utils: Honor "week-start-day" gsetting on startup

This lets users override the "first day of the week" through a
GSetting, instead of being forced to use whatever is defined by
the locale, which often may not be correct for their culture,
language or location.

Currently this would apply only upon (re)starting the app.

Fixes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/160
Co-authored-by: Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
Co-authored-by: Titouan Real <titouan.real@gmail.com>
Co-authored-by: Adrien Plazas <aplazas@gnome.org>

src/utils/gcal-utils.c

@@ -285,6 +285,89 @@ get_uuid_from_component (ESource       *source,
   return uuid;
 }
 
+static gboolean
+read_first_weekday_from_portal (gint *out_first_weekday)
+{
+  static gsize first_weekday_init = FALSE;
+  static gsize first_weekday = G_MAXSIZE;
+
+  if (g_once_init_enter (&first_weekday_init))
+    {
+      g_autoptr (GDBusProxy) settings_portal = NULL;
+      g_autoptr (GVariant) result = NULL;
+      g_autoptr (GVariant) aux2 = NULL;
+      g_autoptr (GVariant) aux = NULL;
+      g_autoptr (GError) error = NULL;
+      const gchar *day;
+
+      /*
+       * If the value is not "default" then there is a user preference
+       * override, in that case skip the autodetection dance.
+       */
+      static const gchar *portal_weekdays[] = {
+        "sunday",
+        "monday",
+        "tuesday",
+        "wednesday",
+        "thursday",
+        "friday",
+        "saturday",
+      };
+
+      settings_portal = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SESSION,
+                                                       G_DBUS_PROXY_FLAGS_NONE,
+                                                       NULL,
+                                                       "org.freedesktop.portal.Desktop",
+                                                       "/org/freedesktop/portal/desktop",
+                                                       "org.freedesktop.portal.Settings",
+                                                       NULL,
+                                                       &error);
+
+      if (error)
+        {
+          g_warning ("Failed to load portals: %s. Aborting...", error->message);
+          goto out;
+        }
+
+      result = g_dbus_proxy_call_sync (settings_portal,
+                                       "ReadOne",
+                                       g_variant_new ("(ss)", "org.gnome.desktop.calendar", "week-start-day"),
+                                       G_DBUS_CALL_FLAGS_NONE,
+                                       G_MAXINT,
+                                       NULL,
+                                       &error);
+
+      if (error)
+        {
+          g_warning ("Failed to load first weekday settings: %s", error->message);
+          goto out;
+        }
+
+      aux = g_variant_get_child_value (result, 0);
+      aux2 = g_variant_get_variant (aux);
+      day = g_variant_get_string (aux2, NULL);
+
+      g_debug ("Setting 'week-start-day' is currently: %s", day);
+
+      for (int i = 0; i < G_N_ELEMENTS (portal_weekdays); i++)
+        {
+          if (g_strcmp0 (day, portal_weekdays[i]) == 0)
+            {
+              first_weekday = i;
+              break;
+            }
+        }
+
+out:
+      g_once_init_leave (&first_weekday_init, TRUE);
+    }
+
+  if (first_weekday != G_MAXSIZE && out_first_weekday)
+    *out_first_weekday = first_weekday;
+
+  return first_weekday != G_MAXSIZE;
+}
+
 /**
  * get_first_weekday:
  *
@@ -296,7 +379,10 @@ get_uuid_from_component (ESource       *source,
 gint
 get_first_weekday (void)
 {
-  int week_start;
+  gint week_start;
+
+  if (read_first_weekday_from_portal (&week_start))
+    return week_start;
 
 #ifdef HAVE__NL_TIME_FIRST_WEEKDAY