Add a test case, and improve the NEWS.d

Author: zangjiucheng

Lib/test/test_thread.py

@@ -337,6 +337,18 @@ def func():
             handle = thread.start_joinable_thread(func, handle=None)
             handle.join()
 
+    def test_forked_not_started(self):
+        handle = thread._ThreadHandle()
+        pid = os.fork()
+        if pid == 0:
+            # child process
+            try:
+                self.assertTrue(handle.is_alive())
+                self.assertFalse(handle.is_done())
+            finally:
+                os._exit(0)
+        else:
+            support.wait_process(pid, exitcode=0)
 
 class Barrier:
     def __init__(self, num_threads):

Misc/NEWS.d/next/Core_and_Builtins/2025-05-22-14-48-19.gh-issue-134381.2BXhth.rst

@@ -1 +1 @@
-Improve thread safety in :class:`_PyThread_AfterFork` by preserving not-started handles
+Fix :exc:`RuntimeError` when using a not-started :class:`threading.Thread` after calling :func:`os.fork`