bpo-32176: Set CO_NOFREE in the code object constructor (GH-4675)

Previously, CO_NOFREE was set in the compiler, which meant
it could end up being set incorrectly when code objects
were created directly. Setting it in the constructor based
on freevars and cellvars ensures it is always accurate,
regardless of how the code object is defined.

Lib/test/test_code.py

@@ -102,6 +102,7 @@ consts: ('None',)
 
 """
 
+import inspect
 import sys
 import threading
 import unittest
@@ -130,6 +131,10 @@ def dump(co):
         print("%s: %s" % (attr, getattr(co, "co_" + attr)))
     print("consts:", tuple(consts(co.co_consts)))
 
+# Needed for test_closure_injection below
+# Defined at global scope to avoid implicitly closing over __class__
+def external_getitem(self, i):
+    return f"Foreign getitem: {super().__getitem__(i)}"
 
 class CodeTest(unittest.TestCase):
 
@@ -141,6 +146,46 @@ class CodeTest(unittest.TestCase):
         self.assertEqual(co.co_name, "funcname")
         self.assertEqual(co.co_firstlineno, 15)
 
+    @cpython_only
+    def test_closure_injection(self):
+        # From https://bugs.python.org/issue32176
+        from types import FunctionType, CodeType
+
+        def create_closure(__class__):
+            return (lambda: __class__).__closure__
+
+        def new_code(c):
+            '''A new code object with a __class__ cell added to freevars'''
+            return CodeType(
+                c.co_argcount, c.co_kwonlyargcount, c.co_nlocals,
+                c.co_stacksize, c.co_flags, c.co_code, c.co_consts, c.co_names,
+                c.co_varnames, c.co_filename, c.co_name, c.co_firstlineno,
+                c.co_lnotab, c.co_freevars + ('__class__',), c.co_cellvars)
+
+        def add_foreign_method(cls, name, f):
+            code = new_code(f.__code__)
+            assert not f.__closure__
+            closure = create_closure(cls)
+            defaults = f.__defaults__
+            setattr(cls, name, FunctionType(code, globals(), name, defaults, closure))
+
+        class List(list):
+            pass
+
+        add_foreign_method(List, "__getitem__", external_getitem)
+
+        # Ensure the closure injection actually worked
+        function = List.__getitem__
+        class_ref = function.__closure__[0].cell_contents
+        self.assertIs(class_ref, List)
+
+        # Ensure the code correctly indicates it accesses a free variable
+        self.assertFalse(function.__code__.co_flags & inspect.CO_NOFREE,
+                         hex(function.__code__.co_flags))
+
+        # Ensure the zero-arg super() call in the injected method works
+        obj = List([1, 2, 3])
+        self.assertEqual(obj[0], "Foreign getitem: 1")
 
 def isinterned(s):
     return s is sys.intern(('_' + s + '_')[1:-1])