Minor tweaks to ffi addendum

[Malcolm Wallace]

> >   It's going to be really hard to implement ForeignPtr as specified.
> >   The problem is that invocation of the cleanup function is triggered
> >   by the garbage collector.
> 
> Malcolm can comment on exactly how nhc98 handles the finalizers, but I
> believe that there's a list of pending finalizers maintained by the RTS
> which are run on re-entry to Haskell land.

That's the theory, yes.  In practice, I've never been able to get it
to work correctly, so the only finalisers that are guaranteed to run
in nhc98's runtime system are ones written in C rather than Haskell.
The underlying problem is when do you run a Haskell finaliser in a
non-concurrent system?  You can't run it when you discover it (in
the middle of a GC).  When the GC is called, the mutator is probably
right in the middle of some reduction step.  If I run the finalisers
immediately the GC finishes, then I'll probably corrupt the reduction
that was already in progress.  So the pending finalisers need to be
held off until a "safe" moment, but I haven't yet been able to find a
way to determine when that is.

In any case the most likely definition of a Haskell finaliser is just
a call into C-land anyway, so cut out the middle-man is what I say!

Regards,
    Malcolm