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953bf4604f
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).
181 lines
8.8 KiB
XML
181 lines
8.8 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2013 Lennart Poettering
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Copyright 2016 Zbigniew Jędrzejewski-Szmek
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="systemd.offline-updates">
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<refentryinfo>
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<title>systemd.offline-updates</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.offline-updates</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.offline-updates</refname>
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<refpurpose>Implementation of offline updates in systemd</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Implementing Offline System Updates</title>
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<para>This man page describes how to implement "offline" system updates with systemd. By "offline"
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OS updates we mean package installations and updates that are run with the system booted into a
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special system update mode, in order to avoid problems related to conflicts of libraries and
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services that are currently running with those on disk. This document is inspired by this
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<ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>.
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</para>
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<para>The logic:</para>
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<orderedlist>
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<listitem>
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<para>The package manager prepares system updates by downloading all (RPM or DEB or
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whatever) packages to update off-line in a special directory
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<filename noindex="true">/var/lib/system-update</filename> (or
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another directory of the package/upgrade manager's choice).</para>
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</listitem>
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<listitem>
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<para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
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created that points to <filename noindex="true">/var/lib/system-update</filename> (or
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wherever the directory with the upgrade files is located) and the system is rebooted. This
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symlink is in the root directory, since we need to check for it very early at boot, at a
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time where <filename>/var</filename> is not available yet.</para>
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</listitem>
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<listitem>
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<para>Very early in the new boot
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<citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
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this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
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<filename>system-update.target</filename>, a special target that is pulls in the base system
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(i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
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else) and the system update units.</para>
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</listitem>
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<listitem>
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<para>The system now continues to boot into <filename>default.target</filename>, and
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thus into <filename>system-update.target</filename>. This target pulls in all system
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update units. Only one service should perform an update (see the next point), and all
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the other ones should exit cleanly with a "success" return code and without doing
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anything. Update services should be ordered after <filename>sysinit.target</filename>
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so that the update starts after after all file systems have been mounted.</para>
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</listitem>
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<listitem>
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<para>As the first step, an update service should check if the
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<filename>/system-update</filename> symlink points to the location used by that update
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service. In case it does not exist or points to a different location, the service must exit
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without error. It is possible for multiple update services to be installed, and for multiple
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update services to be launched in parallel, and only the one that corresponds to the tool
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that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
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is unsafe to run multiple updates in parallel.</para>
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</listitem>
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<listitem>
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<para>The update service should now do its job. If applicable and possible, it should
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create a file system snapshot, then install all packages. After completion (regardless
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whether the update succeeded or failed) the machine must be rebooted, for example by
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calling <command>systemctl reboot</command>. In addition, on failure the script should
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revert to the old file system snapshot (without the symlink).</para>
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</listitem>
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<listitem>
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<para>The upgrade scripts should exit only after the update is finished. It is expected
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that the service which performs the upgrade will cause the machine to reboot after it
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is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
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all update services have run, and the <filename>/system-update</filename> symlink still
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exists, it will be removed and the machine rebooted as a safety measure.</para>
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</listitem>
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<listitem>
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<para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
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the generator won't redirect <filename>default.target</filename> anymore and the system
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now boots into the default target again.</para>
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</listitem>
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</orderedlist>
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</refsect1>
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<refsect1>
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<title>Recommendations</title>
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<orderedlist>
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<listitem>
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<para>To make things a bit more robust we recommend hooking the update script into
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<filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename>
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symlink in the distribution package, rather than depending on <command>systemctl
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enable</command> in the postinst scriptlets of your package. More specifically, for your
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update script create a .service file, without [Install] section, and then add a symlink like
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<filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename>
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→ <filename noindex='true'>../foobar.service</filename> to your package.</para>
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</listitem>
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<listitem>
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<para>Make sure to remove the <filename>/system-update</filename> symlink as early as
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possible in the update script to avoid reboot loops in case the update fails.</para>
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</listitem>
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<listitem>
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<para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
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to ensure that a reboot is automatically triggered if the update fails.
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<varname>FailureAction=</varname> makes sure that the specified unit is activated if your
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script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds
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you should trigger the reboot in your own code, for example by invoking logind's
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<command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
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<ulink url="http://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
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for details.</para>
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</listitem>
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<listitem>
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<para>The update service should declare <varname>DefaultDependencies=false</varname>,
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<varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
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and explicitly pull in any other services it requires.</para>
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</listitem>
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</orderedlist>
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</refsect1>
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<refsect1>
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<title>See also</title>
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<para>
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<ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/">Implementing Offline System Updates</ulink>,
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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