gh-137017: Fix Thread.is_alive() to only return False after the underlying OS thread exits

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

@@ -711,6 +711,10 @@ PyThreadHandleObject_is_done(PyObject *op, PyObject *Py_UNUSED(dummy))
 {
     PyThreadHandleObject *self = PyThreadHandleObject_CAST(op);
     if (_PyEvent_IsSet(&self->handle->thread_is_exiting)) {
+        if (_PyOnceFlag_CallOnce(&self->handle->once, join_thread, self->handle) == -1) {
+            PyErr_SetString(PyExc_RuntimeError, "failed to join thread");
+            return NULL;
+        }
         Py_RETURN_TRUE;
     }
     else {