gh-154937: Fix _thread._shutdown() racing with _thread.start_joinable_thread on ThreadHandle.ident (GH-155085)

(cherry picked from commit a4095ee)

Co-authored-by: Edward Xu <xuxiangad@gmail.com>

Author: LindaSummer

Misc/NEWS.d/next/Core_and_Builtins/2026-08-02-17-55-29.gh-issue-154937.m9IBba.rst

@@ -0,0 +1,3 @@
+Fix a data race on thread handle identifiers when ``_thread._shutdown()``
+runs concurrently with the startup of non-daemon threads in the
+:term:`free-threaded build`.

Modules/_threadmodule.c

@@ -2349,7 +2349,7 @@ thread_shutdown(PyObject *self, PyObject *args)
         struct llist_node *node;
         llist_for_each_safe(node, &state->shutdown_handles) {
             ThreadHandle *cur = llist_data(node, ThreadHandle, shutdown_node);
-            if (cur->ident != ident) {
+            if (ThreadHandle_ident(cur) != ident) {
                 ThreadHandle_incref(cur);
                 handle = cur;
                 break;