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Mon Feb 18 20:20:35 CET 2008 · https://mail.python.org/pipermail/python-dev/2008-February/076948.html

On Feb 17, 2008 12:29 PM, "Martin v. Löwis" <martin at v.loewis.de> wrote:
> Jeffrey Yasskin wrote:
> > On Feb 16, 2008 3:12 PM, Amaury Forgeot d'Arc <amauryfa at gmail.com> wrote:
> >> Should we however intensively search and correct all of them?
> >> Is there a clever way to prevent these problems globally, for example
> >> by delaying finalizers "just a little"?
> >
> > A simple way to do this would be to push objects whose refcounts had
> > reached 0 onto a list instead of finalizing them immediately, and have
> > PyEval_EvalFrameEx periodically swap in a new to-delete list and
> > delete the objects on the old one.
>
> -1. This would only soften the problem in a shallow way. It should still
> be possible to create cases where the final cleanup of the object comes
> "too early", e.g. when some caller of PyEval_EvalFrameEx still carries
> a pointer to some object that got deleted, and then still some code can
> get hold of the then-deleted object. It will just be more difficult to
> trigger such crashes, depending on the period in which objects are
> cleaned.

Right. Changing introducing these bugs from "easy" to "possible"
sounds like an improvement. Making them harder to trigger has both
good and bad effects: they're harder to exploit and harder to
reproduce. If we delete on every iteration, it only costs a couple
memory accesses more, and should eliminate most of the
non-determinism.

Anyway, I saw an approach like this work well on a server I used to
work on, but there were differences. In particular, there was no way
to call a function to trigger deletions; you had to return out to the
main loop, which made it a little more reliable than it would be in
Python. I suspect it would still fix nearly all of the bugs Amaury is
finding since Py_DECREF would no longer return control to the
interpreter, but I could be wrong.

> The only sane way to never touch deleted objects is to have no
> references to them on heap anymore, and to not touch stack references
> after the DECREF.
>
> Regards,
> Martin
>

-- 
Namasté,
Jeffrey Yasskin
http://jeffrey.yasskin.info/

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[Python-Dev] Py_CLEAR to avoid crashes