fix format error that leads to segfaults

CHANGES/1310.bugfix.rst

@@ -0,0 +1,2 @@
+A segmentation fault that could be triggered when getting an item is now fixed
+-- by :user:`Vizonex`.

multidict/_multidict_py.py

@@ -834,8 +834,22 @@ def _parse_args(
                             f"multidict update sequence element #{pos} "
                             f"has length {len(item)}; 2 is required"
                         )
-                    identity = identity_func(item[0])
-                    yield _Entry(hash(identity), identity, item[0], item[1])
+                    try:
+                        key = item[0]
+                    except Exception as exc:
+                        raise ValueError(
+                            f"multidict update sequence element #{pos}'s "
+                            f"key could not be fetched"
+                        ) from exc
+                    try:
+                        value = item[1]
+                    except Exception as exc:
+                        raise ValueError(
+                            f"multidict update sequence element #{pos}'s "
+                            f"value could not be fetched"
+                        ) from exc
+                    identity = identity_func(key)
+                    yield _Entry(hash(identity), identity, key, value)
         else:
             yield len(kwargs)
             for key, value in kwargs.items():

multidict/_multilib/hashtable.h

@@ -1470,8 +1470,8 @@ _err_cannot_fetch(Py_ssize_t i, const char *name)
     PyErr_Format(PyExc_ValueError,
                  "multidict update sequence element #%zd's "
                  "%s could not be fetched",
-                 name,
-                 i);
+                 i,
+                 name);
 }
 
 static int
@@ -1507,11 +1507,11 @@ _md_parse_item(Py_ssize_t i, PyObject *item, PyObject **pkey,
             goto fail;
         }
         *pkey = PySequence_ITEM(item, 0);
-        *pvalue = PySequence_ITEM(item, 1);
         if (*pkey == NULL) {
             _err_cannot_fetch(i, "key");
             goto fail;
         }
+        *pvalue = PySequence_ITEM(item, 1);
         if (*pvalue == NULL) {
             _err_cannot_fetch(i, "value");
             goto fail;

tests/test_multidict.py

@@ -300,6 +300,42 @@ def test_cannot_create_from_unaccepted(
         ):
             cls([(1, 2, 3)])  # type: ignore[call-arg]
 
+    def test_cannot_create_from_item_with_failing_getitem(
+        self,
+        cls: type[MutableMultiMapping[str]],
+    ) -> None:
+        class BadItem:
+            def __len__(self) -> int:
+                return 2
+
+            def __getitem__(self, i: int) -> object:
+                raise RuntimeError("intentional getitem failure")
+
+        with pytest.raises(
+            ValueError,
+            match=r"^multidict update sequence element #0's key could not be fetched$",
+        ):
+            cls([BadItem()])  # type: ignore[call-arg]
+
+    def test_cannot_create_from_item_with_failing_getitem_value(
+        self,
+        cls: type[MutableMultiMapping[str]],
+    ) -> None:
+        class BadValueItem:
+            def __len__(self) -> int:
+                return 2
+
+            def __getitem__(self, i: int) -> object:
+                if i == 0:
+                    return "key"
+                raise RuntimeError("intentional getitem failure")
+
+        with pytest.raises(
+            ValueError,
+            match=r"^multidict update sequence element #0's value could not be fetched$",
+        ):
+            cls([BadValueItem()])  # type: ignore[call-arg]
+
     def test_keys_is_set_less(self, cls: type[MultiDict[str]]) -> None:
         d = cls([("key", "value1")])