units: add first-boot-complete.target for first boot ordering

Add a new target for synchronizing units that wish to run once during
the first boot of the system.  The machine-id will be committed to disk
only after the target has been reached, thus ensuring that all units
ordered before it had a chance to complete.

man/systemd.special.xml

@@ -32,6 +32,7 @@
     <filename>emergency.target</filename>,
     <filename>exit.target</filename>,
     <filename>final.target</filename>,
+    <filename>first-boot-complete.target</filename>,
     <filename>getty.target</filename>,
     <filename>getty-pre.target</filename>,
     <filename>graphical.target</filename>,
@@ -878,6 +879,17 @@
             stopped.</para>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term><filename>first-boot-complete.target</filename></term>
+          <listitem>
+            <para>This passive target is intended as a synchronization point for units that need to run once
+            during the first boot.  Only after all units ordered before this target have finished, will the
+            <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+            be committed to disk, marking the first boot as completed.  If the boot is aborted at any time
+            before that, the next boot will re-run any units with <varname>ConditionFirstBoot=yes</varname>.
+            </para>
+          </listitem>
+        </varlistentry>
         <varlistentry>
           <term><filename>getty-pre.target</filename></term>
           <listitem>

units/first-boot-complete.target

@@ -0,0 +1,14 @@
+#  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=First Boot Complete
+Documentation=man:systemd.special(7)
+RefuseManualStart=yes
+ConditionFirstBoot=yes

units/meson.build

@@ -17,6 +17,7 @@ units = [
         ['emergency.target',                    ''],
         ['exit.target',                         ''],
         ['final.target',                        ''],
+        ['first-boot-complete.target',          ''],
         ['getty.target',                        '',
          'multi-user.target.wants/'],
         ['getty-pre.target',                    ''],

units/systemd-machine-id-commit.service

@@ -12,8 +12,8 @@ Description=Commit a transient machine-id on disk
 Documentation=man:systemd-machine-id-commit.service(8)
 DefaultDependencies=no
 Conflicts=shutdown.target
-Before=sysinit.target shutdown.target
-After=local-fs.target
+Before=shutdown.target
+After=local-fs.target first-boot-complete.target
 ConditionPathIsReadWrite=/etc
 ConditionPathIsMountPoint=/etc/machine-id