gh-90494: Reject 6th element of the __reduce__() tuple (GH-93609)

copy.copy() and copy.deepcopy() now always raise a TypeError if
__reduce__() returns a tuple with length 6 instead of silently ignore
the 6th item or produce incorrect result.

Lib/copy.py

@@ -258,7 +258,7 @@ def _keep_alive(x, memo):
 
 def _reconstruct(x, memo, func, args,
                  state=None, listiter=None, dictiter=None,
-                 deepcopy=deepcopy):
+                 *, deepcopy=deepcopy):
     deep = memo is not None
     if deep and args:
         args = (deepcopy(arg, memo) for arg in args)

Lib/pickle.py

@@ -619,7 +619,7 @@ class _Pickler:
                     "persistent IDs in protocol 0 must be ASCII strings")
 
     def save_reduce(self, func, args, state=None, listitems=None,
-                    dictitems=None, state_setter=None, obj=None):
+                    dictitems=None, state_setter=None, *, obj=None):
         # This API is called by some subclasses
 
         if not isinstance(args, tuple):

Lib/test/test_copy.py

@@ -678,6 +678,28 @@ class TestCopy(unittest.TestCase):
         self.assertIsNot(x, y)
         self.assertIsNot(x["foo"], y["foo"])
 
+    def test_reduce_6tuple(self):
+        def state_setter(*args, **kwargs):
+            self.fail("shouldn't call this")
+        class C:
+            def __reduce__(self):
+                return C, (), self.__dict__, None, None, state_setter
+        x = C()
+        with self.assertRaises(TypeError):
+            copy.copy(x)
+        with self.assertRaises(TypeError):
+            copy.deepcopy(x)
+
+    def test_reduce_6tuple_none(self):
+        class C:
+            def __reduce__(self):
+                return C, (), self.__dict__, None, None, None
+        x = C()
+        with self.assertRaises(TypeError):
+            copy.copy(x)
+        with self.assertRaises(TypeError):
+            copy.deepcopy(x)
+
     def test_copy_slots(self):
         class C(object):
             __slots__ = ["foo"]

Misc/NEWS.d/next/Library/2022-06-08-20-11-02.gh-issue-90494.LIZT85.rst

@@ -0,0 +1,3 @@
+:func:`copy.copy` and :func:`copy.deepcopy` now always raise a TypeError if
+``__reduce__()`` returns a tuple with length 6 instead of silently ignore
+the 6th item or produce incorrect result.