Re: Weak observers (was Re: The taming of the before-gc-hook)

[Jost Boekemeier <jostobfe@calvados.zrz.TU-Berlin.DE>]

Mikael Djurfeldt <mdj@mdj-pc.nada.kth.se> writes:
> Ah, you need it for the module system?
> Then it is probably highly time critical code.

Yes, and no, it is not very time critical.  An observer is a scheme
function observing an environment. The function is called whenever a
location of a (symbol . value) binding changes (either because it
has been removed or re-exported).

The environment points to a list of observer smobs. Each observer smob
holds the observer (`scheme'), a flag that indicates whether `scheme'
is weak (must be marked or not) and a forward/backward pointer. Whenever one of
the following happens the observer-smob must remove itself from the list:

1. The environment has been garbage collected
2. The observer smob is removed via (environment-unobserve <smob>)
3. The scheme function stored in the observer smob has
   been garbage collected.

Case 1:  When the environment has been gc'ed the observer-smobs
         will no longer be marked and will go away. 
Case 2:  Obvious :)
Case 3:  That's the real problem:

In the environment mark function:

  scm_gc_mark (environment->obarray);

  /* now mark all observer smobs */
  node = environment->root.link.right;
  while (SCM_NIMP(node)) 
    {
      struct environment_observer *observer =  SCM_OBSERVER_STRUCT(node);
      if(!observer->weak_p || !is_zombie(observer->scheme, observer_guard))
	{
	  /* mark the whole observer smob, scheme will be marked later by the
             smob's mark fkt */
	  scm_gc_mark(node);
	}

      node = observer->link.right;
    }

The problem is the function is_zombie() which iterates over all
all zombies until the right one has been found.

The following would be better:

register_gc_callback_for(observer->scheme, remove_observer_smob)

This registers the finit() function 'remove_observer_smob' for the
object observer->scheme.  The garbage collector must call this
function after scheme has been garbage collected.

All I need is a simple notification that "this object has been garbage
collected".


Jost