pygobject-object: Fix Python GC collecting a ref cycle too early

PyGObject traverses its closures in tp_traverse, but the lifetime of the closures
is tied to the lifetime of the GObject and not the wrapper. This confuses
the Python GC when it sees a ref cycle and tries to break it up with tp_clear.
Since tp_clear will not invalidate the closure and only invalidate the Python
wrapper the closure callback gets called with the now cleared/invalid object.

Instead let the GC only check the Python objects referenced by the closure when tp_clear
would actually free them and as a result break the cycle. This is only the case when
the wrapped object would be freed by tp_clear which is when its reference count is at 1.

Original patch by Gustavo Carneiro:
    https://bugzilla.gnome.org/show_bug.cgi?id=546802#c5

https://bugzilla.gnome.org/show_bug.cgi?id=731501

1 files changed, 4 insertions, 2 deletions

diff --git a/gi/pygobject-object.c b/gi/pygobject-object.c
index ca82ffb9..729bb4dc 100644
--- a/gi/pygobject-object.c
+++ b/gi/pygobject-object.c
@@ -1168,8 +1168,10 @@ pygobject_traverse(PyGObject *self, visitproc visit, void *arg)
     if (self->inst_dict) ret = visit(self->inst_dict, arg);
     if (ret != 0) return ret;
 
-    if (data) {
-
+    /* Only let the GC track the closures when tp_clear() would free them.
+     * https://bugzilla.gnome.org/show_bug.cgi?id=731501
+     */
+    if (data && self->obj->ref_count == 1) {
         for (tmp = data->closures; tmp != NULL; tmp = tmp->next) {
             PyGClosure *closure = tmp->data;