gh-136655: ensure cancelled futures are notified on process pool shutdown

At present when a process pool executor shuts down it is cancelling pending
work items, but failing to notify any waiting threads. Fix this.

See also gh-109934, which is a similar bug in the thread pool executor.

Author: duaneg

Lib/concurrent/futures/process.py

@@ -518,7 +518,9 @@ def flag_executor_shutting_down(self):
                 # to only have futures that are currently running.
                 new_pending_work_items = {}
                 for work_id, work_item in self.pending_work_items.items():
-                    if not work_item.future.cancel():
+                    if work_item.future.cancel():
+                        work_item.future.set_running_or_notify_cancel()
+                    else:
                         new_pending_work_items[work_id] = work_item
                 self.pending_work_items = new_pending_work_items
                 # Drain work_ids_queue since we no longer need to

Lib/test/test_concurrent_futures/executor.py

@@ -247,3 +247,35 @@ def test_swallows_falsey_exceptions(self):
         msg = 'lenlen'
         with self.assertRaisesRegex(FalseyLenException, msg):
             self.executor.submit(raiser, FalseyLenException, msg).result()
+
+    def test_shutdown_notifies_cancelled_futures(self):
+
+        # TODO: remove when gh-109934 is fixed
+        if self.executor_type is futures.ThreadPoolExecutor:
+            self.skipTest("gh-109934: skipping thread pool executor")
+
+        # gh-136655: ensure cancelled futures are notified
+        count = self.worker_count * 2
+        barrier = self.create_barrier(self.worker_count + 1)
+        with self.executor as exec:
+            fs = [exec.submit(blocking_raiser,
+                              barrier if index < self.worker_count else None)
+                  for index in range(count)]
+
+            exec.shutdown(wait=False, cancel_futures=True)
+            try:
+                barrier.wait()
+            except threading.BrokenBarrierError:
+                pass
+
+            for future in fs:
+                self.assertRaises(
+                    (FalseyBoolException, futures.CancelledError, threading.BrokenBarrierError),
+                    future.result)
+
+            self.assertIn('CANCELLED_AND_NOTIFIED', [f._state for f in fs])
+
+def blocking_raiser(barrier=None):
+    if barrier is not None:
+        barrier.wait(1)
+    raise FalseyBoolException()

Lib/test/test_concurrent_futures/util.py

@@ -80,6 +80,9 @@ def get_context(self):
 class ThreadPoolMixin(ExecutorMixin):
     executor_type = futures.ThreadPoolExecutor
 
+    def create_barrier(self, count):
+        return threading.Barrier(count)
+
     def create_event(self):
         return threading.Event()
 
@@ -88,6 +91,9 @@ def create_event(self):
 class InterpreterPoolMixin(ExecutorMixin):
     executor_type = futures.InterpreterPoolExecutor
 
+    def create_barrier(self, count):
+        self.skipTest("InterpreterPoolExecutor doesn't support barriers")
+
     def create_event(self):
         self.skipTest("InterpreterPoolExecutor doesn't support events")
 
@@ -107,6 +113,9 @@ def get_context(self):
             self.skipTest("TSAN doesn't support threads after fork")
         return super().get_context()
 
+    def create_barrier(self, count):
+        return self.manager.Barrier(count)
+
     def create_event(self):
         return self.manager.Event()
 
@@ -122,6 +131,9 @@ def get_context(self):
             self.skipTest("ProcessPoolExecutor unavailable on this system")
         return super().get_context()
 
+    def create_barrier(self, count):
+        return self.manager.Barrier(count)
+
     def create_event(self):
         return self.manager.Event()
 
@@ -141,6 +153,9 @@ def get_context(self):
             self.skipTest("TSAN doesn't support threads after fork")
         return super().get_context()
 
+    def create_barrier(self, count):
+        return self.manager.Barrier(count)
+
     def create_event(self):
         return self.manager.Event()
 

Misc/NEWS.d/next/Library/2025-10-13-13-05-15.gh-issue-136655.R8fBtC.rst

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