bpo-39349: Add *cancel_futures* to Executor.shutdown() (GH-18057)

3 files changed, 75 insertions, 2 deletions

diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 9e2ab9d..fd9f572 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -435,6 +435,24 @@ def _queue_management_worker(executor_reference,
                 # is not gc-ed yet.
                 if executor is not None:
                     executor._shutdown_thread = True
+                    # Unless there are pending work items, we have nothing to cancel.
+                    if pending_work_items and executor._cancel_pending_futures:
+                        # Cancel all pending futures and update pending_work_items
+                        # to only have futures that are currently running.
+                        new_pending_work_items = {}
+                        for work_id, work_item in pending_work_items.items():
+                            if not work_item.future.cancel():
+                                new_pending_work_items[work_id] = work_item
+
+                        pending_work_items = new_pending_work_items
+                        # Drain work_ids_queue since we no longer need to
+                        # add items to the call queue.
+                        while True:
+                            try:
+                                work_ids_queue.get_nowait()
+                            except queue.Empty:
+                                break
+
                 # Since no new work items can be added, it is safe to shutdown
                 # this thread if there are no pending work items.
                 if not pending_work_items:
@@ -546,6 +564,7 @@ class ProcessPoolExecutor(_base.Executor):
         self._broken = False
         self._queue_count = 0
         self._pending_work_items = {}
+        self._cancel_pending_futures = False
 
         # Create communication channels for the executor
         # Make the call queue slightly larger than the number of processes to
@@ -660,9 +679,11 @@ class ProcessPoolExecutor(_base.Executor):
                               timeout=timeout)
         return _chain_from_iterable_of_lists(results)
 
-    def shutdown(self, wait=True):
+    def shutdown(self, wait=True, *, cancel_futures=False):
         with self._shutdown_lock:
+            self._cancel_pending_futures = cancel_futures
             self._shutdown_thread = True
+
         if self._queue_management_thread:
             # Wake up queue management thread
             self._queue_management_thread_wakeup.wakeup()
diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index b89f8f2..be79161 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -215,9 +215,22 @@ class ThreadPoolExecutor(_base.Executor):
                 if work_item is not None:
                     work_item.future.set_exception(BrokenThreadPool(self._broken))
 
-    def shutdown(self, wait=True):
+    def shutdown(self, wait=True, *, cancel_futures=False):
         with self._shutdown_lock:
             self._shutdown = True
+            if cancel_futures:
+                # Drain all work items from the queue, and then cancel their
+                # associated futures.
+                while True:
+                    try:
+                        work_item = self._work_queue.get_nowait()
+                    except queue.Empty:
+                        break
+                    if work_item is not None:
+                        work_item.future.cancel()
+
+            # Send a wake-up to prevent threads calling
+            # _work_queue.get(block=True) from permanently blocking.
             self._work_queue.put(None)
         if wait:
             for t in self._threads:
diff --git a/Lib/test/test_concurrent_futures.py b/Lib/test/test_concurrent_futures.py
index c8fa35e..af77f81 100644
--- a/Lib/test/test_concurrent_futures.py
+++ b/Lib/test/test_concurrent_futures.py
@@ -342,6 +342,29 @@ class ExecutorShutdownTest:
         for f in fs:
             f.result()
 
+    def test_cancel_futures(self):
+        executor = self.executor_type(max_workers=3)
+        fs = [executor.submit(time.sleep, .1) for _ in range(50)]
+        executor.shutdown(cancel_futures=True)
+        # We can't guarantee the exact number of cancellations, but we can
+        # guarantee that *some* were cancelled. With setting max_workers to 3,
+        # most of the submitted futures should have been cancelled.
+        cancelled = [fut for fut in fs if fut.cancelled()]
+        self.assertTrue(len(cancelled) >= 35, msg=f"{len(cancelled)=}")
+
+        # Ensure the other futures were able to finish.
+        # Use "not fut.cancelled()" instead of "fut.done()" to include futures
+        # that may have been left in a pending state.
+        others = [fut for fut in fs if not fut.cancelled()]
+        for fut in others:
+            self.assertTrue(fut.done(), msg=f"{fut._state=}")
+            self.assertIsNone(fut.exception())
+
+        # Similar to the number of cancelled futures, we can't guarantee the
+        # exact number that completed. But, we can guarantee that at least
+        # one finished.
+        self.assertTrue(len(others) > 0, msg=f"{len(others)=}")
+
     def test_hang_issue39205(self):
         """shutdown(wait=False) doesn't hang at exit with running futures.
 
@@ -422,6 +445,22 @@ class ThreadPoolShutdownTest(ThreadPoolMixin, ExecutorShutdownTest, BaseTestCase
             self.assertRegex(t.name, r'ThreadPoolExecutor-\d+_[0-4]$')
             t.join()
 
+    def test_cancel_futures_wait_false(self):
+        # Can only be reliably tested for TPE, since PPE often hangs with
+        # `wait=False` (even without *cancel_futures*).
+        rc, out, err = assert_python_ok('-c', """if True:
+            from concurrent.futures import ThreadPoolExecutor
+            from test.test_concurrent_futures import sleep_and_print
+            if __name__ == "__main__":
+                t = ThreadPoolExecutor()
+                t.submit(sleep_and_print, .1, "apple")
+                t.shutdown(wait=False, cancel_futures=True)
+            """.format(executor_type=self.executor_type.__name__))
+        # Errors in atexit hooks don't change the process exit code, check
+        # stderr manually.
+        self.assertFalse(err)
+        self.assertEqual(out.strip(), b"apple")
+
 
 class ProcessPoolShutdownTest(ExecutorShutdownTest):
     def _prime_executor(self):