bug#46767: /run/booted-system can be removed by ‘guix system delete-generations’

["Ludovic Courtès" <ludo@gnu.org>]

Before rebooting, I had:

--8<---------------cut here---------------start------------->8---
$ ls -l /run/{current,booted}-system 
lrwxrwxrwx 1 root root 33 Nov  2 16:06 /run/booted-system -> /var/guix/profiles/system-68-link
lrwxrwxrwx 1 root root 50 Feb 21 01:34 /run/current-system -> /gnu/store/qq4rz2fprvnsgqhj24v735hhmp189jl8-system
--8<---------------cut here---------------end--------------->8---

After rebooting:

--8<---------------cut here---------------start------------->8---
$ ls -l /run/{current,booted}-system 
lrwxrwxrwx 1 root root 33 Feb 25 10:28 /run/booted-system -> /var/guix/profiles/system-86-link
lrwxrwxrwx 1 root root 33 Feb 25 10:28 /run/current-system -> /var/guix/profiles/system-86-link
--8<---------------cut here---------------end--------------->8---

/run/booted-system is symlinked from /run/current-system in
‘shepherd-boot-gexp’:

--8<---------------cut here---------------start------------->8---
(define (shepherd-boot-gexp config)
  "Return a gexp starting the shepherd service."
  (let ((shepherd (shepherd-configuration-shepherd config))
        (services (shepherd-configuration-services config)))
  #~(begin
      ;; Keep track of the booted system.
      (false-if-exception (delete-file "/run/booted-system"))
      (symlink (readlink "/run/current-system")
               "/run/booted-system")
      …)))
--8<---------------cut here---------------end--------------->8---

So the solution is to make sure /run/current-system always points to the
store item rather than to the /var/guix symlink in the first place.

/run/current-system is created from (gnu build activation).  When
reconfiguring or deploying, the symlink points to $GUIX_NEW_SYSTEM,
which is set to the store item in (guix scripts system reconfigure).

But when booting, /run/current-system is symlinked to the ‘--system’
kernel command-line argument, which is /var/guix/….  To address that, we
need to throw a ‘canonicalize-path’ call.

Done in 412e4f081e9cdf38db9859e1548ef2362cde678e.

Ludo’.