From db33aee4d8aec3d4b9f98ab2605e288ddca3e050 Mon Sep 17 00:00:00 2001
From: dzaramelcone <134235821+dzaramelcone@users.noreply.github.com>
Date: Thu, 20 Aug 2026 10:22:24 -0400
Subject: [PATCH] [3.14] gh-149816: #96 Fix a race condition in
 invoke_gc_callback with free threading (GH-150029)

(cherry picked from commit 91d71dd67074d4599b6bd49cc933f41f8bd57058)
---
 Lib/test/test_free_threading/test_gc.py       | 21 ++++++++++++
 ...-05-18-12-32-33.gh-issue-149816.v18Ypf.rst |  2 ++
 Python/gc_free_threading.c                    | 34 +++++++++++--------
 3 files changed, 42 insertions(+), 15 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-05-18-12-32-33.gh-issue-149816.v18Ypf.rst

diff --git a/Lib/test/test_free_threading/test_gc.py b/Lib/test/test_free_threading/test_gc.py
index 399010234509408..d1522a1d6da14e0 100644
--- a/Lib/test/test_free_threading/test_gc.py
+++ b/Lib/test/test_free_threading/test_gc.py
@@ -2,6 +2,7 @@
 
 import threading
 from threading import Thread
+import time
 from unittest import TestCase
 import gc
 
@@ -94,6 +95,26 @@ def evil():
         thread.start()
         thread.join()
 
+    def test_gc_callbacks_race_with_mutation(self):
+        def collect():
+            b.wait()
+            while not stop.is_set():
+                gc.collect()
+
+        def mutate():
+            b.wait()
+            while not stop.is_set():
+                gc.callbacks[:] = [lambda *_: _ for _ in range(16)]
+                time.sleep(0)
+                gc.callbacks.clear()
+
+        threads = [threading.Thread(target=f) for f in (collect, mutate) * 4]
+        b = threading.Barrier(len(threads) + 1)
+        stop = threading.Event()
+
+        with threading_helper.start_threads(threads, stop.set):
+            b.wait()
+            time.sleep(0.2)
     def test_set_threshold(self):
         # GH-148613: Setting the GC threshold from another thread could cause a
         # race between the `gc_should_collect` and `gc_set_threshold` functions.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-05-18-12-32-33.gh-issue-149816.v18Ypf.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-05-18-12-32-33.gh-issue-149816.v18Ypf.rst
new file mode 100644
index 000000000000000..bf7e4a624250e3e
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-05-18-12-32-33.gh-issue-149816.v18Ypf.rst
@@ -0,0 +1,2 @@
+Fix race conditions in ``invoke_gc_callback`` iterating ``gc.callbacks``
+in free-threading mode.
diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index d1b8d282415337a..0a00fb4647f04b5 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -9,6 +9,7 @@
 #include "pycore_initconfig.h"    // _PyStatus_NO_MEMORY()
 #include "pycore_interp.h"        // PyInterpreterState.gc
 #include "pycore_interpframe.h"   // _PyFrame_GetLocalsArray()
+#include "pycore_list.h"          // _PyList_GetItemRef()
 #include "pycore_object_alloc.h"  // _PyObject_MallocWithType()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_tstate.h"        // _PyThreadStateImpl
@@ -1885,22 +1886,23 @@ invoke_gc_callback(PyThreadState *tstate, const char *phase,
 
     /* The local variable cannot be rebound, check it for sanity */
     assert(PyList_CheckExact(gcstate->callbacks));
-    PyObject *info = NULL;
-    if (PyList_GET_SIZE(gcstate->callbacks) != 0) {
-        info = Py_BuildValue("{sisnsn}",
-            "generation", generation,
-            "collected", collected,
-            "uncollectable", uncollectable);
-        if (info == NULL) {
-            PyErr_FormatUnraisable("Exception ignored while "
-                                   "invoking gc callbacks");
-            return;
-        }
+    if (PyList_GET_SIZE(gcstate->callbacks) == 0) {
+        return;
+    }
+
+    PyObject *info = Py_BuildValue("{sisnsn}",
+        "generation", generation,
+        "collected", collected,
+        "uncollectable", uncollectable);
+    if (info == NULL) {
+        PyErr_FormatUnraisable("Exception ignored while "
+                               "invoking gc callbacks");
+        return;
     }
 
     PyObject *phase_obj = PyUnicode_FromString(phase);
     if (phase_obj == NULL) {
-        Py_XDECREF(info);
+        Py_DECREF(info);
         PyErr_FormatUnraisable("Exception ignored while "
                                "invoking gc callbacks");
         return;
@@ -1908,8 +1910,10 @@ invoke_gc_callback(PyThreadState *tstate, const char *phase,
 
     PyObject *stack[] = {phase_obj, info};
     for (Py_ssize_t i=0; i<PyList_GET_SIZE(gcstate->callbacks); i++) {
-        PyObject *r, *cb = PyList_GET_ITEM(gcstate->callbacks, i);
-        Py_INCREF(cb); /* make sure cb doesn't go away */
+        PyObject *r, *cb = _PyList_GetItemRef((PyListObject *)gcstate->callbacks, i);
+        if (cb == NULL) {
+            break;
+        }
         r = PyObject_Vectorcall(cb, stack, 2, NULL);
         if (r == NULL) {
             PyErr_FormatUnraisable("Exception ignored while "
@@ -1921,7 +1925,7 @@ invoke_gc_callback(PyThreadState *tstate, const char *phase,
         Py_DECREF(cb);
     }
     Py_DECREF(phase_obj);
-    Py_XDECREF(info);
+    Py_DECREF(info);
     assert(!_PyErr_Occurred(tstate));
 }