[3.13] gh-137017: Ensure Thread.is_alive() only returns False after the underlying OS thread exits (gh-137315) (gh-138917)

(cherry picked from commit aa9ceb1)

Co-authored-by: Abdul <[email protected]>
Co-authored-by: Sam Gross <[email protected]>

Author: 3 people

Misc/NEWS.d/next/Library/2025-08-01-23-11-25.gh-issue-137017.0yGcNc.rst

@@ -0,0 +1,3 @@
+Fix :obj:`threading.Thread.is_alive` to remain ``True`` until the underlying OS
+thread is fully cleaned up. This avoids false negatives in edge cases
+involving thread monitoring or premature :obj:`threading.Thread.is_alive` calls.

Modules/_threadmodule.c

@@ -453,8 +453,9 @@ ThreadHandle_start(ThreadHandle *self, PyObject *func, PyObject *args,
 }
 
 static int
-join_thread(ThreadHandle *handle)
+join_thread(void *arg)
 {
+    ThreadHandle *handle = (ThreadHandle*)arg;
     assert(get_thread_handle_state(handle) == THREAD_HANDLE_RUNNING);
     PyThread_handle_t os_handle;
     if (ThreadHandle_get_os_handle(handle, &os_handle)) {
@@ -528,8 +529,7 @@ ThreadHandle_join(ThreadHandle *self, PyTime_t timeout_ns)
         }
     }
 
-    if (_PyOnceFlag_CallOnce(&self->once, (_Py_once_fn_t *)join_thread,
-                             self) == -1) {
+    if (_PyOnceFlag_CallOnce(&self->once, join_thread, self) == -1) {
         return -1;
     }
     assert(get_thread_handle_state(self) == THREAD_HANDLE_DONE);
@@ -657,6 +657,9 @@ PyThreadHandleObject_is_done(PyThreadHandleObject *self,
                              PyObject *Py_UNUSED(ignored))
 {
     if (_PyEvent_IsSet(&self->handle->thread_is_exiting)) {
+        if (_PyOnceFlag_CallOnce(&self->handle->once, join_thread, self->handle) == -1) {
+            return NULL;
+        }
         Py_RETURN_TRUE;
     }
     else {