gh-123431: Harmonize extension code checks in pickle (GH-123434)

This checks are redundant in normal circumstances and can only work if
the extension registry was intentionally broken.

* The Python implementation now raises exception for the extension code
  with false boolean value.
* Simplify the C code. RuntimeError is now raised in explicit checks.
* Add many tests.

Lib/test/pickletester.py

@@ -1311,6 +1311,35 @@ class AbstractUnpickleTests:
         self.assertEqual(loads(b'cmath\nlog\n.'), ('math', 'log'))
         self.assertEqual(loads(b'\x8c\x04math\x8c\x03log\x93.'), ('math', 'log'))
 
+    def test_bad_ext_code(self):
+        # unregistered extension code
+        self.check_unpickling_error(ValueError, b'\x82\x01.')
+        self.check_unpickling_error(ValueError, b'\x82\xff.')
+        self.check_unpickling_error(ValueError, b'\x83\x01\x00.')
+        self.check_unpickling_error(ValueError, b'\x83\xff\xff.')
+        self.check_unpickling_error(ValueError, b'\x84\x01\x00\x00\x00.')
+        self.check_unpickling_error(ValueError, b'\x84\xff\xff\xff\x7f.')
+        # EXT specifies code <= 0
+        self.check_unpickling_error(pickle.UnpicklingError, b'\x82\x00.')
+        self.check_unpickling_error(pickle.UnpicklingError, b'\x83\x00\x00.')
+        self.check_unpickling_error(pickle.UnpicklingError, b'\x84\x00\x00\x00\x00.')
+        self.check_unpickling_error(pickle.UnpicklingError, b'\x84\x00\x00\x00\x80.')
+        self.check_unpickling_error(pickle.UnpicklingError, b'\x84\xff\xff\xff\xff.')
+
+    @support.cpython_only
+    def test_bad_ext_inverted_registry(self):
+        code = 1
+        def check(key, exc):
+            with support.swap_item(copyreg._inverted_registry, code, key):
+                with self.assertRaises(exc):
+                    self.loads(b'\x82\x01.')
+        check(None, ValueError)
+        check((), ValueError)
+        check((__name__,), (TypeError, ValueError))
+        check((__name__, "MyList", "x"), (TypeError, ValueError))
+        check((__name__, None), (TypeError, ValueError))
+        check((None, "MyList"), (TypeError, ValueError))
+
     def test_bad_reduce(self):
         self.assertEqual(self.loads(b'cbuiltins\nint\n)R.'), 0)
         self.check_unpickling_error(TypeError, b'N)R.')
@@ -2163,6 +2192,28 @@ class AbstractPicklingErrorTests:
             check({Clearer(): 1, Clearer(): 2})
             check({1: Clearer(), 2: Clearer()})
 
+    @support.cpython_only
+    def test_bad_ext_code(self):
+        # This should never happen in normal circumstances, because the type
+        # and the value of the extesion code is checked in copyreg.add_extension().
+        key = (__name__, 'MyList')
+        def check(code, exc):
+            assert key not in copyreg._extension_registry
+            assert code not in copyreg._inverted_registry
+            with (support.swap_item(copyreg._extension_registry, key, code),
+                  support.swap_item(copyreg._inverted_registry, code, key)):
+                for proto in protocols[2:]:
+                    with self.assertRaises(exc):
+                        self.dumps(MyList, proto)
+
+        check(object(), TypeError)
+        check(None, TypeError)
+        check(-1, (RuntimeError, struct.error))
+        check(0, RuntimeError)
+        check(2**31, (RuntimeError, OverflowError, struct.error))
+        check(2**1000, (OverflowError, struct.error))
+        check(-2**1000, (OverflowError, struct.error))
+
 
 class AbstractPickleTests:
     # Subclass must define self.dumps, self.loads.