Fix regression in marshalling partial() objects

In a4880dbc4575fadc0e3 a special case for partial() was added to handle gtk4 template
callbacks. This in turn broken normal usage of partial objects.

To work around that add a special marker in the gtk template code for now until
we find a better fix.

Also adds a test so this doesn't happen again.

Fixes #464

2 files changed, 6 insertions, 3 deletions

diff --git a/gi/_gtktemplate.py b/gi/_gtktemplate.py
index a631a6eb..925aa0dd 100644
--- a/gi/_gtktemplate.py
+++ b/gi/_gtktemplate.py
@@ -78,9 +78,12 @@ def define_builder_scope():
             handler, args = _extract_handler_and_args(current_object, handler_name)
 
             if obj:
-                return partial(handler, *args, swap_data=obj)
+                p = partial(handler, *args, swap_data=obj)
+            else:
+                p = partial(handler, *args)
 
-            return partial(handler, *args)
+            p.__gtk_template__ = True
+            return p
 
     return BuilderScope
 
diff --git a/gi/pygi-struct-marshal.c b/gi/pygi-struct-marshal.c
index 0e190b8e..a3eb2802 100644
--- a/gi/pygi-struct-marshal.c
+++ b/gi/pygi-struct-marshal.c
@@ -191,7 +191,7 @@ pygi_arg_gclosure_from_py_marshal (PyObject   *py_arg,
             Py_DECREF (functools);
         }
 
-        if (partial && PyObject_IsInstance (py_arg, partial) > 0) {
+        if (partial && PyObject_IsInstance (py_arg, partial) > 0 && PyObject_HasAttrString (py_arg, "__gtk_template__")) {
             PyObject *partial_func;
             PyObject *partial_args;
             PyObject *partial_keywords;