units: make system-update-pre.target a passive unit (#9349)

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.

1 files changed, 9 insertions, 6 deletions

diff --git a/man/systemd.special.xml b/man/systemd.special.xml
index 2776276a1d..fb12805fff 100644
--- a/man/systemd.special.xml
+++ b/man/systemd.special.xml
@@ -78,6 +78,7 @@
     <filename>swap.target</filename>,
     <filename>sysinit.target</filename>,
     <filename>system-update.target</filename>,
+    <filename>system-update-pre.target</filename>,
     <filename>time-sync.target</filename>,
     <filename>timers.target</filename>,
     <filename>umount.target</filename>,
@@ -658,6 +659,7 @@
       </varlistentry>
       <varlistentry>
         <term><filename>system-update.target</filename></term>
+        <term><filename>system-update-pre.target</filename></term>
         <term><filename>system-update-cleanup.service</filename></term>
         <listitem>
           <para>A special target unit that is used for offline system updates.
@@ -667,12 +669,13 @@
           <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
           </para>
 
-          <para>Updates should happen before the <filename>system-update.target</filename> is
-          reached, and the services which implement them should cause the machine to reboot. As
-          a safety measure, if this does not happen, and <filename>/system-update</filename>
-          still exists after <filename>system-update.target</filename> is reached,
-          <filename>system-update-cleanup.service</filename> will remove this symlink and
-          reboot the machine.</para>
+          <para>Updates should happen before the <filename>system-update.target</filename> is reached, and the services
+          which implement them should cause the machine to reboot. The main units executing the update should order
+          themselves after <filename>system-update-pre.target</filename> but not pull it in. Services which want to run
+          during system updates only, but before the actual system update is executed should order themselves before
+          this unit and pull it in. As a safety measure, if this does not happen, and
+          <filename>/system-update</filename> still exists after <filename>system-update.target</filename> is reached,
+          <filename>system-update-cleanup.service</filename> will remove this symlink and reboot the machine.</para>
         </listitem>
       </varlistentry>
       <varlistentry>