systemd mount unit fails: "Mount process finished, but there is no mount."

[James Dingwall]

Upstream issue: https://github.com/systemd/systemd/issues/10872

The upstream issue is noted in bug #1772016 but that issue is talking about adding a workaround in snapd rather than fixing the root cause in systemd hence this report.

Our configuration is a vm environment with bionic dom0 and ~15 bionic guests. In around 1% of guest startups we get a boot failure (different guests, different mounts affected). I don't know if it matters but the mount unit is generated from /etc/fstab.

Aug 16 08:13:07 zsecprobe0000-0068-00-00 systemd[1]: var-log.mount: Mount process exited, code=exited status=32
Aug 16 08:13:07 zsecprobe0000-0068-00-00 systemd[1]: Failed unmounting /var/log.
-- Subject: Unit var-log.mount has finished shutting down
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- Unit var-log.mount has finished shutting down.

[FAILED] Failed to mount /var/log.
See 'systemctl status var-log.mount' for details.
[DEPEND] Dependency failed for Flush Journal to Persistent Storage.
[DEPEND] Dependency failed for Update UTMP about System Runlevel Changes.
[DEPEND] Dependency failed for Local File Systems.
[DEPEND] Dependency failed for Update UTMP about System Boot/Shutdown.