Re: Order of weak ref removal and __gc

[Pablo Pissanetzky <pablo@...>]

> The object has been marked for finalization. It will be finalized no
> matter what, so you should not use it anymore. That is why it is removed
> from weak tables.

There is no way for me to know that the object has been marked for
finalization - so I can't stop using it.
I rely on __gc being called to give me this information. The fact that
its corresponding key is no longer in
the weak value table can give me a hint, but it is not reliable. It is
entirely possible that the key has been
re-used and now points to a completely different user data.

> Why do you need that reference?

In my system, we are using a toggle reference to bind to ref counted
objects (GObject). The user data has a
strong ref to the underlying object. The object, in turn, toggles
between a weak ref and a strong ref to the
user data. (It is weak when the user data's ref is the only one and
strong when there are others). This works
well but relies on the weak ref to the user data sticking around until
__gc is called.

See http://www.mail-archive.com/gtk-devel-list@gnome.org/msg00544.html




On Mon, Aug 9, 2010 at 2:44 PM, Pablo Pissanetzky <pablo@trickplay.com> wrote:
> This is what I'm seeing:
>
> 1. In one pass of my event loop, I create a user data and add it to a
> table as a weak value (the key is an integer).
>
> 2. In the next pass of the event loop, I create another one and add it
> to the same table. At this point, the first one I added is removed,
> but that user data's __gc has not been called. Control, returns to me.
>
> 3. In the next pass of the event loop, I expect the first one to still
> be there (because it has not been finalized), so I look for its
> (integer) key in the table and it is not there.
>
>
> It seems like there are two distinct steps: 1. Remove weak k/v pairs
> when the value is weak. 2. Call __gc later.
>
>
> I'm trying to maintain a weak reference to the user data until _gc is
> called. Is there another way to do it?
>
> Pablo
>
> On Fri, Aug 6, 2010 at 5:31 PM, Pablo Pissanetzky <pablo@trickplay.com> wrote:
>> (In 5.1.4)
>>
>> If I have a weak ref to a user data (as a value) in a table, is it
>> possible for the weak ref to be removed long before the userdata's
>> __gc metamethod is called? Does this happen in multiple passes of the
>> collector?
>>
>> Thanks in advance,
>> Pablo Pissanetzky
>>
>