gh-94207: Fix struct module leak (GH-94239)

Make _struct.Struct a GC type

This fixes a memory leak in the _struct module, where as soon
as a Struct object is stored in the cache, there's a cycle from
the _struct module to the cache to Struct objects to the Struct
type back to the module. If _struct.Struct is not gc-tracked, that
cycle is never collected.

This PR makes _struct.Struct GC-tracked, and adds a regression test.

1 files changed, 20 insertions, 2 deletions

diff --git a/Modules/_struct.c b/Modules/_struct.c
index 4daf952..20307ad 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1496,10 +1496,26 @@ Struct___init___impl(PyStructObject *self, PyObject *format)
     return ret;
 }
 
+static int
+s_clear(PyStructObject *s)
+{
+    Py_CLEAR(s->s_format);
+    return 0;
+}
+
+static int
+s_traverse(PyStructObject *s, visitproc visit, void *arg)
+{
+    Py_VISIT(Py_TYPE(s));
+    Py_VISIT(s->s_format);
+    return 0;
+}
+
 static void
 s_dealloc(PyStructObject *s)
 {
     PyTypeObject *tp = Py_TYPE(s);
+    PyObject_GC_UnTrack(s);
     if (s->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)s);
     if (s->s_codes != NULL) {
@@ -2078,13 +2094,15 @@ static PyType_Slot PyStructType_slots[] = {
     {Py_tp_getattro, PyObject_GenericGetAttr},
     {Py_tp_setattro, PyObject_GenericSetAttr},
     {Py_tp_doc, (void*)s__doc__},
+    {Py_tp_traverse, s_traverse},
+    {Py_tp_clear, s_clear},
     {Py_tp_methods, s_methods},
     {Py_tp_members, s_members},
     {Py_tp_getset, s_getsetlist},
     {Py_tp_init, Struct___init__},
     {Py_tp_alloc, PyType_GenericAlloc},
     {Py_tp_new, s_new},
-    {Py_tp_free, PyObject_Del},
+    {Py_tp_free, PyObject_GC_Del},
     {0, 0},
 };
 
@@ -2092,7 +2110,7 @@ static PyType_Spec PyStructType_spec = {
     "_struct.Struct",
     sizeof(PyStructObject),
     0,
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
     PyStructType_slots
 };