units: add time-set.target

time-sync.target is supposed to indicate system clock is synchronized
with a remote clock, but as used through 241 it only provided a system
clock that was updated based on a locally-maintained timestamp.  Systems
that are powered off for extended periods would not come up with
accurate time.

Retain the existing behavior using a new time-set.target leaving
time-sync.target for cases where accuracy is required.

Closes #8861

man/systemd.special.xml

@@ -77,6 +77,7 @@
     <filename>sysinit.target</filename>,
     <filename>system-update.target</filename>,
     <filename>system-update-pre.target</filename>,
+    <filename>time-set.target</filename>,
     <filename>time-sync.target</filename>,
     <filename>timers.target</filename>,
     <filename>umount.target</filename>,
@@ -950,6 +951,18 @@
             the <literal>$portmap</literal> facility.</para>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term><filename>time-set.target</filename></term>
+          <listitem>
+            <para>Services responsible for setting the system clock from
+            a local source (such as a maintained timestamp file or
+            imprecise real-time clock) should pull in this target and
+            order themselves before it.  Services where approximate time
+            is desired should be ordered after this unit, but not pull
+            it in.  This target does not provide the accuracy guarantees
+            of <filename>time-sync.target</filename>.</para>
+          </listitem>
+        </varlistentry>
         <varlistentry>
           <term><filename>time-sync.target</filename></term>
           <listitem>

units/meson.build

@@ -104,6 +104,7 @@ units = [
          'sockets.target.wants/'],
         ['systemd-udevd-kernel.socket',         '',
          'sockets.target.wants/'],
+        ['time-set.target',                     ''],
         ['time-sync.target',                    ''],
         ['timers.target',                       ''],
         ['tmp.mount',                           '',

units/systemd-timesyncd.service.in

@@ -14,9 +14,9 @@ ConditionCapability=CAP_SYS_TIME
 ConditionVirtualization=!container
 DefaultDependencies=no
 After=systemd-remount-fs.service systemd-sysusers.service
-Before=time-sync.target sysinit.target shutdown.target
+Before=time-set.target sysinit.target shutdown.target
 Conflicts=shutdown.target
-Wants=time-sync.target
+Wants=time-set.target time-sync.target
 
 [Service]
 AmbientCapabilities=CAP_SYS_TIME

units/systemd-tmpfiles-clean.service.in

@@ -12,7 +12,7 @@ Description=Cleanup of Temporary Directories
 Documentation=man:tmpfiles.d(5) man:systemd-tmpfiles(8)
 DefaultDependencies=no
 Conflicts=shutdown.target
-After=local-fs.target time-sync.target
+After=local-fs.target time-set.target
 Before=shutdown.target
 
 [Service]

units/time-set.target

@@ -0,0 +1,13 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=System Time Set
+Documentation=man:systemd.special(7)
+RefuseManualStart=yes

units/time-sync.target

@@ -11,3 +11,5 @@
 Description=System Time Synchronized
 Documentation=man:systemd.special(7)
 RefuseManualStart=yes
+After=time-set.target
+Wants=time-set.target