gh-109047: concurrent.futures catches PythonFinalizationError

concurrent.futures: The 'executor manager thread' now catches PythonFinalizationError, it calls terminate_broken(). The exception occurs while Python is being finalized when adding an item to the 'call queue' tries to create a new 'queue feeder' thread. Add test_python_finalization_error() to test_concurrent_futures. concurrent.futures._ExecutorManagerThread changes: * terminate_broken() no longer calls shutdown_workers() since the queue is no longer working anymore (read and write ends of the queue pipe are closed). * terminate_broken() now terminates child processes. * wait_result_broken_or_wakeup() now uses the short form of traceback.format_exception(). multiprocessing.Queue changes: * Add _terminate_broken() method. * _start_thread() sets _thread to None on exception to prevent leaking "dangling threads" even if the thread was not started yet.

Author: vstinner

Lib/concurrent/futures/process.py

@@ -341,7 +341,12 @@ def run(self):
  # Main loop for the executor manager thread.
 
  while True:
- self.add_call_item_to_queue()
+ try:
+ self.add_call_item_to_queue()
+ except PythonFinalizationError as exc:
+ cause = format_exception(exc)
+ self.terminate_broken(cause)
+ return
 
  result_item, is_broken, cause = self.wait_result_broken_or_wakeup()
 
@@ -425,8 +430,8 @@ def wait_result_broken_or_wakeup(self):
  try:
  result_item = result_reader.recv()
  is_broken = False
- except BaseException as e:
- cause = format_exception(type(e), e, e.__traceback__)
+ except BaseException as exc:
+ cause = format_exception(exc)
 
  elif wakeup_reader in ready:
  is_broken = False
@@ -515,16 +520,10 @@ def terminate_broken(self, cause):
  for p in self.processes.values():
  p.terminate()
 
- # Prevent queue writing to a pipe which is no longer read.
- # https://github.com/python/cpython/issues/94777
- self.call_queue._reader.close()
-
- # gh-107219: Close the connection writer which can unblock
- # Queue._feed() if it was stuck in send_bytes().
- self.call_queue._writer.close()
+ self.call_queue._terminate_broken()
 
  # clean up resources
- self.join_executor_internals()
+ self._join_executor_internals(broken=True)
 
  def flag_executor_shutting_down(self):
  # Flag the executor as shutting down and cancel remaining tasks if
@@ -567,7 +566,11 @@ def shutdown_workers(self):
  break
 
  def join_executor_internals(self):
- self.shutdown_workers()
+ self._join_executor_internals()
+
+ def _join_executor_internals(self, broken=False):
+ if not broken:
+ self.shutdown_workers()
  # Release the queue's resources as soon as possible.
  self.call_queue.close()
  self.call_queue.join_thread()
@@ -576,6 +579,8 @@ def join_executor_internals(self):
  # If .join() is not called on the created processes then
  # some ctx.Queue methods may deadlock on Mac OS X.
  for p in self.processes.values():
+ if broken:
+ p.terminate()
  p.join()
 
  def get_n_children_alive(self):

Lib/multiprocessing/queues.py

@@ -158,6 +158,19 @@ def cancel_join_thread(self):
  except AttributeError:
  pass
 
+ def _terminate_broken(self):
+ # Close a Queue on error.
+
+ # gh-94777: Prevent queue writing to a pipe which is no longer read.
+ self._reader.close()
+
+ # gh-107219: Close the connection writer which can unblock
+ # Queue._feed() if it was stuck in send_bytes().
+ self._writer.close()
+
+ self.close()
+ self.join_thread()
+
  def _start_thread(self):
  debug('Queue._start_thread()')
 
@@ -169,13 +182,17 @@ def _start_thread(self):
  self._wlock, self._reader.close, self._writer.close,
  self._ignore_epipe, self._on_queue_feeder_error,
  self._sem),
- name='QueueFeederThread'
+ name='QueueFeederThread',
+ daemon=True,
  )
- self._thread.daemon = True
 
- debug('doing self._thread.start()')
- self._thread.start()
- debug('... done self._thread.start()')
+ try:
+ debug('doing self._thread.start()')
+ self._thread.start()
+ debug('... done self._thread.start()')
+ except:
+ self._thread = None
+ raise
 
  if not self._joincancelled:
  self._jointhread = Finalize(

Lib/test/test_concurrent_futures/test_process_pool.py

@@ -1,5 +1,6 @@
 import os
 import sys
+import threading
 import time
 import unittest
 from concurrent import futures
@@ -187,6 +188,27 @@ def test_max_tasks_early_shutdown(self):
  for i, future in enumerate(futures):
  self.assertEqual(future.result(), mul(i, i))
 
+ def test_python_finalization_error(self):
+ context = self.get_context()
+
+ # Create _ExecutorManagerThread, but block QueueFeederThread
+ orig_start_new_thread = threading._start_new_thread
+ nthread = 0
+ def mock_start_new_thread(func, *args):
+ nonlocal nthread
+ if nthread >= 1:
+ raise PythonFinalizationError()
+ nthread += 1
+ return orig_start_new_thread(func, *args)
+
+ with support.swap_attr(threading, '_start_new_thread',
+ mock_start_new_thread):
+ executor = self.executor_type(max_workers=2, mp_context=context)
+ with executor:
+ with self.assertRaises(BrokenProcessPool):
+ list(executor.map(mul, [(2, 3)] * 10))
+ executor.shutdown()
+
 
 create_executor_tests(globals(), ProcessPoolExecutorTest,
  executor_mixins=(ProcessPoolForkMixin,

Misc/NEWS.d/next/Library/2023-09-25-02-11-14.gh-issue-109047.b1TrqG.rst

@@ -0,0 +1,4 @@
+:mod:`concurrent.futures`: The *executor manager thread* now catches
+:exc:`PythonFinalizationError`, it calls ``terminate_broken()``. The exception
+occurs while Python is being finalized when adding an item to the *call queue*
+tries to create a new *queue feeder* thread. Patch by Victor Stinner.