bug#32336: herd stop mcron yields instant shutdown of computer

[Tonton]

Triggering action:
from bash: sudo herd stop mcron

Result: Exactly the same behaviour as when I call halt/poweroff/reboot from
bash. Except one of the messages clearly seen midscreen on the TTY is
shepherds message that mcron has been stopped. It then scrolls off as the
system is doing it's regular shutdown routine. And reboots, ending at the
normal login screen.

Expected result: mcron service stopped with maybe a message to stdout or log.
System still running.

Guix pull was at:
Generation 9    Jul 28 2018 22:59:26
  guix 48d7ac1
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: 48d7ac175f69fea587eaa0358eddb5c76205e8ad

The configuration on the host at the time can be found at:
https://notabug.org/thomassgn/guixsd-configuration/src/e8404bd617547b41d57bceaaa880ca8109360e03/config.scm
(I'm guessing most of it can be stripped out, I'll see if I can reproduce in
a minimal os)

I was implementing the battery-check job for mcron (line 71). Which I know
does not work as is in the config above. It has since been improved.


Here is /var/log/messages around the time (removed ip's and some ntpd lines
because ip's):

750117:Jul 31 18:24:55 localhost shepherd[1]: Service mcron has been started. 
750118:Jul 31 18:24:55 localhost shepherd[1]: Service mcron has been disabled. 
750119:Jul 31 18:24:55 localhost shepherd[1]:   (Respawning too fast.) 
750172:Jul 31 18:37:42 localhost shepherd[1]: Enabled service mcron. 
750173:Jul 31 18:37:42 localhost shepherd[1]: Service mcron has been started. 
750174:Jul 31 18:37:42 localhost shepherd[1]: Respawning mcron. 
750175:Jul 31 18:37:42 localhost shepherd[1]: Service mcron has been started. 
750176:Jul 31 18:37:42 localhost shepherd[1]: Respawning mcron. 
750177:Jul 31 18:37:42 localhost shepherd[1]: Service mcron has been started.  
750178:Jul 31 18:38:20 localhost shepherd[1]: [No documentation.] 
750203:Jul 31 18:43:54 localhost shepherd[1]: Service mcron has been stopped. 
750204:Jul 31 18:43:54 localhost shepherd[1]: Exiting shepherd... 
750205:Jul 31 18:43:54 localhost shepherd[1]: Service console-font-tty2 has 
been stopped. 
750206:Jul 31 18:43:54 localhost shepherd[1]: Service term-tty2 has been 
stopped. 
750207:Jul 31 18:43:54 localhost shepherd[1]: Service cups has been stopped. 
750208:Jul 31 18:43:54 localhost shepherd[1]: Service rpcbind-daemon has been 
stopped. 
750209:Jul 31 18:43:54 localhost avahi-daemon[833]: Got SIGTERM, quitting.
750210:Jul 31 18:43:54 localhost shepherd[1]: Service avahi-daemon has been 
stopped. 
750211:Jul 31 18:43:54 localhost avahi-daemon[833]: Leaving mDNS multicast 
group on interface wlp0s20u2.IPv6 with address
750212:Jul 31 18:43:54 localhost avahi-daemon[833]: Leaving mDNS multicast 
group on interface wlp0s20u2.IPv4 with address
750213:Jul 31 18:43:54 localhost ntpd[827]: ntpd exiting on signal 15 
(Terminated)
750218:Jul 31 18:43:54 localhost shepherd[1]: Service ntpd has been stopped. 
750219:Jul 31 18:43:54 localhost connmand[826]: Terminating

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