Subject: bug#10668: closed (Re: bug#10668: FFI: ‘set-pointer-finalizer!’overrides the previous finalizer)

From: [email-masked] (GNU bug Tracking System)
Date: Fri, 06 Jul 2012 18:32:02 +0000

Your bug report

#10668: FFI: ‘set-pointer-finalizer!’ overrides the previous finalizer

which was filed against the guile package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to [email-masked].

-- 
10668: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10668
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: bug#10668: FFI: ‘set-pointer-finalizer!’ overrides the previous finalizer
From: Andy Wingo <[email-masked]>
To: [email-masked] (Ludovic Courtès)
Cc: [email-masked]
Date: Fri, 06 Jul 2012 20:26:20 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

On Mon 30 Jan 2012 23:31, [email-masked] (Ludovic Courtès) writes:

> 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

This has worked since 75ba64d6797f5857cc9885eb753126119a8c8b68.

Andy
-- 
http://wingolog.org/

Subject: FFI: ‘set-pointer-finalizer!’ overrides the previous finalizer
From: [email-masked] (Ludovic Courtès)
To: [email-masked]
Date: Mon, 30 Jan 2012 23:31:59 +0100
List-Id: <debbugs-submit.debbugs.gnu.org>

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’.