bug#28590: Weak tables in 2.2.2 grow indefinitely

[Ludovic Courtès]

Consider this program:

--8<---------------cut here---------------start------------->8---
(use-modules (ice-9 format))

(define loops 3000000)

(define table
  (make-weak-key-hash-table))

(let loop ((i loops))
  (unless #f ;(zero? i)
    (when (zero? (modulo i 100000))
      (format #t "heap-size: ~,2h MiB  table: ~s~%"
              (/ (assoc-ref (gc-stats) 'heap-size) (expt 2. 20))
              table))

    (hashq-set! table (cons 1 2) #t)
    (loop (1- i))))
--8<---------------cut here---------------end--------------->8---

On 2.0.14, the heap size stays at around 24 MiB, and the table size is
stable at 224,717 buckets (?).

On 2.2.2, the heap grows indefinitely (though logarithmically).  It’s
not deterministic though: sometimes the heap size stabilizes in the
140–300 MiB range, and sometimes it keeps growing endlessly even though
the table size reaches a maxium at 7,190,537 entries.

Ludo’.