nixpkgs/pkgs/desktops/xfce/core/xfce4-light-locker.patch

--- ./scripts/xflock4.orig	2017-08-06 23:05:53.807688995 +0100
+++ ./scripts/xflock4	2017-08-06 23:09:06.171789989 +0100
@@ -24,12 +24,19 @
 PATH=/bin:/usr/bin
 export PATH
 
-# Lock by xscreensaver or gnome-screensaver, if a respective daemon is running
+# First test for the command set in the session's xfconf channel
+LOCK_CMD=$(xfconf-query -c xfce4-session -p /general/LockCommand)
+
+# Lock by xscreensaver, gnome-screensaver, or light-locker, if a respective daemon is running
 for lock_cmd in \
+    "$LOCK_CMD" \
     "xscreensaver-command -lock" \
-    "gnome-screensaver-command --lock"
+    "gnome-screensaver-command --lock" \
+    "light-locker-command -l"
 do
-    $lock_cmd >/dev/null 2>&1 && exit
+    if [ ! -z "$lock_cmd" ]; then
+        $lock_cmd >/dev/null 2>&1 && exit
+    fi
 done
 
 # else run another access locking utility, if installed