GSettingsBackend: add read_user_value() API

This will get the 'user' value from the database (ie: the one that the user has
control over).

Provide a default implementation that chains to ->read().  That will work for
all of our internal backends which don't have a concept of layering or
lockdown.

The delayed backend implments "user value" by returning anything that's
in the changeset (incuding an explicit NULL) or chaining up otherwise.

We will use this for g_settings_get_user_value().

https://bugzilla.gnome.org/show_bug.cgi?id=668233

gio/gsettingsbackend.c

@@ -745,6 +745,43 @@ g_settings_backend_read (GSettingsBackend   *backend,
   return value;
 }
 
+/*< private >
+ * g_settings_backend_read_user_value:
+ * @backend: a #GSettingsBackend implementation
+ * @key: the key to read
+ * @expected_type: a #GVariantType
+ *
+ * Reads the 'user value' of a key.
+ *
+ * This is the value of the key that the user has control over and has
+ * set for themselves.  Put another way: if the user did not set the
+ * value for themselves, then this will return %NULL (even if the
+ * sysadmin has provided a default value).
+ *
+ * Returns: the value that was read, or %NULL
+ */
+GVariant *
+g_settings_backend_read_user_value (GSettingsBackend   *backend,
+                                    const gchar        *key,
+                                    const GVariantType *expected_type)
+{
+  GVariant *value;
+
+  value = G_SETTINGS_BACKEND_GET_CLASS (backend)
+    ->read_user_value (backend, key, expected_type);
+
+  if (value != NULL)
+    value = g_variant_take_ref (value);
+
+  if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))
+    {
+      g_variant_unref (value);
+      value = NULL;
+    }
+
+  return value;
+}
+
 /*< private >
  * g_settings_backend_write:
  * @backend: a #GSettingsBackend implementation
@@ -903,6 +940,14 @@ ignore_subscription (GSettingsBackend *backend,
 {
 }
 
+static GVariant *
+g_settings_backend_real_read_user_value (GSettingsBackend   *backend,
+                                         const gchar        *key,
+                                         const GVariantType *expected_type)
+{
+  return g_settings_backend_read (backend, key, expected_type, FALSE);
+}
+
 static void
 g_settings_backend_init (GSettingsBackend *backend)
 {
@@ -918,6 +963,8 @@ g_settings_backend_class_init (GSettingsBackendClass *class)
   class->subscribe = ignore_subscription;
   class->unsubscribe = ignore_subscription;
 
+  class->read_user_value = g_settings_backend_real_read_user_value;
+
   gobject_class->finalize = g_settings_backend_finalize;
 }