‘set-cdr!’ and weak-cdr pairs

[Ludovic Courtès]

Hello,

A bit of explanation for commit
ca33b501a93f8de389c1e3e1bc987f63b6912029...

Try this:

--8<---------------cut here---------------start------------->8---
(use-modules (srfi srfi-1)
             (srfi srfi-9))

(define-record-type <foo>
  (make-foo x)
  foo?
  (x foo-x))

(define register!
  (let ((t (make-weak-value-hash-table 10)))
    (lambda (x)
      (let ((k+v (hash-create-handle! t x #f)))
        (or (cdr k+v)
            (let ((o (make-foo x)))
              (set-cdr! k+v o)
              o))))))

(every (lambda (x)
         (make-foo #f)
         (let ((o (register! x)))
           (or (foo? o)
               (pk 'bad! o))))
       (circular-list 1 2 3 4 5 6 7 8 9))
--8<---------------cut here---------------end--------------->8---

Eventually ‘(foo? o)’ fails and Guile segfaults while trying to display O.

Changing ‘register!’ to the following works:

--8<---------------cut here---------------start------------->8---
(define register!
  (let ((t (make-weak-value-hash-table 10)))
    (lambda (x)
      (or (hash-ref t x)
          (let ((o (make-foo x)))
            (hash-set! t x o)
            o)))))
--8<---------------cut here---------------end--------------->8---

The problem is that ‘hash-create-handle!’ above created a weak-cdr
pair—i.e., a pair whose cdr is /not/ scanned for pointers—but ‘set-cdr!’
did not register a disappearing link from O to K+V.  Consequently, O
eventually gets collected, but K+V remains; the storage of O then gets
reused, and the cdr of K+V ends up containing either an unrelated or >an
invalid Scheme object.

This problem is explicitly addressed in ‘scm_hash_fn_set_x’.  AFAICS
‘set-cdr!’ has no way of knowing whether its passed a normal pair or a
weak-cdr one, so it cannot be changed to handle weak-cdr pairs
gracefully.

And of course, we have the same problem with weak-car pairs and
‘set-car!’, but ‘set-car!’ is unlikely to be used on weak-car pairs.

Thoughts?

Ludo’.