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

[Antoine Pitrou]

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?

Regards

Antoine.