Bug#764607: systemd: systemctl does not re-create display-manager.service symlink

[Didier Roche]

Here is an updated version of the patch (ensuring we echo a warning if 
systemctl enable fails) after discussing with Laurent.

There seems to be one case failing due to the autogenerated gdm3 service 
script from the LSB version which is making systemctl enable --force 
gdm3 failing, investigating.

Cheers,
Didier
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diff -Nru gdm3-3.14.1/debian/changelog gdm3-3.14.1/debian/changelog
--- gdm3-3.14.1/debian/changelog	2014-11-09 18:16:03.000000000 +0100
+++ gdm3-3.14.1/debian/changelog	2014-11-20 09:44:14.000000000 +0100
@@ -1,3 +1,11 @@
+gdm3 (3.14.1-4) UNRELEASED; urgency=medium
+
+  * debian/patches/92_systemd_unit.patch, debian/gdm3.postinst:
+    - Using Alias and systemctl to handle systemd unit alternatives.
+      Closes: #764607
+
+ -- Didier Roche <didrocks at ubuntu.com>  Thu, 20 Nov 2014 09:40:25 +0100
+
 gdm3 (3.14.1-3) unstable; urgency=medium
 
   * 18_all_displays_transient.patch: fix autologin for the initial 
diff -Nru gdm3-3.14.1/debian/gdm3.postinst gdm3-3.14.1/debian/gdm3.postinst
--- gdm3-3.14.1/debian/gdm3.postinst	2014-04-27 15:07:16.000000000 +0200
+++ gdm3-3.14.1/debian/gdm3.postinst	2014-11-20 09:45:01.000000000 +0100
@@ -40,21 +40,17 @@
   fi
 fi
 
-DEFAULT_SERVICE=/etc/systemd/system/display-manager.service
+DEFAULT_SERVICE=display-manager.service
+SERVICE=$(basename $(cat "$DEFAULT_DISPLAY_MANAGER_FILE")).service
 # set default-display-manager systemd service link according to our config
-if [ "$1" = configure ] && [ -d /etc/systemd/system/ ]; then
-  if [ -e "$DEFAULT_DISPLAY_MANAGER_FILE" ]; then
-    SERVICE=/lib/systemd/system/$(basename $(cat "$DEFAULT_DISPLAY_MANAGER_FILE")).service
-    if [ -h "$DEFAULT_SERVICE" ] && [ $(readlink "$DEFAULT_SERVICE") = /dev/null ]; then
-      echo "Display manager service is masked" >&2
-    elif [ -e "$SERVICE" ]; then
-      ln -sf "$SERVICE" "$DEFAULT_SERVICE"
-    else
-      echo "WARNING: $SERVICE is the selected default display manager but does not exist" >&2
-      rm -f "$DEFAULT_SERVICE"
-    fi
+if [ "$1" = configure ] && [ -x /bin/systemctl ]; then
+  if [ $(systemctl is-enabled "$DEFAULT_SERVICE") = masked ]; then
+    echo "Display manager service is masked" >&2
   else
-    rm -f "$DEFAULT_SERVICE"
+    [ -d /run/systemd/system ] && systemctl daemon-reload
+    if [ ! `systemctl enable --force $SERVICE 2>/dev/null` ]; then
+      echo "WARNING: $SERVICE is the selected default display manager but does not have a systemd service" >&2
+    fi
   fi
 fi
 
diff -Nru gdm3-3.14.1/debian/patches/92_systemd_unit.patch gdm3-3.14.1/debian/patches/92_systemd_unit.patch
--- gdm3-3.14.1/debian/patches/92_systemd_unit.patch	2014-04-27 14:44:32.000000000 +0200
+++ gdm3-3.14.1/debian/patches/92_systemd_unit.patch	2014-11-20 09:43:34.000000000 +0100
@@ -1,8 +1,8 @@
-Index: gdm3-3.12.1/data/gdm.service.in
+Index: gdm3-3.14.1/data/gdm.service.in
 ===================================================================
---- gdm3-3.12.1.orig/data/gdm.service.in	2014-04-27 14:40:14.210580120 +0200
-+++ gdm3-3.12.1/data/gdm.service.in	2014-04-27 14:43:22.350149176 +0200
-@@ -4,12 +4,15 @@ Conflicts=getty at tty@GDM_INITIAL_VT at .serv
+--- gdm3-3.14.1.orig/data/gdm.service.in
++++ gdm3-3.14.1/data/gdm.service.in
+@@ -4,10 +4,16 @@ Conflicts=getty at tty@GDM_INITIAL_VT at .serv
  After=systemd-user-sessions.service getty at tty@GDM_INITIAL_VT at .service plymouth-quit.service
  
  [Service]
@@ -20,6 +20,4 @@
 +#BusName=org.gnome.DisplayManager
  StandardOutput=syslog
  StandardError=inherit
--
--[Install]
--Alias=display-manager.service
+