d/rules, d/systemd/*: Add systemd user session, but only install it if building for Ubuntu.

debian/rules

@@ -9,6 +9,10 @@ export DEB_BUILD_MAINT_OPTIONS=hardening=+all
 override_dh_install:
 	chmod 755 debian/xfce4-session/etc/xdg/xfce4/xinitrc
 	find debian/xfce4-session/usr/lib -name '*.la' -delete
+ifeq ($(shell dpkg-vendor --derives-from ubuntu && echo 1),1)
+	dh_install debian/systemd/xfce4-session.service /usr/lib/systemd/user/
+	dh_install debian/systemd/run-systemd-session /usr/share/xfce4/scripts/
+endif
 	dh_install
 
 # default backend for Linux and Hurd

debian/systemd/run-systemd-session

+#!/bin/sh
+set -e
+
+# robustness: if the previous graphical session left some failed units,
+# reset them so that they don't break this startup
+for unit in $(systemctl --user --no-legend --state=failed list-units | cut -f1 -d' '); do
+if [ "$(systemctl --user show -p PartOf --value)" = "graphical-session.target" ]; then
+    systemctl --user reset-failed $unit
+fi
+done
+
+systemctl --user restart graphical-session-pre.target
+systemctl --user restart "$1"
+
+# Wait until the session gets closed
+# FIXME: replace with an event-based waiting instead of polling (needs new systemctl functionality)
+while systemctl --user --quiet is-active "$1"; do sleep 1; done
+
+# Delay killing the X server until all graphical units stopped
+# FIXME: we currently cannot make targets wait on its dependencies going to
+# "inactive", only to "deactivating"
+while [ -n "$(systemctl --user --no-legend --state=deactivating list-units)" ]; do sleep 0.2; done

debian/systemd/xfce4-session.service

+[Unit]
+Description=Xfce 4 Session Manager
+PartOf=graphical-session.target
+
+[Service]
+ExecStart=/usr/bin/startxfce4