Python: add weakref support to builtin types

These changes add a weakreflist member to SwigPyObject, and set
tp_weaklistoffset to its offset. This fixes #1792.

The Py_TPFLAGS_MANAGED_WEAKREF introduced in Python 3.12 requires
Py_TPFLAGS_HAVE_GC to be set as well, which it currently isn't for
SwigPyObjectType. (See https://github.com/python/cpython/issues/134786).

Examples/test-suite/python/Makefile.in

@@ -111,6 +111,7 @@ C_TEST_CASES += \
 	li_cwstring \
 	python_nondynamic \
 	python_varargs_typemap \
+	python_weakref \
 
 MULTI_CPP_TEST_CASES += \
 	import_callback \

Examples/test-suite/python/python_weakref_runme.py

@@ -0,0 +1,6 @@
+import weakref
+
+import python_weakref
+
+ts = python_weakref.TestStruct()
+ts_ref = weakref.ref(ts)

Examples/test-suite/python_weakref.i

@@ -0,0 +1,9 @@
+// Test if SwigPyObject derived class can be weakrefed
+
+%module python_weakref
+
+%inline %{
+struct TestStruct {
+    int x;
+};
+%}

Lib/python/builtin.swg

@@ -540,6 +540,8 @@ SWIGINTERN void
 SwigPyBuiltin_destructor_closure(SwigPyWrapperFunction wrapper, const char *wrappername, PyObject *a) {
   SwigPyObject *sobj;
   sobj = (SwigPyObject *)a;
+  if (sobj->weakreflist != NULL)
+    PyObject_ClearWeakRefs(a);
   SWIG_Py_XDECREF(sobj->swigdict);
   if (sobj->own) {
     PyObject *o;

Lib/python/pyrun.swg

@@ -627,6 +627,7 @@ typedef struct {
   int own;
   PyObject *next;
   PyObject *swigdict;
+  PyObject *weakreflist;
 } SwigPyObject;
 
 
@@ -1019,7 +1020,7 @@ SwigPyObject_TypeOnce(void) {
       0,                                    /* tp_traverse */
       0,                                    /* tp_clear */
       (richcmpfunc)SwigPyObject_richcompare,/* tp_richcompare */
-      0,                                    /* tp_weaklistoffset */
+      offsetof(SwigPyObject, weakreflist),  /* tp_weaklistoffset */
       0,                                    /* tp_iter */
       0,                                    /* tp_iternext */
       SwigPyObject_methods,                 /* tp_methods */
@@ -1108,6 +1109,8 @@ SwigPyObject_TypeOnce(void) {
     ((PyTypeObject *)pytype)->tp_dictoffset = offsetof(SwigPyObject, swigdict);
 #endif
 #endif
+  if (pytype)
+    ((PyTypeObject *)pytype)->tp_weaklistoffset = offsetof(SwigPyObject, weakreflist);
   if (pytype && PyModule_AddObject(runtime_data_module, "SwigPyObject", pytype) == 0)
     SWIG_Py_INCREF(pytype);
   return (PyTypeObject *)pytype;
@@ -1124,6 +1127,7 @@ SwigPyObject_New(void *ptr, swig_type_info *ty, int own)
     sobj->own  = own;
     sobj->next = 0;
     sobj->swigdict = 0;
+    sobj->weakreflist = 0;
     if (own == SWIG_POINTER_OWN) {
       /* Obtain a reference to the Python capsule wrapping the module information, so that the
        * module information is correctly destroyed after all SWIG python objects have been freed
@@ -1789,11 +1793,13 @@ SWIG_Python_NewPointerObj(PyObject *self, void *ptr, swig_type_info *type, int f
         newobj->next = next_self;
         newobj = (SwigPyObject *)next_self;
         newobj->swigdict = 0;
+        newobj->weakreflist = 0;
       }
     } else {
       newobj = PyObject_New(SwigPyObject, clientdata->pytype);
       if (newobj) {
         newobj->swigdict = 0;
+        newobj->weakreflist = 0;
       }
     }
     if (newobj) {