gh-134381: Improve thread safety in _PyThread_AfterFork by preserving not-started handles

Lib/test/test_thread.py

@@ -439,6 +439,21 @@ def fork_thread(read_fd, write_fd):
         self.assertIsNotNone(pid)
         support.wait_process(pid, exitcode=0)
 
+    @support.requires_fork()
+    @threading_helper.reap_threads
+    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)
+
     def tearDown(self):
         try:
             os.close(self.read_fd)

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

@@ -0,0 +1 @@
+Fix :exc:`RuntimeError` when using a not-started :class:`threading.Thread` after calling :func:`os.fork`

Modules/_threadmodule.c

@@ -296,6 +296,11 @@ _PyThread_AfterFork(struct _pythread_runtime_state *state)
             continue;
         }
 
+        // keep not_start handles – they are safe to start in the child
+        if (handle->state == THREAD_HANDLE_NOT_STARTED){
+            continue;
+        }
+
         // Mark all threads as done. Any attempts to join or detach the
         // underlying OS thread (if any) could crash. We are the only thread;
         // it's safe to set this non-atomically.