girepository: add singleton GIRepository back

This is needed to avoid situations where you cannot coordinate between
bindings and libraries which also need to interact with introspection
which could affect bindings.

For example, a Python application using libpeas to load plugins, also in
Python.

Fixes: #3664

girepository/girepository.c

@@ -2271,3 +2271,33 @@ gi_typelib_blob_type_to_info_type (GITypelibBlobType blob_type)
       return (GIInfoType) blob_type;
     }
 }
+
+/**
+ * gi_repository_dup_default:
+ *
+ * Gets the singleton process-global default `GIRepository`.
+ *
+ * The singleton is needed for situations where you must coordinate between
+ * bindings and libraries which also need to interact with introspection which
+ * could affect the bindings. For example, a Python application using a
+ * GObject-based library through `GIRepository` to load plugins also written in
+ * Python.
+ *
+ * Returns: (transfer full): the global singleton repository
+ *
+ * Since: 2.86
+ */
+GIRepository *
+gi_repository_dup_default (void)
+{
+  static GIRepository *instance;
+
+  if (g_once_init_enter (&instance))
+    {
+      GIRepository *repository = gi_repository_new ();
+      g_object_add_weak_pointer (G_OBJECT (repository), (gpointer *)&instance);
+      g_once_init_leave (&instance, repository);
+    }
+
+  return g_object_ref (instance);
+}

girepository/girepository.h

@@ -242,6 +242,9 @@ void gi_cclosure_marshal_generic (GClosure       *closure,
                                   void           *invocation_hint,
                                   void           *marshal_data);
 
+GI_AVAILABLE_IN_2_86
+GIRepository *gi_repository_dup_default (void);
+
 #include <girepository/girepository-autocleanups.h>
 
 G_END_DECLS

girepository/tests/repository.c

@@ -895,6 +895,20 @@ test_repository_loaded_namespaces (RepositoryFixture *fx,
   g_strfreev (namespaces);
 }
 
+static void
+test_repository_dup_default (void)
+{
+  GIRepository *repository1 = gi_repository_dup_default ();
+  GIRepository *repository2 = gi_repository_dup_default ();
+
+  g_assert_nonnull (repository1);
+  g_assert_nonnull (repository2);
+  g_assert_true (repository1 == repository2);
+
+  g_clear_object (&repository1);
+  g_clear_object (&repository2);
+}
+
 int
 main (int   argc,
       char *argv[])
@@ -924,6 +938,7 @@ main (int   argc,
   ADD_REPOSITORY_TEST ("/repository/vfunc-info-with-invoker-on-object", test_repository_vfunc_info_with_invoker_on_object, &typelib_load_spec_gio);
   ADD_REPOSITORY_TEST ("/repository/find-by-gtype", test_repository_find_by_gtype, &typelib_load_spec_gio_platform);
   ADD_REPOSITORY_TEST ("/repository/loaded-namespaces", test_repository_loaded_namespaces, &typelib_load_spec_gio_platform);
+  g_test_add_func ("/repository/dup_default", test_repository_dup_default);
 
   return g_test_run ();
 }