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