gh-137992: Stop the world when calling PyRefTracer_SetTracer

Misc/NEWS.d/next/Core_and_Builtins/2025-08-20-14-17-47.gh-issue-137992.fcL3SK.rst

@@ -0,0 +1,2 @@
+Ensure that  :c:func:`PyRefTracer_SetTracer` sync with all existing threads when called
+to avoid races in the free threaded build. Patch by Pablo Galindo

Objects/object.c

@@ -3286,8 +3286,10 @@ _Py_SetRefcnt(PyObject *ob, Py_ssize_t refcnt)
 
 int PyRefTracer_SetTracer(PyRefTracer tracer, void *data) {
     _Py_AssertHoldsTstate();
+    _PyEval_StopTheWorldAll(&_PyRuntime);
     _PyRuntime.ref_tracer.tracer_func = tracer;
     _PyRuntime.ref_tracer.tracer_data = data;
+    _PyEval_StartTheWorldAll(&_PyRuntime);
     return 0;
 }