[displaymanager] Allow configuring the preferred-greeter-list

src/modules/displaymanager/displaymanager.conf

@@ -64,6 +64,12 @@ sysconfigSetup: false
 #
 # greetd has configurable user and group; the user and group is created if it
 # does not exist, and the user is set as default-session user.
+#
+# lightdm has a list of greeters to look for, preferring them in order if
+# they are installed (if not, picks the alphabetically first greeter that is installed).
+#
 greetd:
   greeter_user: "tom_bombadil"
   greeter_group: "wheel"
+lightdm:
+  preferred_greeters: ["lightdm-greeter.desktop", "slick-greeter.desktop"]

src/modules/displaymanager/displaymanager.schema.yaml

@@ -26,4 +26,7 @@ properties:
             greeter_user: { type: string }
             greeter_group: { type: string }
         additionalProperties: false
-
+    lightdm:
+        type: object
+        properties:
+            preferred_greeters: { type: array, items: { type: string } }

src/modules/displaymanager/main.py

@@ -544,6 +544,9 @@ class DMlightdm(DisplayManager):
     name = "lightdm"
     executable = "lightdm"
 
+    # Can be overridden in the .conf file
+    preferred_greeters = ["lightdm-greeter.desktop"]
+
     def set_autologin(self, username, do_autologin, default_desktop_environment):
         # Systems with LightDM as Desktop Manager
         # Ideally, we should use configparser for the ini conf file,
@@ -645,10 +648,9 @@ class DMlightdm(DisplayManager):
         """
         greeters_dir = "usr/share/xgreeters"
         greeters_target_path = os.path.join(self.root_mount_point, greeters_dir)
-        preferred_names = ("lightdm-greeter.desktop", )
         available_names = os.listdir(greeters_target_path)
         available_names.sort()
-        desktop_names = [n for n in preferred_names if n in available_names] # Preferred ones
+        desktop_names = [n for n in self.preferred_greeters if n in available_names] # Preferred ones
         if desktop_names:
             return desktop_names[0]
         desktop_names = [n for n in available_names if n.endswith(".desktop")] # .. otherwise any .desktop