Fix data race for list capacity

Author: bkap123

Lib/test/test_list.py

@@ -6,6 +6,7 @@
 from test.support.import_helper import import_module
 from test.support.script_helper import assert_python_failure, assert_python_ok
 import pickle
+from threading import Thread
 import unittest
 
 class ListTest(list_tests.CommonTest):
@@ -381,6 +382,30 @@ def foo(x):
 
         self.assertEqual(foo(list(range(10))), 45)
 
+    # gh-145036: race condition with list.__sizeof__()
+    def test_list_sizeof_free_threaded_build(self):
+        L = []
+
+        def test1():
+            for _ in range(100):
+                L.append(1)
+                L.pop()
+
+        def test2():
+            for _ in range(100):
+                L.__sizeof__()
+
+        threads = []
+        for _ in range(4):
+            threads.append(Thread(target=test1))
+            threads.append(Thread(target=test2))
+
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        self.assertEqual(len(L), 0)
 
 if __name__ == "__main__":
     unittest.main()

Objects/listobject.c

@@ -3558,8 +3558,14 @@ list___sizeof___impl(PyListObject *self)
 /*[clinic end generated code: output=3417541f95f9a53e input=b8030a5d5ce8a187]*/
 {
     size_t res = _PyObject_SIZE(Py_TYPE(self));
-    Py_ssize_t allocated = FT_ATOMIC_LOAD_SSIZE_RELAXED(self->allocated);
-    res += (size_t)allocated * sizeof(void*);
+#ifdef Py_GIL_DISABLED
+    PyObject **ob_item = _Py_atomic_load_ptr(&self->ob_item);
+    if (ob_item != NULL) {
+        res += list_capacity(ob_item) * sizeof(PyObject *);
+    }
+#else
+    res += (size_t)self->allocated * sizeof(PyObject *);
+#endif
     return PyLong_FromSize_t(res);
 }