Subject: FFI: ‘set-pointer-finalizer!’ overrides the previous finalizer

From: [email-masked] (Ludovic Courtès)
Date: Mon, 30 Jan 2012 23:31:59 +0100

Hi,

The ‘set-pointer-finalizer!’ procedure is harmful because it erases any
previously set finalizer.

Said finalizer could come from a guardian:

   (define ptr (make-pointer 123))
   (define g (make-guardian))
   (g ptr)
   (set-pointer-finalizer! ptr (dynamic-func "scm_is_pair" (dynamic-link)))
   (set! ptr #f)
   (gc)

   (g)          ;; Should return PTR.
   => #f

or from a weak hash table:

   (define ptr (make-pointer 123))
   (define t (make-weak-value-hash-table))
   (hashq-set! t ptr #t)
   (set-pointer-finalizer! ptr (dynamic-func "scm_is_pair" (dynamic-link)))
   (set! ptr #f)
   (gc)
   (gc)

   (hash-fold alist-cons '() t)
   => ((#<pointer 0x7b> . #t))   ;; Should be the empty list.

There are several ways to fix this:

  1. Deprecate ‘set-pointer-finalizer!’ in favor of guardians, with the
     problem that it would be up to the user to actually call the
     guardians.

  2. Implement ‘set-pointer-finalizer!’ in terms of guardians, with a
     gc-hook to automatically call the guardians and invoke any
     finalizers.

  3. Fix ‘set-pointer-finalizer!’ to honor the previously-set finalizer,
     if any.  This means (a) finding a way to keep previous-finalizer
     info around, and (b) duplicating part of the (hairy) code from
     guardians.c to honor the previous finalizer.

None of these solutions is really pleasant.

Thoughts?

Thanks,
Ludo’.