[3.15] gh-159739: Release replaced type alias values (GH-160167) (#160242)

A type alias can be evaluated again before an earlier evaluation finishes, either recursively or in another thread. The later completion overwrites the cached value without releasing it, permanently leaking that value; synchronize cache access and release the replaced value outside the lock. Backport to 3.15 and 3.14.
(cherry picked from commit d36a3f6)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Lib/test/test_type_aliases.py

@@ -1,7 +1,8 @@
 import pickle
 import types
 import unittest
-from test.support import check_syntax_error, run_code
+import weakref
+from test.support import check_syntax_error, gc_collect, run_code
 from test.typinganndata import mod_generics_cache
 
 from typing import (
@@ -131,6 +132,24 @@ def more_generic[T, *Ts, **P]():
         T, Ts, P = more_generic.__type_params__
         self.assertEqual(alias.__value__, tuple[Callable[P, tuple[T, *Ts]], Callable[P2, tuple[T2, *Ts2]]])
 
+    def test_reentrant_value_does_not_leak(self):
+        class Value:
+            pass
+
+        refs = []
+
+        def compute():
+            value = Value()
+            refs.append(weakref.ref(value))
+            if len(refs) == 1:
+                Alias.__value__
+            return value
+
+        type Alias = compute()
+        self.assertIs(Alias.__value__, refs[0]())
+        gc_collect()
+        self.assertIsNone(refs[1]())
+
     def test_subscripting(self):
         type NonGeneric = int
         type Generic[A] = dict[A, A]

Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-19-55-00.gh-issue-159739.tali01.rst

@@ -0,0 +1,2 @@
+Fix a reference leak when the value of a type alias is evaluated
+recursively or concurrently.

Objects/typevarobject.c

@@ -1899,14 +1899,23 @@ typealias_dealloc(PyObject *self)
 static PyObject *
 typealias_get_value(typealiasobject *ta)
 {
-    if (ta->value != NULL) {
-        return Py_NewRef(ta->value);
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(ta);
+    result = Py_XNewRef(ta->value);
+    Py_END_CRITICAL_SECTION();
+    if (result != NULL) {
+        return result;
     }
-    PyObject *result = PyObject_CallNoArgs(ta->compute_value);
+    result = PyObject_CallNoArgs(ta->compute_value);
     if (result == NULL) {
         return NULL;
     }
+    PyObject *old_value;
+    Py_BEGIN_CRITICAL_SECTION(ta);
+    old_value = ta->value;
     ta->value = Py_NewRef(result);
+    Py_END_CRITICAL_SECTION();
+    Py_XDECREF(old_value);
     return result;
 }