units: add graphical-session-pre.target user unit (#3848)

This complements graphical-session.target for services which set up the
environment (e. g. dbus-update-activation-environment) and need to run before
the actual graphical session.

Makefile.am

@@ -574,7 +574,8 @@ dist_userunit_DATA = \
 	units/user/basic.target \
 	units/user/default.target \
 	units/user/exit.target \
-	units/user/graphical-session.target
+	units/user/graphical-session.target \
+	units/user/graphical-session-pre.target
 
 nodist_userunit_DATA = \
 	units/user/systemd-exit.service

man/systemd.special.xml

@@ -927,6 +927,19 @@ PartOf=graphical-session.target
         </programlisting>
       </example>
     </refsect2>
+
+    <refsect2>
+      <title>graphical-session-pre.target</title>
+
+      <para>This target contains services which set up the environment or
+      global configuration of a graphical session, such as SSH/GPG agents
+      (which need to export an environment variable into all desktop processes)
+      or migration of obsolete d-conf keys after an OS upgrade (which needs to
+      happen before starting any process that might use them). This target must
+      be started before starting a graphical session
+      like <filename>gnome-session.target</filename>.</para>
+    </refsect2>
+
   </refsect1>
 
   <refsect1>

units/user/graphical-session-pre.target

@@ -0,0 +1,14 @@
+#  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=Session services which should run early before the graphical session is brought up
+Documentation=man:systemd.special(7)
+Requires=basic.target
+Before=graphical-session.target
+RefuseManualStart=yes
+StopWhenUnneeded=yes