gh-93353: Add test.support.late_deletion() (#93774)

Author: vstinner

Lib/test/support/__init__.py

@@ -2213,3 +2213,40 @@ def requires_venv_with_pip():
 # True if Python is built with the Py_DEBUG macro defined: if
 # Python is built in debug mode (./configure --with-pydebug).
 Py_DEBUG = hasattr(sys, 'gettotalrefcount')
+
+
+def late_deletion(obj):
+    """
+    Keep a Python alive as long as possible.
+
+    Create a reference cycle and store the cycle in an object deleted late in
+    Python finalization. Try to keep the object alive until the very last
+    garbage collection.
+
+    The function keeps a strong reference by design. It should be called in a
+    subprocess to not mark a test as "leaking a reference".
+    """
+
+    # Late CPython finalization:
+    # - finalize_interp_clear()
+    # - _PyInterpreterState_Clear(): Clear PyInterpreterState members
+    #   (ex: codec_search_path, before_forkers)
+    # - clear os.register_at_fork() callbacks
+    # - clear codecs.register() callbacks
+
+    ref_cycle = [obj]
+    ref_cycle.append(ref_cycle)
+
+    # Store a reference in PyInterpreterState.codec_search_path
+    import codecs
+    def search_func(encoding):
+        return None
+    search_func.reference = ref_cycle
+    codecs.register(search_func)
+
+    if hasattr(os, 'register_at_fork'):
+        # Store a reference in PyInterpreterState.before_forkers
+        def atfork_func():
+            pass
+        atfork_func.reference = ref_cycle
+        os.register_at_fork(before=atfork_func)

Lib/test/test_gc.py

@@ -1440,19 +1440,13 @@ def test_ast_fini(self):
         code = textwrap.dedent("""
             import ast
             import codecs
+            from test import support
 
             # Small AST tree to keep their AST types alive
             tree = ast.parse("def f(x, y): return 2*x-y")
-            x = [tree]
-            x.append(x)
-
-            # Put the cycle somewhere to survive until the last GC collection.
-            # Codec search functions are only cleared at the end of
-            # interpreter_clear().
-            def search_func(encoding):
-                return None
-            search_func.a = x
-            codecs.register(search_func)
+
+            # Store the tree somewhere to survive until the last GC collection
+            support.late_deletion(tree)
         """)
         assert_python_ok("-c", code)