dconf-editor: Respect NIX_GSETTINGS_OVERRIDES_DIR variable

We patch GLib to recognize NIX_GSETTINGS_OVERRIDES_DIR
environment variable as a high priority source of settings
schemas. This is used by NixOS modules for desktop environments
like GNOME to set default values for stuff like background image.

Since dconf-editor uses custom GSettingsSchemaSource,
it bypassed that patch, leading to the dconf-editor
showing different values from what other apps
using the GLib’s default source saw.

Let’s mirror our GLib patch in dconf-editor to fix that.

pkgs/desktops/gnome/core/dconf-editor/default.nix

@@ -27,6 +27,12 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-Vxr0x9rU8Em1PmzXKLea3fCMJ92ra8V7OW0hGGbueeM=";
   };
 
+  patches = [
+    # Look for compiled schemas in NIX_GSETTINGS_OVERRIDES_DIR
+    # environment variable, to match what we patched GLib to do.
+    ./schema-override-variable.patch
+  ];
+
   nativeBuildInputs = [
     meson
     ninja

pkgs/desktops/gnome/core/dconf-editor/schema-override-variable.patch

@@ -0,0 +1,14 @@
+diff --git a/editor/source-manager.vala b/editor/source-manager.vala
+index 27b2b17a..87f7ba86 100644
+--- a/editor/source-manager.vala
++++ b/editor/source-manager.vala
+@@ -121,6 +121,9 @@ private class SourceManager : Object
+             source = try_prepend_dir (source, Path.build_filename (system_data_dirs [i], "glib-2.0", "schemas"));
+         string user_data_dir = GLib.Environment.get_user_data_dir ();
+         source = try_prepend_dir (source, Path.build_filename (user_data_dir, "glib-2.0", "schemas"));
++        string? nix_var_schema_dir = GLib.Environment.get_variable ("NIX_GSETTINGS_OVERRIDES_DIR");
++        if (nix_var_schema_dir != null)
++            source = try_prepend_dir (source, (!) nix_var_schema_dir);
+         string? var_schema_dir = GLib.Environment.get_variable ("GSETTINGS_SCHEMA_DIR");
+         if (var_schema_dir != null)
+             source = try_prepend_dir (source, (!) var_schema_dir);