Unordered finalization, SMOBs, finalizers, and mark functions

[Andy Wingo]

Hi,

Guile currently uses the Boehm-Demers-Weiser collector with "Java-style
finalization", which is to say that finalizers are unordered.  This is
good in the sense that Guile's GC can collect cycles of objects with
finalizers that point to each other.  It also has some understudied
disadvantages or strange behavior.

One disadvantage is that with the default topological finalization
order of BDW-GC, cycles of objects with finalizers are uncollectable.  I
think that BDW-GC will warn to the console when this is the case, in the
default mode, so perhaps it's not that bad.

Another disadvantage is that running the finalizer of object O1 which
links to finalizable object O2 could see O2 after O2's finalizer has
been run, because finalizers are unordered.  This is almost certainly
not what you would expect.

Also, if O1 has a mark function, you could mark O1 when O1 is on the
finalization queue but not yet finalized, and O2 has already been
finalized -- so your mark functions also need to be ready to deal with
unordered finalization.

Keep in mind that in Guile 2.x, finalizers are uncommon.  They are
probably more common in user code than in Guile code, in the form of
SMOB free functions.  This is especially the case in code ported from
earlier Guile versions.

Useful links:

  Finalization
  http://hboehm.info/gc/finalization.html

  Finalizers, Threads, and the Java Memory Model
  http://hboehm.info/misc_slides/java_finalizers.pdf

  Foreign Object Memory Management
  
http://www.gnu.org/software/guile/docs/master/guile.html/Foreign-Object-Memory-Management.html#Foreign-Object-Memory-Management

Open question: should Guile configure the BDW GC in a different way?
Topological finalization is desirable for all the reasons Boehm links in
that first article.  Should it allow the user to configure it?  I
believe it is currently unordered due to issues with guardians, but I
don't recall correctly.

Thoughts?

Andy
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