[meld/windows-fixes] settings: Handle missing GNOME desktop schemas for font settings

[Kai Willadsen <kaiw src gnome org>]

commit 58ae26abf21b9b75dce60520ecb51ade1f90e87d
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sun Jul 24 08:11:51 2022 +1000

    settings: Handle missing GNOME desktop schemas for font settings
    
    We only use the desktop schemas to get the default font, and even then
    we do not currently use this on Windows anyway. Rather than writing
    another Windows-specific workaround here, this commit makes us handle
    missing desktop schemas with a simple fallback to a monospace font.

 meld/settings.py | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)
---
diff --git a/meld/settings.py b/meld/settings.py
index 942269c8..fc2d6c40 100644
--- a/meld/settings.py
+++ b/meld/settings.py
@@ -14,6 +14,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import sys
+from typing import Optional
 
 from gi.repository import Gio, GObject, Gtk, GtkSource, Pango
 
@@ -78,9 +79,11 @@ class MeldSettings(GObject.GObject):
         if settings.get_boolean('use-system-font'):
             if sys.platform == 'win32':
                 font_string = 'Consolas 11'
-            else:
+            elif interface_settings:
                 font_string = interface_settings.get_string(
                     'monospace-font-name')
+            else:
+                font_string = 'monospace'
         else:
             font_string = settings.get_string('custom-font')
         return Pango.FontDescription(font_string)
@@ -101,11 +104,23 @@ def load_settings_schema(schema_id):
     return settings
 
 
+def load_interface_settings() -> Optional[Gio.Settings]:
+
+    # We conditionally load these since they're only used for default
+    # fonts and can sometimes be missing (e.g., in some Windows setups)
+    default_source = Gio.SettingsSchemaSource.get_default()
+    schema = default_source.lookup("org.gnome.desktop.interface", False)
+    if not schema:
+        return None
+
+    return Gio.Settings.new_full(schema=schema, backend=None, path=None)
+
+
 def create_settings():
     global settings, interface_settings, _meldsettings
 
     settings = load_settings_schema(meld.conf.SETTINGS_SCHEMA_ID)
-    interface_settings = Gio.Settings.new('org.gnome.desktop.interface')
+    interface_settings = load_interface_settings()
     _meldsettings = MeldSettings()