[Python-Dev] PEP 442: Safe object finalization

[Antoine Pitrou]

On Sat, 18 May 2013 16:22:55 +0200
Armin Rigo <arigo at tunes.org> wrote:
> Hi Antoine,
> 
> On Sat, May 18, 2013 at 3:45 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> >> How is this done?  I don't see a clear way to determine it by looking
> >> only at the objects in the CI, given that arbitrary modifications of
> >> the object graph may have occurred.
> >
> > The same way a generation is traversed, but restricted to the CI.
> >
> > First the gc_refs field of each CI object is initialized to its
> > ob_refcnt (again).
> >
> > Then, tp_traverse is called on each CI object, and each visited
> > CI object has its gc_refs decremented. This substracts CI-internal
> > references from the gc_refs fields.
> >
> > At the end of the traversal, if all CI objects have their gc_refs equal
> > to 0, then the CI has no external reference to it and can be cleared.
> > If at least one CI object has non-zero gc_refs, the CI cannot be
> > cleared.
> 
> Ok, indeed.  Then you really should call finalizers only once: in case
> one of the finalizers in a cycle did a trivial change like I
> described, the algorithm above will conservatively assume the cycle
> should be kept alive.  At the next GC collection we must not call the
> finalizer again, because it's likely to just do a similar trivial
> change.

Well, the finalizer will only be called if the resurrected object is
dereferenced again; otherwise the object won't be considered by the GC.
So, this will only happen if someone keeps trying to destroy a
resurrected object.

Calling finalizers only once is fine with me, but it would be a change
in behaviour; I don't know if it may break existing code.

(for example, say someone is using __del__ to manage a freelist)

Regards

Antoine.