systemd-machine-tags.service: apply machine-tags.d/ configuration at boot

The service runs "hostnamectl tags --apply" before sysinit.target, so that
the tags declared in the machine-tags.d/ directories are in place before
most units are started and may be matched by ConditionMachineTag=. It is
skipped if no configuration exists.

man/machine-tags.d.xml

@@ -46,6 +46,9 @@
     stored in <filename>/etc/machine-info</filename>, thus applying the same configuration repeatedly has no
     further effect.</para>
 
+    <para>During boot, the configuration is applied automatically by
+    <citerefentry><refentrytitle>systemd-machine-tags.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
+
     <para>This allows OS vendors, image builders and system extensions (see
     <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>) to
     declare tags in a drop-in directory hierarchy, instead of setting them imperatively.</para>
@@ -132,6 +135,7 @@ local.rack=7</programlisting>
       <member><citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
       <member><citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
       <member><citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
+      <member><citerefentry><refentrytitle>systemd-machine-tags.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
       <member><citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
     </simplelist></para>
   </refsect1>

man/rules/meson.build

@@ -1158,6 +1158,7 @@ manpages = [
  ['systemd-machine-id-commit.service', '8', [], ''],
  ['systemd-machine-id-setup', '1', [], ''],
  ['systemd-machine-tag@.service', '8', [], 'ENABLE_HOSTNAMED'],
+ ['systemd-machine-tags.service', '8', [], 'ENABLE_HOSTNAMED'],
  ['systemd-machined.service', '8', ['systemd-machined'], 'ENABLE_MACHINED'],
  ['systemd-makefs@.service',
   '8',

man/systemd-machine-tags.service.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0"?>
+<!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd-machine-tags.service" conditional='ENABLE_HOSTNAMED'
+          xmlns:xi="http://www.w3.org/2001/XInclude">
+
+  <refentryinfo>
+    <title>systemd-machine-tags.service</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd-machine-tags.service</refentrytitle>
+    <manvolnum>8</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd-machine-tags.service</refname>
+    <refpurpose>Apply the machine tags configuration at boot</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <para><filename>systemd-machine-tags.service</filename></para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><filename>systemd-machine-tags.service</filename> is an early boot service that applies the machine
+    tag configuration declared in the <filename>machine-tags.d/</filename> directories, see
+    <citerefentry><refentrytitle>machine-tags.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, by
+    invoking <command>hostnamectl tags --apply</command>, see
+    <citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>. The
+    additions and removals declared there are applied on top of the tags already configured in the
+    <varname>TAGS=</varname> field of
+    <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
+    <para>The service is ordered before <filename>sysinit.target</filename>, so that the configured tags are
+    in place before most other units are started and may be used in their
+    <varname>ConditionMachineTag=</varname> settings, see
+    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
+    <xi:include href="version-info.xml" xpointer="v263"/>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para><simplelist type="inline">
+      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
+      <member><citerefentry><refentrytitle>machine-tags.d</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
+      <member><citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
+      <member><citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
+      <member><citerefentry><refentrytitle>systemd-machine-tag@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
+      <member><citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
+    </simplelist></para>
+  </refsect1>
+</refentry>

test/units/TEST-71-HOSTNAME.sh

@@ -609,6 +609,11 @@ EOF
     hostnamectl tags --apply
     assert_eq "$(hostnamectl tags)" "local.rack=7:manual:role=b"
 
+    # The boot-time service applies the configuration as well.
+    echo "from-service" >/etc/machine-tags.d/86-service-test71.tags
+    systemctl restart systemd-machine-tags.service
+    assert_eq "$(hostnamectl tags)" "from-service:local.rack=7:manual:role=b"
+
     # "-*" resets the list.
     echo "-* fresh" >/etc/machine-tags.d/90-reset-test71.tags
     varlinkctl call /run/systemd/io.systemd.Hostname io.systemd.Hostname.ApplyTags '{}'

units/meson.build

@@ -579,6 +579,11 @@ units = [
           'file' : 'systemd-machine-tag@.service',
           'conditions' : ['ENABLE_HOSTNAMED'],
         },
+        {
+          'file' : 'systemd-machine-tags.service',
+          'conditions' : ['ENABLE_HOSTNAMED'],
+          'symlinks' : ['sysinit.target.wants/'],
+        },
         {
           'file' : 'systemd-machined.service.in',
           'conditions' : ['ENABLE_MACHINED'],

units/systemd-machine-tag@.service

@@ -12,7 +12,7 @@ Description=Add Machine Tag %i
 Documentation=man:systemd-machine-tag@.service(8)
 DefaultDependencies=no
 ConditionPathExists=!/etc/initrd-release
-After=local-fs.target systemd-hostnamed.socket
+After=local-fs.target systemd-hostnamed.socket systemd-machine-tags.service
 Requisite=local-fs.target systemd-hostnamed.socket
 Before=shutdown.target
 Conflicts=shutdown.target

units/systemd-machine-tags.service

@@ -0,0 +1,32 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  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=Apply Machine Tags Configuration
+Documentation=man:systemd-machine-tags.service(8) man:machine-tags.d(5)
+DefaultDependencies=no
+ConditionPathExists=!/etc/initrd-release
+ConditionPathIsReadWrite=/etc
+ConditionDirectoryNotEmpty=|/etc/machine-tags.d
+ConditionDirectoryNotEmpty=|/run/machine-tags.d
+ConditionDirectoryNotEmpty=|/usr/local/lib/machine-tags.d
+ConditionDirectoryNotEmpty=|/usr/lib/machine-tags.d
+
+Requires=systemd-hostnamed.socket
+After=systemd-hostnamed.socket
+After=local-fs.target
+After=systemd-firstboot.service
+After=systemd-sysext.service systemd-confext.service
+Before=sysinit.target shutdown.target
+Conflicts=shutdown.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=hostnamectl tags --apply