[displaymanager] Look for variant gdm config files

FIXES #2335

src/modules/displaymanager/main.py

@@ -376,14 +376,30 @@ class DMgdm(DisplayManager):
         GDM exists with different executable names, so search
         for one of them and use it.
         """
-        for executable, config in (
+        candidates = (
             ( "gdm", "etc/gdm/custom.conf" ),
-            ( "gdm3", "etc/gdm3/daemon.conf" )
-        ):
+            ( "gdm3", "etc/gdm3/daemon.conf" ),
+            ( "gdm3", "etc/gdm3/custom.conf" ),
+        )
+
+        def have_executable(executable : str):
             bin_path = "{!s}/usr/bin/{!s}".format(self.root_mount_point, executable)
             sbin_path = "{!s}/usr/sbin/{!s}".format(self.root_mount_point, executable)
-            if os.path.exists(bin_path) or os.path.exists(sbin_path):
-                # Keep the found-executable name around for later
+            return os.path.exists(bin_path) or os.path.exists(sbin_path)
+
+        def have_config(config : str):
+            config_path = "{!s}/{!s}".format(self.root_mount_point, config)
+            return os.path.exists(config_path)
+
+        # Look for an existing configuration file as a hint, then
+        # keep the found-executable name and config around for later.
+        for executable, config in candidates:
+            if have_config(config) and have_executable(executable):
+                self.executable = executable
+                self.config = config
+                return True
+        for executable, config in candidates:
+            if have_executable(executable):
                 self.executable = executable
                 self.config = config
                 return True

src/modules/displaymanager/tests/CMakeTests.txt

@@ -4,7 +4,7 @@
 # We have tests to load (some) of the DMs specifically, to test their
 # configuration code. Those tests conventionally live in Python
 # files here in the tests/ directory. Add them.
-foreach(_dmname greetd sddm)
+foreach(_dmname greetd sddm gdm)
     add_test(
         NAME configure-displaymanager-${_dmname}
         COMMAND env PYTHONPATH=.: python3 ${CMAKE_CURRENT_LIST_DIR}/test-dm-${_dmname}.py

src/modules/displaymanager/tests/test-dm-gdm.py

@@ -0,0 +1,19 @@
+#   SPDX-FileCopyrightText: no
+#   SPDX-License-Identifier: CC0-1.0
+#
+# Calamares Boilerplate
+import libcalamares
+libcalamares.globalstorage = libcalamares.GlobalStorage(None)
+libcalamares.globalstorage.insert("testing", True)
+
+# Module prep-work
+from src.modules.displaymanager import main
+default_desktop_environment = main.DesktopEnvironment("startplasma-x11", "kde-plasma.desktop")
+
+# Specific DM test
+d = main.DMgdm("/tmp")
+d.have_dm()
+d.set_autologin("d", True, default_desktop_environment)
+# .. and again (this time checks load/save)
+d.set_autologin("d", True, default_desktop_environment)
+d.set_autologin("d", True, default_desktop_environment)