bug#68087: Signal handlers not called after ‘primitive-fork’

In 3.0.9 and current ‘main’, I get this:

--8<---------------cut here---------------start------------->8---
$ cat sigaction-fork.scm
(use-modules (ice-9 match))

;; This call spawns the signal delivery thread as a side effect.
(sigaction SIGALRM
  (lambda (signal)
    (pk 'got-signal! signal)))

(match (primitive-fork)
  (0
   (pk 'child (getpid))
   (sigaction SIGALRM
     (lambda (signal)
       ;; This handler is never called!
       (pk 'got-signal-child! signal)))
   (kill 0 SIGALRM)
   (pk 'alarm-sent))
  (_
   (primitive-exit 0)))
$ guile sigaction-fork.scm

;;; (child 30308)

;;; (alarm-sent)
--8<---------------cut here---------------end--------------->8---

Everything works fine if we remove the pre-fork ‘sigaction’ call.

Ludo’.