Fix segfault when a single arg is passed to setdefault (#1160)

Author: bdraco

CHANGES/1160.bugfix.rst

@@ -0,0 +1 @@
+Fixed a segmentation fault when calling :py:meth:`multidict.MultiDict.setdefault` with a single argument -- by :user:`bdraco`.

multidict/_multidict.c

@@ -573,6 +573,10 @@ multidict_setdefault(MultiDictObject *self, PyObject *const *args,
                 "key", &key, "default", &_default) < 0) {
         return NULL;
     }
+    if (_default == NULL) {
+        // fixme, _default is potentially dangerous borrowed ref here
+        _default = Py_None;
+    }
     return pair_list_set_default(&self->pairs, key, _default);
 }
 

tests/test_mutable_multidict.py

@@ -156,6 +156,15 @@ def test_set_default(
         assert "otherkey" in d
         assert "three" == d["otherkey"]
 
+    def test_set_default_single_arg(
+        self,
+        case_sensitive_multidict_class: type[MultiDict[str]],
+    ) -> None:
+        d = case_sensitive_multidict_class([("key", "one"), ("key", "two")], foo="bar")
+        assert d.setdefault("key") == "one"  # type: ignore[call-arg]
+        assert d.setdefault("noexist") is None  # type: ignore[call-arg]
+        assert d["noexist"] is None
+
     def test_popitem(
         self,
         case_sensitive_multidict_class: type[MultiDict[str]],