GH-100892: Fix race in clearing `threading.local` (#100922)

2 files changed, 56 insertions, 15 deletions

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index c777c3e..486988b 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1879,12 +1879,19 @@ temporary_c_thread(void *data)
     PyThread_release_lock(test_c_thread->exit_event);
 }
 
+static test_c_thread_t test_c_thread;
+
 static PyObject *
-call_in_temporary_c_thread(PyObject *self, PyObject *callback)
+call_in_temporary_c_thread(PyObject *self, PyObject *args)
 {
     PyObject *res = NULL;
-    test_c_thread_t test_c_thread;
+    PyObject *callback = NULL;
     long thread;
+    int wait = 1;
+    if (!PyArg_ParseTuple(args, "O|i", &callback, &wait))
+    {
+        return NULL;
+    }
 
     test_c_thread.start_event = PyThread_allocate_lock();
     test_c_thread.exit_event = PyThread_allocate_lock();
@@ -1910,6 +1917,10 @@ call_in_temporary_c_thread(PyObject *self, PyObject *callback)
     PyThread_acquire_lock(test_c_thread.start_event, 1);
     PyThread_release_lock(test_c_thread.start_event);
 
+    if (!wait) {
+        Py_RETURN_NONE;
+    }
+
     Py_BEGIN_ALLOW_THREADS
         PyThread_acquire_lock(test_c_thread.exit_event, 1);
         PyThread_release_lock(test_c_thread.exit_event);
@@ -1919,13 +1930,32 @@ call_in_temporary_c_thread(PyObject *self, PyObject *callback)
 
 exit:
     Py_CLEAR(test_c_thread.callback);
-    if (test_c_thread.start_event)
+    if (test_c_thread.start_event) {
         PyThread_free_lock(test_c_thread.start_event);
-    if (test_c_thread.exit_event)
+        test_c_thread.start_event = NULL;
+    }
+    if (test_c_thread.exit_event) {
         PyThread_free_lock(test_c_thread.exit_event);
+        test_c_thread.exit_event = NULL;
+    }
     return res;
 }
 
+static PyObject *
+join_temporary_c_thread(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    Py_BEGIN_ALLOW_THREADS
+        PyThread_acquire_lock(test_c_thread.exit_event, 1);
+        PyThread_release_lock(test_c_thread.exit_event);
+    Py_END_ALLOW_THREADS
+    Py_CLEAR(test_c_thread.callback);
+    PyThread_free_lock(test_c_thread.start_event);
+    test_c_thread.start_event = NULL;
+    PyThread_free_lock(test_c_thread.exit_event);
+    test_c_thread.exit_event = NULL;
+    Py_RETURN_NONE;
+}
+
 /* marshal */
 
 static PyObject*
@@ -3275,8 +3305,9 @@ static PyMethodDef TestMethods[] = {
      METH_VARARGS | METH_KEYWORDS},
     {"with_tp_del",             with_tp_del,                     METH_VARARGS},
     {"create_cfunction",        create_cfunction,                METH_NOARGS},
-    {"call_in_temporary_c_thread", call_in_temporary_c_thread, METH_O,
+    {"call_in_temporary_c_thread", call_in_temporary_c_thread, METH_VARARGS,
      PyDoc_STR("set_error_class(error_class) -> None")},
+    {"join_temporary_c_thread", join_temporary_c_thread, METH_NOARGS},
     {"pymarshal_write_long_to_file",
         pymarshal_write_long_to_file, METH_VARARGS},
     {"pymarshal_write_object_to_file",
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index d323ca8..4fbf0e5 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -839,6 +839,11 @@ local_traverse(localobject *self, visitproc visit, void *arg)
     return 0;
 }
 
+#define HEAD_LOCK(runtime) \
+    PyThread_acquire_lock((runtime)->interpreters.mutex, WAIT_LOCK)
+#define HEAD_UNLOCK(runtime) \
+    PyThread_release_lock((runtime)->interpreters.mutex)
+
 static int
 local_clear(localobject *self)
 {
@@ -849,18 +854,23 @@ local_clear(localobject *self)
     /* Remove all strong references to dummies from the thread states */
     if (self->key) {
         PyInterpreterState *interp = _PyInterpreterState_GET();
+        _PyRuntimeState *runtime = &_PyRuntime;
+        HEAD_LOCK(runtime);
         PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
-        for(; tstate; tstate = PyThreadState_Next(tstate)) {
-            if (tstate->dict == NULL) {
-                continue;
-            }
-            PyObject *v = _PyDict_Pop(tstate->dict, self->key, Py_None);
-            if (v != NULL) {
-                Py_DECREF(v);
-            }
-            else {
-                PyErr_Clear();
+        HEAD_UNLOCK(runtime);
+        while (tstate) {
+            if (tstate->dict) {
+                PyObject *v = _PyDict_Pop(tstate->dict, self->key, Py_None);
+                if (v != NULL) {
+                    Py_DECREF(v);
+                }
+                else {
+                    PyErr_Clear();
+                }
             }
+            HEAD_LOCK(runtime);
+            tstate = PyThreadState_Next(tstate);
+            HEAD_UNLOCK(runtime);
         }
     }
     return 0;