gh-94207: Fix struct module leak (GH-94239) (GH-94265)

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.
(cherry picked from commit 6b86534)
Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

Author: miss-islington

Lib/test/test_struct.py

@@ -1,10 +1,12 @@
 fromcollectionsimportabc
 importarray
+importgc
 importmath
 importoperator
 importunittest
 importstruct
 importsys
+importweakref
 
 fromtestimportsupport
 fromtest.supportimportimport_helper
@@ -672,6 +674,21 @@ def __del__(self):
 self.assertIn(b"Exception ignored in:", stderr)
 self.assertIn(b"C.__del__", stderr)
 
+deftest__struct_reference_cycle_cleaned_up(self):
+# Regression test for python/cpython#94207.
+
+# When we create a new struct module, trigger use of its cache,
+# and then delete it ...
+_struct_module=import_helper.import_fresh_module("_struct")
+module_ref=weakref.ref(_struct_module)
+_struct_module.calcsize("b")
+del_struct_module
+
+# Then the module should have been garbage collected.
+gc.collect()
+self.assertIsNone(
+module_ref(), "_struct module was not garbage collected")
+
 deftest_issue35714(self):
 # Embedded null characters should not be allowed in format strings.
 forsin'\0', '2\0i', b'\0':

Misc/NEWS.d/next/Library/2022-06-24-19-23-59.gh-issue-94207.VhS1eS.rst

@@ -0,0 +1,2 @@
+Made :class:`_struct.Struct` GC-tracked in order to fix a reference leak in
+the :mod:`_struct` module.

Modules/_struct.c

@@ -1496,10 +1496,26 @@ Struct___init___impl(PyStructObject *self, PyObject *format)
 returnret;
 }
 
+staticint
+s_clear(PyStructObject*s)
+{
+Py_CLEAR(s->s_format);
+return0;
+}
+
+staticint
+s_traverse(PyStructObject*s, visitprocvisit, void*arg)
+{
+Py_VISIT(Py_TYPE(s));
+Py_VISIT(s->s_format);
+return0;
+}
+
 staticvoid
 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) {
@@ -2078,21 +2094,23 @@ 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},
 };
 
 staticPyType_SpecPyStructType_spec= {
 "_struct.Struct",
 sizeof(PyStructObject),
 0,
-Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
 PyStructType_slots
 };