bug#63190: [Shepherd] Nested calls lead to a hang

[Bruno Victal]

Original discussion (IRC): 
<https://logs.guix.gnu.org/guix/2023-04-29.log#180735>


Minimal example (annotated):

test-system.scm:
--8<---------------cut here---------------start------------->8---
(use-modules (gnu)
             (gnu tests)
             (gnu packages)
             (gnu packages base) ; coreutils/sleep
             (gnu packages admin)  ; shepherd
             (gnu services shepherd))

;; Some dummy service whose start action simply waits for some seconds,
;; about enough to check with herd status before it exits.
(define dummy-service-type
  (shepherd-service-type
   'dummy
   (lambda (cfg)
     (shepherd-service
      (documentation "Dummy action to start service.")
      (provision '(dummy-service))
      (respawn? #f)    ; <<<<<< note, this disables the service!
      (modules (cons* '(gnu services herd)
                      %default-modules))
      (start #~(lambda _
                 (format #t "Starting a delay on dummy service.~%")
                 (fork+exec-command (list #$(file-append coreutils "/bin/sleep")
                                          "30"))))
      (stop #~(make-kill-destructor))
      (actions
       (list (shepherd-action
              (name 'my-action)
              (documentation "lorem ipsum")
              (procedure
               #~(lambda (x)
                   ;; Scenario 1: using code from (gnu services herd), this 
hangs shepherd
                   #;(start-service 'dummy)  ; hangs shepherd
                   ;; Scenario 2: this doesn't hang shepherd but do note that 
the service has to be re-enabled either manually or automatically here
                   #;(system* #$(file-append shepherd "/bin/herd") "start" 
"dummy-service")
                   ;; Scenario 3: use the already imported (shepherd service) 
module, doesn't hang shepherd
                   ;;             Like Scenario 2, the service must be 
re-enabled since (respawn? #f) disabled this.
                   ;;             Comment: Won't re-enabling mean that this 
service will relaunch once it quits?
                   ;;                      That means the service has to 
disable itself on a successful exit, perhaps within the (stop ...) field?
                   (start 'dummy-service))))))))
   #f  ; no config
   (description "lorem ipsum.")))

(operating-system
  (inherit %simple-os)
  (services
   (cons*
    (service dummy-service-type)
    %base-services)))

--8<---------------cut here---------------end--------------->8---


Required modifications to gnu/services/shepherd.scm for scenario 1:

--8<---------------cut here---------------start------------->8---
diff --git a/gnu/services/shepherd.scm b/gnu/services/shepherd.scm
index b2601c0128..158806f421 100644
--- a/gnu/services/shepherd.scm
+++ b/gnu/services/shepherd.scm
@@ -282,7 +282,8 @@ (define (shepherd-service-file-name service)
 
 (define (shepherd-service-file service)
   "Return a file defining SERVICE."
   (scheme-file (shepherd-service-file-name service)
-               (with-imported-modules %default-imported-modules
+               (with-imported-modules (cons '(gnu services herd)
+                                            %default-imported-modules)
                  #~(begin
                      (use-modules #$@(shepherd-service-modules service))
--8<---------------cut here---------------end--------------->8---