gh-128421: add critical section around traceback.tb_next (#131322)

Author: kumaraditya303

Python/clinic/traceback.c.h

@@ -99,10 +99,16 @@ tb_dir(PyObject *Py_UNUSED(self), PyObject *Py_UNUSED(ignored))
                                    "tb_lasti", "tb_lineno");
 }
 
+/*[clinic input]
+@critical_section
+@getter
+traceback.tb_next
+[clinic start generated code]*/
+
 static PyObject *
-tb_next_get(PyObject *op, void *Py_UNUSED(_))
+traceback_tb_next_get_impl(PyTracebackObject *self)
+/*[clinic end generated code: output=963634df7d5fc837 input=8f6345f2b73cb965]*/
 {
-    PyTracebackObject *self = _PyTracebackObject_CAST(op);
     PyObject* ret = (PyObject*)self->tb_next;
     if (!ret) {
         ret = Py_None;
@@ -133,37 +139,48 @@ tb_lineno_get(PyObject *op, void *Py_UNUSED(_))
     return PyLong_FromLong(lineno);
 }
 
+/*[clinic input]
+@critical_section
+@setter
+traceback.tb_next
+[clinic start generated code]*/
+
 static int
-tb_next_set(PyObject *op, PyObject *new_next, void *Py_UNUSED(_))
+traceback_tb_next_set_impl(PyTracebackObject *self, PyObject *value)
+/*[clinic end generated code: output=d4868cbc48f2adac input=ce66367f85e3c443]*/
 {
-    if (!new_next) {
+    if (!value) {
         PyErr_Format(PyExc_TypeError, "can't delete tb_next attribute");
         return -1;
     }
 
     /* We accept None or a traceback object, and map None -> NULL (inverse of
        tb_next_get) */
-    if (new_next == Py_None) {
-        new_next = NULL;
-    } else if (!PyTraceBack_Check(new_next)) {
+    if (value == Py_None) {
+        value = NULL;
+    } else if (!PyTraceBack_Check(value)) {
         PyErr_Format(PyExc_TypeError,
                      "expected traceback object, got '%s'",
-                     Py_TYPE(new_next)->tp_name);
+                     Py_TYPE(value)->tp_name);
         return -1;
     }
 
     /* Check for loops */
-    PyTracebackObject *self = _PyTracebackObject_CAST(op);
-    PyTracebackObject *cursor = (PyTracebackObject *)new_next;
+    PyTracebackObject *cursor = (PyTracebackObject *)value;
+    Py_XINCREF(cursor);
     while (cursor) {
         if (cursor == self) {
             PyErr_Format(PyExc_ValueError, "traceback loop detected");
+            Py_DECREF(cursor);
             return -1;
         }
-        cursor = cursor->tb_next;
+        Py_BEGIN_CRITICAL_SECTION(cursor);
+        Py_XINCREF(cursor->tb_next);
+        Py_SETREF(cursor, cursor->tb_next);
+        Py_END_CRITICAL_SECTION();
     }
 
-    Py_XSETREF(self->tb_next, (PyTracebackObject *)Py_XNewRef(new_next));
+    Py_XSETREF(self->tb_next, (PyTracebackObject *)Py_XNewRef(value));
 
     return 0;
 }
@@ -181,7 +198,7 @@ static PyMemberDef tb_memberlist[] = {
 };
 
 static PyGetSetDef tb_getsetters[] = {
-    {"tb_next", tb_next_get, tb_next_set, NULL, NULL},
+    TRACEBACK_TB_NEXT_GETSETDEF
     {"tb_lineno", tb_lineno_get, NULL, NULL, NULL},
     {NULL}      /* Sentinel */
 };