[Python-Dev] subtype_dealloc needs rethinking

[Tim Peters]

[Tim]
> ...
> There are two identifiable (so far) problems in subtype_dealloc

Make that one; I'm convinced the first is bogus.

> ...
> 1. A weakref callback can resurrect self, but the code isn't aware of
>    that now.

I no longer believe that's possible.  While a weakref callback can resurrect
objects in dead cycles, a weakref callback called *as a result* of anything
subtype_dealloc does cannot resurrect the object subtype_dealloc is tearing
down (because self's refcount is legitimately 0 then -- Python code can't
get to self, even if it was in a dead cycle, and the weakref callback
doesn't pass the dying object to the callback either).

That just leaves subtype_dealloc with its problem of allowing gc to believe
that self is collectible.  I'm feeling more confident about that too after
staring at the code more, but a complete fix remains strained.