[Python-Dev] Re: weakref callback vs gc vs threads

[Neil Schemenauer]

[Taking this to email.  Carrying out discussions via the SF bug tracker
sucks.]

Comment By: Tim Peters (tim_one)
> [Neil] 
> > I had to change _PyWeakref_ClearRef() since it was also
> > clearing the weakref list of the trash object.
> 
> That was really its *purpose*.  If a trash weakref with a 
> callback isn't removed from the referent's list of weakrefs, 
> then the callback will trigger when PyObject_ClearWeakRefs() 
> is invoked on the referent.  The purpose of 
> _PyWeakref_ClearRef() was to ensure that the callback never 
> triggers.

But it's okay of the callback triggers, as long as the callback
doesn't reference trash.

> > Now it just sets wr_object to Py_None.
> 
> That won't stop the callback from triggering.  It also means 
> (see earlier comment) that PyObject_ClearWeakRefs() will 
> never removed the weakref from the list either, although I'm 
> not sure that does real harm.

I'm trying to figure out PyObject_ClearWeakRefs() right now.

> > I also made some serious simplifications to gcmodule by
> > just treating trash weakref objects with callbacks the same
> > as objects with __del__ methods (i.e. move them to the
> > finalizers list and then do the transitive closure of that set).
> 
> Does that mean they can end up in gc.garbage too?  If so, I 
> don't think that's sellable.

I think so.  That can be easily changed though.  What we can't do is
invoke those callbacks.

  Neil