[Python-Dev] PEP 489: module m_traverse called with NULL module state

Petr Viktorin encukou at gmail.com
Tue Dec 19 10:10:06 EST 2017

On Thu, Dec 14, 2017 at 12:00 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Thu, 14 Dec 2017 17:00:10 +1000
> Nick Coghlan <ncoghlan at gmail.com> wrote:
>> On 14 Dec. 2017 9:19 am, "Antoine Pitrou" <solipsis at pitrou.net> wrote:
>> Hello,
>> After debugging a crash on AppVeyor for a submitter's PR
>> (see https://github.com/python/cpython/pull/4611 ), I came to the
>> following diagnosis: converting the "atexit" module (which is a
>> built-in C extension) to PEP 489 multiphase initialization can lead to
>> its m_traverse function (and presumably also m_clear and m_free) to be
>> called while not module state is yet registered: that is,
>> `PyModule_GetState(self)` when called from m_traverse returns NULL!
>> Is that an expected or known subtlety?
>> Not that I'm aware of, so I'd be inclined to classify it as a bug in the
>> way we're handling multi-phase initialisation unless/until we determine
>> there's no way to preserve the existing invariant from the single phase
>> case.
> Speaking of which, the doc is not very clear: is PEP 489 required for
> multi-interpreter support or is PyModule_GetState() sufficient?

Yes, it is possible to have proper subinterpreter support without
multi-phase init.

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