Fix segfault in PyType_GetModuleByDef for Python < 3.11 (#1141)

fixes #1140

---------

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Author: bdraco

CHANGES/1141.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed a segmentation fault when creating subclassed :py:class:`~multidict.MultiDict` objects on Python < 3.11 -- by :user:`bdraco`.
+
+The problem first appeared in 6.4.0

docs/spelling_wordlist.txt

@@ -87,6 +87,7 @@ runtime
 runtimes
 str
 subclassable
+subclassed
 subclassing
 subprotocol
 subprotocols

multidict/_multilib/state.h

@@ -69,13 +69,14 @@ PyType_GetModuleByDef(PyTypeObject *tp, PyModuleDef *def)
 
     Py_ssize_t n = PyTuple_GET_SIZE(mro);
     for (Py_ssize_t i = 1; i < n; i++) {
-        mod = PyType_GetModule(tp);
-        if (mod != NULL) {
+        PyObject *super = PyTuple_GET_ITEM(mro, i);
+        if (!PyType_HasFeature((PyTypeObject *)super, Py_TPFLAGS_HEAPTYPE)) {
+            continue;
+        }
+        mod = PyType_GetModule((PyTypeObject*)super);
+        if (mod == NULL) {
             PyErr_Clear();
         } else {
-            if (!PyType_HasFeature(tp, Py_TPFLAGS_HEAPTYPE)) {
-                continue;
-            }
             mod_def = PyModule_GetDef(mod);
             if (mod_def == def) {
                 return mod;

tests/test_multidict.py

@@ -1307,3 +1307,19 @@ def test_init_does_not_alter_refcount(
     original_refcount = sys.getrefcount(original)
     case_sensitive_multidict_class(original)
     assert sys.getrefcount(original) == original_refcount
+
+
+def test_subclassed_multidict(
+    any_multidict_class: type[MultiDict[str]],
+) -> None:
+    """Test that subclassed MultiDicts work as expected."""
+    class SubclassedMultiDict(any_multidict_class):  # type: ignore[valid-type, misc]
+        """Subclassed MultiDict."""
+
+    d1 = SubclassedMultiDict([("key", "value1")])
+    d2 = SubclassedMultiDict([("key", "value2")])
+    d3 = SubclassedMultiDict([("key", "value1")])
+    assert d1 != d2
+    assert d1 == d3
+    assert d1 == SubclassedMultiDict([("key", "value1")])
+    assert d1 != SubclassedMultiDict([("key", "value2")])