bug#71144: Interactive prompt opened upon shepherd config file error

["Ludovic Courtès" <ludovic.courtes@inria.fr>]

Hello,

One problem we noticed in the analysis of the boot problem of bayfront
after the recent downtime¹ is that an interactive REPL would be opened
after an unbound variable was found in the shepherd config file:

--8<---------------cut here---------------start------------->8---
[   13.098907] shepherd[1]: Service root started.
[   13.100711] shepherd[1]: Service root running with value #t.
[   13.103824] shepherd[1]: Service root has been started.
[   13.426102] shepherd[1]: ice-9/boot-9.scm:1685:16: In procedure raise-exception:
[   13.428099] shepherd[1]: Unbound variable: make-forkexec-constructor/container
[   13.429912] shepherd[1]: 
[   13.431108] shepherd[1]: Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
[   13.441983] shepherd[1]: GNU Guile 3.0.9
[   13.442728] shepherd[1]: Copyright (C) 1995-2023 Free Software Foundation, Inc.
[   13.443947] shepherd[1]: 
[   13.444427] shepherd[1]: Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
[   13.445679] shepherd[1]: This program is free software, and you are welcome to redistribute it
[   13.446919] shepherd[1]: under certain conditions; type `,show c' for 
[   13.447072] shepherd[1]: details.
[   13.448737] shepherd[1]: 
[   13.449239] shepherd[1]: Enter `,help' for help.
--8<---------------cut here---------------end--------------->8---

This was unhelpful because we couldn’t interact with that REPL remotely
(no IPMI).  Even when you can interact, it’s of limited use; in this
case, if you type “,q”, it tries to continue and fails:

--8<---------------cut here---------------start------------->8---
Uncaught exception in task:
In fibers.scm:
    172:8  7 (_)
In ice-9/exceptions.scm:
   406:15  6 (_)
In ice-9/boot-9.scm:
  1752:10  5 (with-exception-handler _ _ #:unwind? _ # _)
In shepherd/service.scm:
   824:39  4 (_)


this is because we’re effectively adding #f in the middle of the list
passed to ‘register-services’ (see below).

This REPL-on-error “feature” comes from Guix System, not Shepherd, in
the config file generated from (gnu services shepherd):

    ;; Arrange to spawn a REPL if something goes wrong.  This is better
    ;; than a kernel panic.
    (call-with-error-handling
      (lambda ()
        (register-services
         (parameterize ((current-warning-port
                         (%make-void-port "w")))
           (map (lambda (file)
                  (save-module-excursion
                   (lambda ()
                     (set-current-module (make-user-module))
                     (load-compiled file))))
                '#$(map scm->go files))))))

The rationale mentioned in the comment no longer holds: starting from
Shepherd 0.10.2, the config file is loaded in the background; if it’s
evaluation fails, shepherd keeps running (see
‘tests/config-failure.sh’, which tests this behavior).

I think we should change the above to log and gracefully handle failure
to load an individual service file.

Ludo’.

¹ https://lists.gnu.org/archive/html/info-guix/2024-05/msg00000.html