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,