bpo-39104: Fix hanging ProcessPoolExecutor on shutdown nowait with pickling failure

Lib/concurrent/futures/process.py

@@ -87,7 +87,12 @@ def close(self):
  self._reader.close()
 
  def wakeup(self):
- self._writer.send_bytes(b"")
+ try:
+ self._writer.send_bytes(b"")
+ except OSError:
+ # This can happen if the QueueManagerThread has been shutdown by
+ # another thread before this wakeup call.
+ pass
 
  def clear(self):
  while self._reader.poll():
@@ -160,15 +165,17 @@ def __init__(self, work_id, fn, args, kwargs):
 
 class _SafeQueue(Queue):
  """Safe Queue set exception to the future object linked to a job"""
- def __init__(self, max_size=0, *, ctx, pending_work_items):
+ def __init__(self, max_size=0, *, ctx, pending_work_items, thread_wakeup):
  self.pending_work_items = pending_work_items
+ self.thread_wakeup = thread_wakeup
  super().__init__(max_size, ctx=ctx)
 
  def _on_queue_feeder_error(self, e, obj):
  if isinstance(obj, _CallItem):
  tb = traceback.format_exception(type(e), e, e.__traceback__)
  e.__cause__ = _RemoteTraceback('\n"""\n{}"""'.format(''.join(tb)))
  work_item = self.pending_work_items.pop(obj.work_id, None)
+ self.thread_wakeup.wakeup()
  # work_item can be None if another process terminated. In this case,
  # the queue_manager_thread fails all work_items with BrokenProcessPool
  if work_item is not None:
@@ -339,6 +346,8 @@ def shutdown_worker():
 
  # Release the queue's resources as soon as possible.
  call_queue.close()
+ call_queue.join_thread()
+ thread_wakeup.close()
  # If .join() is not called on the created processes then
  # some ctx.Queue methods may deadlock on Mac OS X.
  for p in processes.values():
@@ -547,29 +556,30 @@ def __init__(self, max_workers=None, mp_context=None,
  self._queue_count = 0
  self._pending_work_items = {}
 
+ # _ThreadWakeup is a communication channel used to interrupt the wait
+ # of the main loop of queue_manager_thread from another thread (e.g.
+ # when calling executor.submit or executor.shutdown). We do not use the
+ # _result_queue to send the wakeup signal to the queue_manager_thread
+ # as it could result in a deadlock if a worker process dies with the
+ # _result_queue write lock still acquired.
+ self._queue_management_thread_wakeup = _ThreadWakeup()
+
  # Create communication channels for the executor
  # Make the call queue slightly larger than the number of processes to
  # prevent the worker processes from idling. But don't make it too big
  # because futures in the call queue cannot be cancelled.
  queue_size = self._max_workers + EXTRA_QUEUED_CALLS
  self._call_queue = _SafeQueue(
  max_size=queue_size, ctx=self._mp_context,
- pending_work_items=self._pending_work_items)
+ pending_work_items=self._pending_work_items,
+ thread_wakeup=self._queue_management_thread_wakeup)
  # Killed worker processes can produce spurious "broken pipe"
  # tracebacks in the queue's own worker thread. But we detect killed
  # processes anyway, so silence the tracebacks.
  self._call_queue._ignore_epipe = True
  self._result_queue = mp_context.SimpleQueue()
  self._work_ids = queue.Queue()
 
- # _ThreadWakeup is a communication channel used to interrupt the wait
- # of the main loop of queue_manager_thread from another thread (e.g.
- # when calling executor.submit or executor.shutdown). We do not use the
- # _result_queue to send the wakeup signal to the queue_manager_thread
- # as it could result in a deadlock if a worker process dies with the
- # _result_queue write lock still acquired.
- self._queue_management_thread_wakeup = _ThreadWakeup()
-
  def _start_queue_management_thread(self):
  if self._queue_management_thread is None:
  # When the executor gets garbarge collected, the weakref callback
@@ -671,16 +681,11 @@ def shutdown(self, wait=True):
  # To reduce the risk of opening too many files, remove references to
  # objects that use file descriptors.
  self._queue_management_thread = None
- if self._call_queue is not None:
- self._call_queue.close()
- if wait:
- self._call_queue.join_thread()
- self._call_queue = None
+ self._call_queue = None
  self._result_queue = None
  self._processes = None
 
  if self._queue_management_thread_wakeup:
- self._queue_management_thread_wakeup.close()
  self._queue_management_thread_wakeup = None
 
  shutdown.__doc__ = _base.Executor.shutdown.__doc__

Lib/test/test_concurrent_futures.py

@@ -1027,6 +1027,32 @@ def test_shutdown_deadlock(self):
  with self.assertRaises(BrokenProcessPool):
  f.result()
 
+ def test_shutdown_deadlock_pickle(self):
+ # Test that the pool calling shutdown with wait=False does not cause
+ # a deadlock if a task fails at pickle after the shutdown call.
+ # Reported in GH#6034.
+ self.executor.shutdown(wait=True)
+ with self.executor_type(max_workers=2,
+ mp_context=get_context(self.ctx)) as executor:
+ self.executor = executor # Allow clean up in fail_on_deadlock
+
+ # Start the executor and get the queue_management_thread to collect
+ # the threads and avoid dangling thread that should be cleaned up
+ # asynchronously.
+ executor.submit(id, 42).result()
+ queue_manager = executor._queue_management_thread
+
+ # Submit a task that fails at pickle and shutdown the executor
+ # without waiting
+ f = executor.submit(id, ErrorAtPickle())
+ executor.shutdown(wait=False)
+ with self.assertRaises(PicklingError):
+ f.result()
+
+ # Make sure the executor is eventually shutdown and do not leave
+ # dangling threads
+ queue_manager.join()
+
 
 create_executor_tests(ExecutorDeadlockTest,
  executor_mixins=(ProcessPoolForkMixin,