Make list_ass_item to be thread-safe

Objects/listobject.c

@@ -853,20 +853,39 @@ list_inplace_repeat(PyObject *_self, Py_ssize_t n)
 }
 
 static int
-list_ass_item(PyObject *aa, Py_ssize_t i, PyObject *v)
+list_ass_item_locked(PyListObject *a, Py_ssize_t i, PyObject *v)
 {
-    PyListObject *a = (PyListObject *)aa;
     if (!valid_index(i, Py_SIZE(a))) {
         PyErr_SetString(PyExc_IndexError,
                         "list assignment index out of range");
         return -1;
     }
-    if (v == NULL)
-        return list_ass_slice(a, i, i+1, v);
-    Py_SETREF(a->ob_item[i], Py_NewRef(v));
+    PyObject *tmp = a->ob_item[i];
+    if (v == NULL) {
+        Py_ssize_t size = Py_SIZE(a);
+        for (Py_ssize_t idx = i; idx < size - 1; idx++) {
+            FT_ATOMIC_STORE_PTR_RELAXED(a->ob_item[idx], a->ob_item[idx + 1]);
+        }
+        Py_SET_SIZE(a, size - 1);
+    }
+    else {
+        FT_ATOMIC_STORE_PTR_RELEASE(a->ob_item[i], Py_NewRef(v));
+    }
+    Py_DECREF(tmp);
     return 0;
 }
 
+static int
+list_ass_item(PyObject *aa, Py_ssize_t i, PyObject *v)
+{
+    int ret;
+    PyListObject *a = (PyListObject *)aa;
+    Py_BEGIN_CRITICAL_SECTION(a);
+    ret = list_ass_item_locked(a, i, v);
+    Py_END_CRITICAL_SECTION();
+    return ret;
+}
+
 /*[clinic input]
 @critical_section
 list.insert