systemd.offline-updates - Implementation of offline updates in
systemd
This man page describes how to implement "offline"
system updates with systemd. By "offline" OS updates we mean
package installations and updates that are run with the system booted into a
special system update mode, in order to avoid problems related to conflicts
of libraries and services that are currently running with those on disk.
This document is inspired by this GNOME design whiteboard[1].
The logic:
1.The package manager prepares system updates by
downloading all (.rpm or .deb or whatever) packages to update off-line in a
special directory /var/lib/system-update (or another directory of the
package/upgrade manager's choice).
2.When the user OK'ed the update, the symlink
/system-update is created that points to /var/lib/system-update (or wherever
the directory with the upgrade files is located) and the system is rebooted.
This symlink is in the root directory, since we need to check for it very
early at boot, at a time where /var/ is not available yet.
3.Very early in the new boot
systemd-system-update-generator(8) checks whether /system-update
exists. If so, it (temporarily and for this boot only) redirects (i.e.
symlinks) default.target to system-update.target, a special target that pulls
in the base system (i.e. sysinit.target, so that all file systems are mounted
but little else) and the system update units.
4.The system now continues to boot into default.target,
and thus into system-update.target. This target pulls in all system update
units. Only one service should perform an update (see the next point), and all
the other ones should exit cleanly with a "success" return code and
without doing anything. Update services should be ordered after sysinit.target
so that the update starts after all file systems have been mounted.
5.As the first step, an update service should check if
the /system-update symlink points to the location used by that update service.
In case it does not exist or points to a different location, the service must
exit without error. It is possible for multiple update services to be
installed, and for multiple update services to be launched in parallel, and
only the one that corresponds to the tool that created the symlink
before reboot should perform any actions. It is unsafe to run multiple updates
in parallel.
6.The update service should now do its job. If
applicable and possible, it should create a file system snapshot, then install
all packages. After completion (regardless whether the update succeeded or
failed) the machine must be rebooted, for example by calling systemctl
reboot. In addition, on failure the script should revert to the old file
system snapshot (without the symlink).
7.The update scripts should exit only after the update
is finished. It is expected that the service which performs the update will
cause the machine to reboot after it is done. If the system-update.target is
successfully reached, i.e. all update services have run, and the
/system-update symlink still exists, it will be removed and the machine
rebooted as a safety measure.
8.After a reboot, now that the /system-update symlink is
gone, the generator won't redirect default.target anymore and the system now
boots into the default target again.
1.To make things a bit more robust we recommend hooking
the update script into system-update.target via a .wants/ symlink in the
distribution package, rather than depending on systemctl enable in the
postinst scriptlets of your package. More specifically, for your update script
create a .service file, without [Install] section, and then add a symlink like
/lib/systemd/system/system-update.target.wants/foobar.service →
../foobar.service to your package.
2.Make sure to remove the /system-update symlink as
early as possible in the update script to avoid reboot loops in case the
update fails.
3.Use
FailureAction=reboot in the service file
for your update script to ensure that a reboot is automatically triggered if
the update fails.
FailureAction= makes sure that the specified unit is
activated if your script exits uncleanly (by non-zero error code, or
signal/coredump). If your script succeeds you should trigger the reboot in
your own code, for example by invoking logind's
Reboot() call or
calling
systemctl reboot. See
org.freedesktop.login1(5) for
details about the logind D-Bus API.
4.The update service should declare
DefaultDependencies=no, Requires=sysinit.target,
After=sysinit.target, After=system-update-pre.target,
Before=system-update.target and explicitly pull in any other services
it requires.
5.It may be desirable to always run an auxiliary unit
when booting into offline-updates mode, which itself does not install updates.
To do this create a .service file with Wants=system-update-pre.target
and Before=system-update-pre.target and add a symlink to that file
under /lib/systemd/system-update.target.wants .
- 1.
- GNOME design whiteboard
https://wiki.gnome.org/Design/OS/SoftwareUpdates