bug#39708: nfs-service-type broken: rpc.mountd respawns too quickly

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

The rpc.mountd service, one of the required services for
nfs-service-type, respawns the process quickly until disabled.

Opening the service file,
/ssh:hurd:/gnu/store/qq1dd9rqa2svxhbkpiq6bkpg6dyxwzqb-shepherd-rpc.mountd.scm:

--8<---------------cut here---------------start------------->8---
(eval-when
    (expand load eval)
  (let
      ((extensions
	(quote
	 ()))
       (prepend
	(lambda
	    (items lst)
	  (let loop
	      ((items items)
	       (lst lst))
	    (if
	     (null? items)
	     lst
	     (loop
	      (cdr items)
	      (cons
	       (car items)
	       (delete
		(car items)
		lst))))))))
    (set! %load-path
      (prepend
       (cons "/gnu/store/wy6fvwp3dfrp9frckdascrrb9rvn3h3g-module-import"
	     (map
	      (lambda
		  (extension)
		(string-append extension "/share/guile/site/"
			       (effective-version)))
	      extensions))
       %load-path))
    (set! %load-compiled-path
      (prepend
       (cons "/gnu/store/xd4lwx10ljn902712sphac3k0k97g1vy-module-import-compiled"
	     (map
	      (lambda
		  (extension)
		(string-append extension "/lib/guile/"
			       (effective-version)
			       "/site-ccache"))
	      extensions))
       %load-compiled-path))))
(begin
  (use-modules
   (shepherd service)
   (oop goops)
   (guix build utils)
   (guix build syscalls))
  (make <service> #:docstring
	(quote "Run the NFS mountd daemon.")
	#:provides
	(quote
	 (rpc.mountd))
	#:requires
	(quote
	 (rpc.statd))
	#:one-shot?
	(quote #f)
	#:respawn?
	(quote #t)
	#:start
	(make-forkexec-constructor
	 (list "/gnu/store/c7kpr1jh5z3mrkz0yw9am86851y57cq7-nfs-utils-2.4.2/sbin/rpc.mountd" "--debug" "all"))
	#:stop
	(make-kill-destructor)
	#:actions
	(make-actions)))
--8<---------------cut here---------------end--------------->8---

The process is launched with the equivalent of:

$ /gnu/store/c7kpr1jh5z3mrkz0yw9am86851y57cq7-nfs-utils-2.4.2/sbin/rpc.mountd \
    --debug all

Which doesn't print any output, but returns cleanly.  The process is
daemonized and runs in the background.

Attempting the equivalent using Shepherd yields the same:

scheme@(guix-user)> ,m (shepherd service)
;;; WARNING: loading compiled file /run/current-system/profile/lib/guile/2.2/site-ccache/shepherd/service.go failed:
;;; In procedure load-thunk-from-memory: incompatible bytecode kind

[...]

scheme@(shepherd service)> (make-forkexec-constructor (list "/gnu/store/c7kpr1jh5z3mrkz0yw9am86851y57cq7-nfs-utils-2.4.2/sbin/rpc.mountd" "--debug" "all"))
$1 = #<procedure 7f472f0dbca0 at ice-9/eval.scm:383:13 rest>
scheme@(shepherd service)> ($1)
$2 = 19655

We get back the PID of the rpc.mountd process, and it lives on.

So, I wonder what is causing Shepherd to respawn the process?

Maxim