реализация офлайн-обновлений в systemd (Implementation of offline updates in systemd)
Имя (Name)
systemd.offline-updates - Implementation of offline updates in
systemd
IMPLEMENTING OFFLINE SYSTEM UPDATES
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.
RECOMMENDATIONS
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
/usr/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 /usr/lib/systemd/system-update.target.wants .
Смотри также (See also)
systemd(1), systemd.generator(7),
systemd-system-update-generator(8), dnf.plugin.system-upgrade
(8)
Примечание (Note)
1. GNOME design whiteboard
https://wiki.gnome.org/Design/OS/SoftwareUpdates