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Re: module GC bug

From: Han-Wen Nienhuys
Subject: Re: module GC bug
Date: Thu, 14 Jul 2005 00:19:28 +0200
User-agent: Mozilla Thunderbird 1.0.2-6 (X11/20050513)

Marius Vollmer wrote:
Han-Wen Nienhuys <address@hidden> writes:

I think the right fix is to change the weak hashtable marking
algorithm to properly cope with circular references like this.  I will
try this and then come back to you.

Interesting. How would you go about doing that?

The marking would would rougly look like this (some special cases are
not considered, like improper alists):

    mark OBJ:
      if mark of OBJ is set:
      set mark of OBJ
      if OBJ is a weak vector
        put it on POSTPONED_OBJECTS
        mark references of OBJ

      mark all root references
      while POSTPONED_OBJECTS not empty
        mark_weak_vector all OBJS
    mark_weak_vector OBJ:
      for all elements ELT of OBJ:
        for all pairs P on list ELT:
          if P is marked, break
          ITEM = car of P
          if ITEM is a pair:
            if (OBJ has weak keys and car of ITEM is unmarked)
            or (OBJ has weak values and cdr of ITEM is unmarked)
              remove P from ELT

what happens if the weak (c[ad]r ITEM) is marked through a postponed weak vector that you haven't processed yet? Then P is removed erroneously, or am I missing something?

 Han-Wen Nienhuys - address@hidden -

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