unit: introduce wait-online@.service for specific interface

This should be useful when a host has multiple interfaces.

Inspired by #22246.

man/rules/meson.build

@@ -932,7 +932,7 @@ manpages = [
  ['systemd-network-generator.service', '8', ['systemd-network-generator'], ''],
  ['systemd-networkd-wait-online.service',
   '8',
-  ['systemd-networkd-wait-online'],
+  ['systemd-networkd-wait-online', 'systemd-networkd-wait-online@.service'],
   'ENABLE_NETWORKD'],
  ['systemd-networkd.service', '8', ['systemd-networkd'], 'ENABLE_NETWORKD'],
  ['systemd-notify', '1', [], ''],

man/systemd-networkd-wait-online.service.xml

@@ -18,12 +18,14 @@
 
   <refnamediv>
     <refname>systemd-networkd-wait-online.service</refname>
+    <refname>systemd-networkd-wait-online@.service</refname>
     <refname>systemd-networkd-wait-online</refname>
     <refpurpose>Wait for network to come online</refpurpose>
   </refnamediv>
 
   <refsynopsisdiv>
     <para><filename>systemd-networkd-wait-online.service</filename></para>
+    <para><filename>systemd-networkd-wait-online@.service</filename></para>
     <para><filename>/usr/lib/systemd/systemd-networkd-wait-online</filename></para>
   </refsynopsisdiv>
 
@@ -38,6 +40,17 @@
     to be fully configured or failed, and for at least one link to be online. Here, online means that
     the link's operational state is equal or higher than <literal>degraded</literal>. The threshold
     can be configured by <option>--operational-state=</option> option.</para>
+
+    <para>The service <filename>systemd-networkd-wait-online.service</filename> invokes
+    <command>systemd-networkd-wait-online</command> without any options. Thus, it waits for all managed
+    interfaces to be configured or failed, and for at least one to be online.</para>
+
+    <para>The service <filename>systemd-networkd-wait-online@.service</filename> takes an interface
+    name, and invokes <command>systemd-networkd-wait-online</command> with <option>-i</option> and the
+    specified interface name. Thus, wait for the specified interface to be configured and online. For
+    example, <filename>systemd-networkd-wait-online@eth0.service</filename> waits for
+    <filename>eth0</filename> to be configured by <command>systemd-networkd</command> and online.
+    </para>
   </refsect1>
 
   <refsect1>

units/meson.build

@@ -220,6 +220,7 @@ in_units = [
         ['systemd-network-generator.service',    ''],
         ['systemd-networkd.service',             'ENABLE_NETWORKD'],
         ['systemd-networkd-wait-online.service', 'ENABLE_NETWORKD'],
+        ['systemd-networkd-wait-online@.service','ENABLE_NETWORKD'],
         ['systemd-nspawn@.service',              ''],
         ['systemd-oomd.service',                 'ENABLE_OOMD'],
         ['systemd-portabled.service',            'ENABLE_PORTABLED',

units/systemd-networkd-wait-online@.service.in

@@ -0,0 +1,25 @@
+#  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=Wait for Network Interface %i to be Configured
+Documentation=man:systemd-networkd-wait-online.service(8)
+DefaultDependencies=no
+Conflicts=shutdown.target
+Requires=systemd-networkd.service
+After=systemd-networkd.service
+Before=network-online.target shutdown.target
+
+[Service]
+Type=oneshot
+ExecStart={{ROOTLIBEXECDIR}}/systemd-networkd-wait-online -i %i
+RemainAfterExit=yes
+
+[Install]
+WantedBy=network-online.target