'skrah', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue11826'
versions = ['Python 3.3']

[skrah]

Valgrind reports a leak (definitely lost) in atexitmodule.c. The
patch fixes the problem.

[amauryfa]

It's the very first usage of PyModuleDef::m_free.
Martin, do you agree with the path?

[pitrou]

Well, if it doesn't crash, it's probably ok ;)
Perhaps check modstate->atexit_callbacks for non-NULL? Or do we trust free() to do the right thing?

[skrah]

Antoine Pitrou <report@bugs.python.org> wrote:

Well, if it doesn't crash, it's probably ok ;)
Perhaps check modstate->atexit_callbacks for non-NULL?
Or do we trust free() to do the right thing?

I was initially surprised by this, but the docs state that it's safe:

http://docs.python.org/dev/c-api/memory.html?highlight=pymem_free#PyMem_Free

The I searched a bit and it appears that free() crashing on NULL is
a pre-ANSI thing.

[skrah]

Actually _iomodule.c already has a freefunc with the same signature.
atexit_free() is properly called in:

static void
module_dealloc(PyModuleObject *m)
{
    PyObject_GC_UnTrack(m);
    if (m->md_def && m->md_def->m_free)
        m->md_def->m_free(m);
    if (m->md_dict != NULL) {
        _PyModule_Clear((PyObject *)m);
        Py_DECREF(m->md_dict);
    }
    if (m->md_state != NULL)
        PyMem_FREE(m->md_state);
    Py_TYPE(m)->tp_free((PyObject *)m);
}

So my only worry is if there's a way to exploit the fact that _PyModule_Clear()
is called after atexit_free(). I tried things like:

>>> import atexit
>>> def g(): pass
...
>>> class silly:
...     def __del__(self): atexit.register(g)
...
>>> atexit.x = silly()
>>> atexit.register(g)
<function g at 0x7fe7ebb83a68>
>>>
Exception AttributeError: "'NoneType' object has no attribute 'register'" in <bound method silly.__del__ of <__main__.silly object at 0x153fc50>> ignored

But I haven't been able to break anything, so I think I'll go ahead and
commit if there aren't any protests.

[python-dev]

New changeset 7c48bb929e6e by Stefan Krah in branch 'default':
Issue bpo-11826: Fix memory leak in atexitmodule.
http://hg.python.org/cpython/rev/7c48bb929e6e