bpo-39104: Fix hanging ProcessPoolExecutor on shutdown nowait with pickling failure (GH-17670)
As reported initially by @rad-pat in #6084, the following script causes a deadlock.
```
from concurrent.futures import ProcessPoolExecutor
class ObjectWithPickleError():
"""Triggers a RuntimeError when sending job to the workers"""
def __reduce__(self):
raise RuntimeError()
if __name__ == "__main__":
e = ProcessPoolExecutor()
f = e.submit(id, ObjectWithPickleError())
e.shutdown(wait=False)
f.result() # Deadlock on get
```
This is caused by the fact that the main process is closing communication channels that might be necessary to the `queue_management_thread` later. To avoid this, this PR let the `queue_management_thread` manage all the closing.
https://bugs.python.org/issue39104
Automerge-Triggered-By: @pitroudiff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index fd9f572..d773228 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -80,18 +80,23 @@
class _ThreadWakeup:
def __init__(self):
+ self._closed = False
self._reader, self._writer = mp.Pipe(duplex=False)
def close(self):
- self._writer.close()
- self._reader.close()
+ if not self._closed:
+ self._closed = True
+ self._writer.close()
+ self._reader.close()
def wakeup(self):
- self._writer.send_bytes(b"")
+ if not self._closed:
+ self._writer.send_bytes(b"")
def clear(self):
- while self._reader.poll():
- self._reader.recv_bytes()
+ if not self._closed:
+ while self._reader.poll():
+ self._reader.recv_bytes()
def _python_exit():
@@ -160,8 +165,9 @@
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):
@@ -169,6 +175,7 @@
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 @@
# 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():
@@ -566,21 +575,6 @@
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
- # 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)
- # 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
@@ -589,6 +583,22 @@
# _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,
+ 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()
+
def _start_queue_management_thread(self):
if self._queue_management_thread is None:
# When the executor gets garbarge collected, the weakref callback
@@ -692,16 +702,11 @@
# 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__
diff --git a/Lib/test/test_concurrent_futures.py b/Lib/test/test_concurrent_futures.py
index af77f81..a7381f9 100644
--- a/Lib/test/test_concurrent_futures.py
+++ b/Lib/test/test_concurrent_futures.py
@@ -415,13 +415,32 @@
def test_del_shutdown(self):
executor = futures.ThreadPoolExecutor(max_workers=5)
- executor.map(abs, range(-5, 5))
+ res = executor.map(abs, range(-5, 5))
threads = executor._threads
del executor
for t in threads:
t.join()
+ # Make sure the results were all computed before the
+ # executor got shutdown.
+ assert all([r == abs(v) for r, v in zip(res, range(-5, 5))])
+
+ def test_shutdown_no_wait(self):
+ # Ensure that the executor cleans up the threads when calling
+ # shutdown with wait=False
+ executor = futures.ThreadPoolExecutor(max_workers=5)
+ res = executor.map(abs, range(-5, 5))
+ threads = executor._threads
+ executor.shutdown(wait=False)
+ for t in threads:
+ t.join()
+
+ # Make sure the results were all computed before the
+ # executor got shutdown.
+ assert all([r == abs(v) for r, v in zip(res, range(-5, 5))])
+
+
def test_thread_names_assigned(self):
executor = futures.ThreadPoolExecutor(
max_workers=5, thread_name_prefix='SpecialPool')
@@ -488,7 +507,7 @@
def test_del_shutdown(self):
executor = futures.ProcessPoolExecutor(max_workers=5)
- list(executor.map(abs, range(-5, 5)))
+ res = executor.map(abs, range(-5, 5))
queue_management_thread = executor._queue_management_thread
processes = executor._processes
call_queue = executor._call_queue
@@ -502,6 +521,31 @@
p.join()
call_queue.join_thread()
+ # Make sure the results were all computed before the
+ # executor got shutdown.
+ assert all([r == abs(v) for r, v in zip(res, range(-5, 5))])
+
+ def test_shutdown_no_wait(self):
+ # Ensure that the executor cleans up the processes when calling
+ # shutdown with wait=False
+ executor = futures.ProcessPoolExecutor(max_workers=5)
+ res = executor.map(abs, range(-5, 5))
+ processes = executor._processes
+ call_queue = executor._call_queue
+ queue_management_thread = executor._queue_management_thread
+ executor.shutdown(wait=False)
+
+ # Make sure that all the executor resources were properly cleaned by
+ # the shutdown process
+ queue_management_thread.join()
+ for p in processes.values():
+ p.join()
+ call_queue.join_thread()
+
+ # Make sure the results were all computed before the executor got
+ # shutdown.
+ assert all([r == abs(v) for r, v in zip(res, range(-5, 5))])
+
create_executor_tests(ProcessPoolShutdownTest,
executor_mixins=(ProcessPoolForkMixin,
@@ -1086,6 +1130,32 @@
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 bpo-39104.
+ 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,