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’.
