Add g_type_get_type_registration_serial()

This lets you cache type lookup information and then know when
the cache information is out of date. In particular, we want this
in order to be able to cache g_type_from_name() lookups in the Gtk+
theme machinery.

https://bugzilla.gnome.org/show_bug.cgi?id=689847

docs/reference/gobject/gobject-sections.txt

@@ -99,6 +99,7 @@ g_type_remove_interface_check
 GTypeInterfaceCheckFunc
 g_type_value_table_peek
 g_type_ensure
+g_type_get_type_registration_serial
 
 G_DEFINE_TYPE
 G_DEFINE_TYPE_WITH_CODE

gobject/gobject.symbols

@@ -350,6 +350,7 @@ g_type_fundamental
 g_type_fundamental_next
 g_type_get_plugin
 g_type_get_qdata
+g_type_get_type_registration_serial
 g_type_init
 g_type_init_with_debug_flags
 g_type_instance_get_private

gobject/gtype.c

@@ -377,6 +377,7 @@ static IFaceCheckFunc *static_iface_check_funcs = NULL;
 static GQuark          static_quark_type_flags = 0;
 static GQuark          static_quark_iface_holder = 0;
 static GQuark          static_quark_dependants_array = 0;
+static guint           type_registration_serial = 0;
 GTypeDebugFlags	       _g_type_debug_flags = 0;
 
 /* --- type nodes --- */
@@ -393,6 +394,25 @@ lookup_type_node_I (register GType utype)
     return static_fundamental_type_nodes[utype >> G_TYPE_FUNDAMENTAL_SHIFT];
 }
 
+/**
+ * g_type_get_type_registration_serial:
+ *
+ * Returns an opaque serial number that represents the state of the set of registered
+ * types. Any time a type is registred this serial changes, which means you can
+ * cache information based on type lookups (such as g_type_from_name) and know if
+ * the cache is still valid at a later time by comparing the current serial with
+ * the one at the type lookup.
+ *
+ * Since: 2.36
+ *
+ * Returns: An unsigned int, representing the state of type registrations.
+ */
+guint
+g_type_get_type_registration_serial (void)
+{
+  return (guint)g_atomic_int_get ((gint *)&type_registration_serial);
+}
+
 static TypeNode*
 type_node_any_new_W (TypeNode             *pnode,
 		     GType                 ftype,
@@ -490,6 +510,9 @@ type_node_any_new_W (TypeNode             *pnode,
   g_hash_table_insert (static_type_nodes_ht,
 		       (gpointer) g_quark_to_string (node->qname),
 		       (gpointer) type);
+
+  g_atomic_int_inc ((gint *)&type_registration_serial);
+
   return node;
 }
 

gobject/gtype.h

@@ -1263,6 +1263,9 @@ gpointer  g_type_class_get_private      (GTypeClass 		    *klass,
 
 GLIB_AVAILABLE_IN_2_34
 void      g_type_ensure                 (GType                       type);
+GLIB_AVAILABLE_IN_2_36
+guint     g_type_get_type_registration_serial (void);
+
 
 /* --- GType boilerplate --- */
 /**