bug#70051: guix system hangs on boot with LUKS /home partition

[Fulbert]

Hello,

Up to guix 9b84b36, my system was properly booting with a LUKS2 partition
mounted on /home. Starting with guix d5f857a (22 mar 2024), the boot hangs on
the same system using the same configuration.scm file. The only way out I found
when it hangs is hardware shutdown. There are no avaible console nor ssh server
started to help troubleshoot and there is nothing written to /var/log/messages
when it hangs.

I have tried to transfer my /home data to a brand new LUKS1 partition, (as well
as removing pointers to the old LUKS2 partition in my config.scm, of course) and
the problem remains exactly the same, including those error messages (obtained 
with
a video capture of the screen at boot, after removing 'quiet' from the kernel
command line in grub) :

#+begin_src boot
shepherd[1]: Starting service device-mapping-luks-homes...
shepherd[1]: Service device-mapping-luks-homes failed to start.
shepherd[1]: Exception caught while while starting device-mapping-luks-homes: 
(unbound-variable #f "Unbound variable: "S" (bytevector?) #f)
#+end_src

Maybe it's worth mentionning that I have then tried one configuration of the
'mapped-device' with 'luks-device-mapping' and another one with
'luks-device-mapping-with-options #:keyfile "/…"'. I also tried one
configuration with the 'source' declared in plain "/dev/..." and another one
declared with the luks '(uuid "…")', but this didnt change anything to the
"symptoms".

So, although I have learned in the process that LUKS2 is not yet fully
supported in guix, this problem also prevents booting using a LUKS1 /home
partition in my case.

Transfering the /home data to a clear (unencrypted) partition is my current
workaround to this problem.

Below is the configuration that has worked for several weeks, if not months, 
using my LUKS2 /home :

  (mapped-devices
    (list
      (mapped-device
        (source (uuid "<the uuid>"))
        (target "luks-homes")
        (type luks-device-mapping))))

  (file-systems
    (append
      (list
        […]
        (file-system (mount-point "/home")
                     (device (file-system-label "luks-homes"))
                     (type "ext4")
                     (dependencies mapped-devices))
        […]

Best regards and thanks for guix !