pythongh-109934: notify cancelled futures on thread pool shutdown

When `ThreadPoolExecutor` shuts down it cancels any pending futures, however at
present it doesn't notify waiters. Thus their state stays as `CANCELLED`
instead of `CANCELLED_AND_NOTIFIED` and any waiters are not awakened.

Call `set_running_or_notify_cancel` on the cancelled futures to fix this.

Author: duaneg

Lib/concurrent/futures/thread.py

@@ -264,6 +264,7 @@ def shutdown(self, wait=True, *, cancel_futures=False):
                         break
                     if work_item is not None:
                         work_item.future.cancel()
+                        work_item.future.set_running_or_notify_cancel()
 
             # Send a wake-up to prevent threads calling
             # _work_queue.get(block=True) from permanently blocking.

Lib/test/test_concurrent_futures/test_thread_pool.py

@@ -112,6 +112,56 @@ def log_n_wait(ident):
         # ident='third' is cancelled because it remained in the collection of futures
         self.assertListEqual(log, ["ident='first' started", "ident='first' stopped"])
 
+    def test_shutdown_cancels_pending_futures(self):
+        def waiter(barrier):
+            barrier.wait(3)
+        def noop():
+            pass
+        barrier = threading.Barrier(2)
+        called_back_1 = threading.Event()
+        called_back_2 = threading.Event()
+        with self.executor_type(max_workers=1) as pool:
+
+            # Submit two futures, the first of which will block and prevent the
+            # second from running
+            f1 = pool.submit(waiter, barrier)
+            f2 = pool.submit(noop)
+            f1.add_done_callback(lambda f: called_back_1.set())
+            f2.add_done_callback(lambda f: called_back_2.set())
+            fs = {f1, f2}
+
+            completed_iter = futures.as_completed(fs, timeout=0)
+            self.assertRaises(TimeoutError, next, completed_iter)
+
+            # Shutdown the pool, cancelling unstarted task
+            pool.shutdown(wait=False, cancel_futures=True)
+            self.assertTrue(f1.running())
+            self.assertTrue(f2.cancelled())
+            self.assertFalse(called_back_1.is_set())
+            self.assertTrue(called_back_2.is_set())
+
+            completed_iter = futures.as_completed(fs, timeout=0)
+            f = next(completed_iter)
+            self.assertIs(f, f2)
+            self.assertRaises(TimeoutError, next, completed_iter)
+
+            result = futures.wait(fs, timeout=0)
+            self.assertEqual(result.not_done, {f1})
+            self.assertEqual(result.done, {f2})
+
+            # Unblock and wait for the first future to complete
+            barrier.wait(3)
+            called_back_1.wait(3)
+            self.assertTrue(f1.done())
+            self.assertTrue(called_back_1.is_set())
+
+            completed = set(futures.as_completed(fs, timeout=0))
+            self.assertEqual(set(fs), completed)
+
+            result = futures.wait(fs, timeout=0)
+            self.assertEqual(result.not_done, set())
+            self.assertEqual(result.done, set(fs))
+
 
 def setUpModule():
     setup_module()

Misc/NEWS.d/next/Library/2025-05-24-15-15-43.gh-issue-109934.WXOdC8.rst

@@ -0,0 +1,2 @@
+Ensure :class:`concurrent.futures.ThreadPoolExecutor` notifies any futures
+it cancels on shutdown.