https://github.com/python/cpython/commit/d36a3f6970407bb91dd863a10f0b42425a7e954e
commit: d36a3f6970407bb91dd863a10f0b42425a7e954e
branch: main
author: Jelle Zijlstra <[email protected]>
committer: JelleZijlstra <[email protected]>
date: 2026-10-22T14:05:16Z
summary:
gh-159739: Release replaced type alias values (#160167)
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.
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-19-55-00.gh-issue-159739.tali01.rst
M Lib/test/test_type_aliases.py
M Objects/typevarobject.c
diff --git a/Lib/test/test_type_aliases.py b/Lib/test/test_type_aliases.py
index 05bc6a80888a06..d80a7fbfd18796 100644
--- a/Lib/test/test_type_aliases.py
+++ b/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]
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-19-55-00.gh-issue-159739.tali01.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-19-55-00.gh-issue-159739.tali01.rst
new file mode 100644
index 00000000000000..a4f672fed87cfe
--- /dev/null
+++
b/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.
diff --git a/Objects/typevarobject.c b/Objects/typevarobject.c
index 398a63257beb4a..11ff8320fd9a48 100644
--- a/Objects/typevarobject.c
+++ b/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;
}
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