gh-84436: Implement Immortal Objects (gh-19474)

This is the implementation of PEP683

Motivation:

The PR introduces the ability to immortalize instances in CPython which bypasses reference counting. Tagging objects as immortal allows up to skip certain operations when we know that the object will be around for the entire execution of the runtime.

Note that this by itself will bring a performance regression to the runtime due to the extra reference count checks. However, this brings the ability of having truly immutable objects that are useful in other contexts such as immutable data sharing between sub-interpreters.

Lib/test/_test_embed_structseq.py

@@ -1,27 +1,31 @@
 import sys
 import types
-import unittest
 
+# Note: This test file can't import `unittest` since the runtime can't
+# currently guarantee that it will not leak memory. Doing so will mark
+# the test as passing but with reference leaks. This can safely import
+# the `unittest` library once there's a strict guarantee of no leaks
+# during runtime shutdown.
 
 # bpo-46417: Test that structseq types used by the sys module are still
 # valid when Py_Finalize()/Py_Initialize() are called multiple times.
-class TestStructSeq(unittest.TestCase):
+class TestStructSeq:
     # test PyTypeObject members
-    def check_structseq(self, obj_type):
+    def _check_structseq(self, obj_type):
         # ob_refcnt
-        self.assertGreaterEqual(sys.getrefcount(obj_type), 1)
+        assert sys.getrefcount(obj_type) > 1
         # tp_base
-        self.assertTrue(issubclass(obj_type, tuple))
+        assert issubclass(obj_type, tuple)
         # tp_bases
-        self.assertEqual(obj_type.__bases__, (tuple,))
+        assert obj_type.__bases__ == (tuple,)
         # tp_dict
-        self.assertIsInstance(obj_type.__dict__, types.MappingProxyType)
+        assert isinstance(obj_type.__dict__, types.MappingProxyType)
         # tp_mro
-        self.assertEqual(obj_type.__mro__, (obj_type, tuple, object))
+        assert obj_type.__mro__ == (obj_type, tuple, object)
         # tp_name
-        self.assertIsInstance(type.__name__, str)
+        assert isinstance(type.__name__, str)
         # tp_subclasses
-        self.assertEqual(obj_type.__subclasses__(), [])
+        assert obj_type.__subclasses__() == []
 
     def test_sys_attrs(self):
         for attr_name in (
@@ -32,23 +36,23 @@ class TestStructSeq(unittest.TestCase):
             'thread_info',    # ThreadInfoType
             'version_info',   # VersionInfoType
         ):
-            with self.subTest(attr=attr_name):
-                attr = getattr(sys, attr_name)
-                self.check_structseq(type(attr))
+            attr = getattr(sys, attr_name)
+            self._check_structseq(type(attr))
 
     def test_sys_funcs(self):
         func_names = ['get_asyncgen_hooks']  # AsyncGenHooksType
         if hasattr(sys, 'getwindowsversion'):
             func_names.append('getwindowsversion')  # WindowsVersionType
         for func_name in func_names:
-            with self.subTest(func=func_name):
-                func = getattr(sys, func_name)
-                obj = func()
-                self.check_structseq(type(obj))
+            func = getattr(sys, func_name)
+            obj = func()
+            self._check_structseq(type(obj))
 
 
 try:
-    unittest.main()
+    tests = TestStructSeq()
+    tests.test_sys_attrs()
+    tests.test_sys_funcs()
 except SystemExit as exc:
     if exc.args[0] != 0:
         raise