python · colesbury · May 23, 2025 · May 22, 2025 · May 22, 2025 · May 22, 2025
@@ -6844,6 +6844,28 @@ def f(x): return x*x
        self.assertEqual("332833500", out.decode('utf-8').strip())
        self.assertFalse(err, msg=err.decode('utf-8'))

+    @support.requires_fork()
+    def test_forked_not_started(self):
+        # gh-134381
+        # Ensure that a forked parent process, which has not been started yet,
+        # can be initiated within the child process.
+
+        ctx = multiprocessing.get_context("fork")   # local “fork” cont
+        q = ctx.Queue()
+        t = threading.Thread(target=lambda: q.put("done"), daemon=True)
+
+        def child():
+            t.start()
+            t.join()
+
+        p = ctx.Process(target=child)
+        p.start()
+        p.join(support.SHORT_TIMEOUT)
+
+        self.assertEqual(p.exitcode, 0)
+        self.assertEqual(q.get_nowait(), "done")
+        close_queue(q)
+

 #
 # Mixins

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-22-14-48-19.gh-issue-134381.2BXhth.rst b/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`
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
@@ -296,6 +296,12 @@ _PyThread_AfterFork(struct _pythread_runtime_state *state)
            continue;
        }

+        // Keep handles for threads that have not been started yet. They are
+        // safe to start in the child process.
+        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.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix :exc:`RuntimeError` when using a not-started :class:`threading.Thread` after calling :func:`os.fork`