This is an additional synchronization point normally not needed. Hence,
let's make it passive, i.e. pull it in from the unit which wants to be
ordered before the update service rather than by the update service
itself.
systemd offline-updates allows dropping multiple system update units
to be added to system-update.target.wants.
As documented in systemd.offline-updates(7) only 1 of these units
should actually be active (based on the /system-update symlink) and
when that unit is done it should reboot the system.
In some cases it is desirable to run a unit whenever booting in
offline-updates mode indepedent of which update unit is going to
handle the update. One example of this is integration with bootloader
code which checks if the previous boot was succesful.
Since the active unit will reboot the system when it is done, there
is no guarantee that adding such a unit to system-update.target.wants
will get it executed always.
This commit adds a system-update-pre.target which can be used for
units which should always run when booting in offline-updates mode.
Note: the name is "system-update-cleanup.service" rather than
"system-update-done.service", because it should not run normally, and also
because there's already "systemd-update-done.service", and having them named
so similarly would be confusing.
In https://bugzilla.redhat.com/show_bug.cgi?id=1395686 the system repeatedly
entered system-update.target on boot. Because of a packaging issue, the tool
that created the /system-update symlink could be installed without the service
unit that was supposed to perform the upgrade (and remove the symlink). In
fact, if there are no units in system-update.target, and /system-update symlink
is created, systemd always "hangs" in system-update.target. This is confusing
for users, because there's no feedback what is happening, and fixing this
requires starting an emergency shell somehow, and also knowing that the symlink
must be removed. We should be more resilient in this case, and remove the
symlink automatically ourselves, if there are no upgrade service to handle it.
This adds a service which is started after system-update.target is reached and
the symlink still exists. It nukes the symlink and reboots the machine. It
should subsequently boot into the default default.target.
This is a more general fix for
https://bugzilla.redhat.com/show_bug.cgi?id=1395686 (the packaging issue was
already fixed).
- use "service" instead of "script", because various offline updaters that we have
aren't really scripts, e.g. dnf-plugin-system-upgrade, packagekit-offline-update,
fwupd-offline-update.
- strongly recommend After=sysinit.target, Wants=sysinit.target
- clarify a bit what should happen when multiple update services are started
- replace links to the wiki with refs to the man page that replaced it.
Yu Watanabe