Adding upstream version 252.22.upstream/252.22upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 163 insertions, 0 deletions

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd.offline-updates">
+  <refentryinfo>
+    <title>systemd.offline-updates</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd.offline-updates</refentrytitle>
+    <manvolnum>7</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd.offline-updates</refname>
+    <refpurpose>Implementation of offline updates in systemd</refpurpose>
+  </refnamediv>
+
+  <refsect1>
+    <title>Implementing Offline System Updates</title>
+
+    <para>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
+    <ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>.
+    </para>
+
+    <para>The logic:</para>
+
+    <orderedlist>
+      <listitem>
+        <para>The package manager prepares system updates by downloading all (.rpm or .deb or
+        whatever) packages to update off-line in a special directory
+        <filename index="false">/var/lib/system-update</filename> (or
+        another directory of the package/upgrade manager's choice).</para>
+      </listitem>
+
+      <listitem>
+        <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
+        created that points to <filename index="false">/var/lib/system-update</filename> (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 <filename>/var/</filename> is not available yet.</para>
+      </listitem>
+
+      <listitem>
+        <para>Very early in the new boot
+        <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+        checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
+        this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
+        <filename>system-update.target</filename>, a special target that pulls in the base system
+        (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
+        else) and the system update units.</para>
+      </listitem>
+
+      <listitem>
+        <para>The system now continues to boot into <filename>default.target</filename>, and
+        thus into <filename>system-update.target</filename>. 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 <filename>sysinit.target</filename>
+        so that the update starts after all file systems have been mounted.</para>
+      </listitem>
+
+      <listitem>
+        <para>As the first step, an update service should check if the
+        <filename>/system-update</filename> 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 <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
+        is unsafe to run multiple updates in parallel.</para>
+      </listitem>
+
+      <listitem>
+        <para>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 <command>systemctl reboot</command>. In addition, on failure the script should
+        revert to the old file system snapshot (without the symlink).</para>
+      </listitem>
+
+      <listitem>
+        <para>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 <filename>system-update.target</filename> is successfully reached, i.e.
+        all update services have run, and the <filename>/system-update</filename> symlink still
+        exists, it will be removed and the machine rebooted as a safety measure.</para>
+      </listitem>
+
+      <listitem>
+        <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
+        the generator won't redirect <filename>default.target</filename> anymore and the system
+        now boots into the default target again.</para>
+      </listitem>
+    </orderedlist>
+  </refsect1>
+
+  <refsect1>
+    <title>Recommendations</title>
+
+    <orderedlist>
+      <listitem>
+        <para>To make things a bit more robust we recommend hooking the update script into
+        <filename>system-update.target</filename> via a <filename index="false">.wants/</filename>
+        symlink in the distribution package, rather than depending on <command>systemctl
+        enable</command> 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
+        <filename index="false">/usr/lib/systemd/system/system-update.target.wants/foobar.service</filename>
+        → <filename index="false">../foobar.service</filename> to your package.</para>
+      </listitem>
+
+      <listitem>
+        <para>Make sure to remove the <filename>/system-update</filename> symlink as early as
+        possible in the update script to avoid reboot loops in case the update fails.</para>
+      </listitem>
+
+      <listitem>
+        <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
+        to ensure that a reboot is automatically triggered if the update fails.
+        <varname>FailureAction=</varname> 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
+        <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
+        <citerefentry><refentrytitle>org.freedesktop.login1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for details about the logind D-Bus API.</para>
+      </listitem>
+
+      <listitem>
+        <para>The update service should declare <varname>DefaultDependencies=no</varname>,
+        <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
+        <varname>After=system-update-pre.target</varname>, <varname>Before=system-update.target</varname>
+        and explicitly pull in any other services it requires.</para>
+      </listitem>
+
+      <listitem>
+        <para>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
+        <varname>Wants=system-update-pre.target</varname> and
+        <varname>Before=system-update-pre.target</varname> and add a symlink
+        to that file under
+        <filename index="false">/usr/lib/systemd/system-update.target.wants</filename>
+        .</para>
+      </listitem>
+    </orderedlist>
+  </refsect1>
+
+  <refsect1>
+    <title>See also</title>
+
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+</refentry>