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gh-90622: Prevent max_tasks_per_child use with a fork mp_context. (#91587)

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Prevent `max_tasks_per_child` use with a "fork" mp_context to avoid deadlocks.

Also defaults to "spawn" when no mp_context is supplied for safe convenience.
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Gregory P. Smith 2022-05-06 00:04:53 -07:00 committed by GitHub
parent 2b563f1ad3
commit fa4f0a134e
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4 changed files with 43 additions and 11 deletions
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7
Doc/library/concurrent.futures.rst
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@ -254,8 +254,11 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
*max_tasks_per_child* is an optional argument that specifies the maximum
number of tasks a single process can execute before it will exit and be
replaced with a fresh worker process. The default *max_tasks_per_child* is
``None`` which means worker processes will live as long as the pool.
replaced with a fresh worker process. By default *max_tasks_per_child* is
``None`` which means worker processes will live as long as the pool. When
a max is specified, the "spawn" multiprocessing start method will be used by
default in absense of a *mp_context* parameter. This feature is incompatible
with the "fork" start method.
.. versionchanged:: 3.3
When one of the worker processes terminates abruptly, a

24
Lib/concurrent/futures/process.py
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@ -617,14 +617,16 @@ class ProcessPoolExecutor(_base.Executor):
execute the given calls. If None or not given then as many
worker processes will be created as the machine has processors.
mp_context: A multiprocessing context to launch the workers. This
object should provide SimpleQueue, Queue and Process.
object should provide SimpleQueue, Queue and Process. Useful
to allow specific multiprocessing start methods.
initializer: A callable used to initialize worker processes.
initargs: A tuple of arguments to pass to the initializer.
max_tasks_per_child: The maximum number of tasks a worker process can
complete before it will exit and be replaced with a fresh
worker process, to enable unused resources to be freed. The
default value is None, which means worker process will live
as long as the executor will live.
max_tasks_per_child: The maximum number of tasks a worker process
can complete before it will exit and be replaced with a fresh
worker process. The default of None means worker process will
live as long as the executor. Requires a non-'fork' mp_context
start method. When given, we default to using 'spawn' if no
mp_context is supplied.
"""
_check_system_limits()
@ -644,7 +646,10 @@ class ProcessPoolExecutor(_base.Executor):
self._max_workers = max_workers
if mp_context is None:
mp_context = mp.get_context()
if max_tasks_per_child is not None:
mp_context = mp.get_context("spawn")
else:
mp_context = mp.get_context()
self._mp_context = mp_context
if initializer is not None and not callable(initializer):
@ -657,6 +662,11 @@ class ProcessPoolExecutor(_base.Executor):
raise TypeError("max_tasks_per_child must be an integer")
elif max_tasks_per_child <= 0:
raise ValueError("max_tasks_per_child must be >= 1")
if self._mp_context.get_start_method(allow_none=False) == "fork":
# https://github.com/python/cpython/issues/90622
raise ValueError("max_tasks_per_child is incompatible with"
" the 'fork' multiprocessing start method;"
" supply a different mp_context.")
self._max_tasks_per_child = max_tasks_per_child
# Management thread

18
Lib/test/test_concurrent_futures.py
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@ -1039,10 +1039,15 @@ class ProcessPoolExecutorTest(ExecutorTest):
executor.shutdown()
def test_max_tasks_per_child(self):
context = self.get_context()
if context.get_start_method(allow_none=False) == "fork":
with self.assertRaises(ValueError):
self.executor_type(1, mp_context=context, max_tasks_per_child=3)
return
# not using self.executor as we need to control construction.
# arguably this could go in another class w/o that mixin.
executor = self.executor_type(
1, mp_context=self.get_context(), max_tasks_per_child=3)
1, mp_context=context, max_tasks_per_child=3)
f1 = executor.submit(os.getpid)
original_pid = f1.result()
# The worker pid remains the same as the worker could be reused
@ -1061,11 +1066,20 @@ class ProcessPoolExecutorTest(ExecutorTest):
executor.shutdown()
def test_max_tasks_per_child_defaults_to_spawn_context(self):
# not using self.executor as we need to control construction.
# arguably this could go in another class w/o that mixin.
executor = self.executor_type(1, max_tasks_per_child=3)
self.assertEqual(executor._mp_context.get_start_method(), "spawn")
def test_max_tasks_early_shutdown(self):
context = self.get_context()
if context.get_start_method(allow_none=False) == "fork":
raise unittest.SkipTest("Incompatible with the fork start method.")
# not using self.executor as we need to control construction.
# arguably this could go in another class w/o that mixin.
executor = self.executor_type(
3, mp_context=self.get_context(), max_tasks_per_child=1)
3, mp_context=context, max_tasks_per_child=1)
futures = []
for i in range(6):
futures.append(executor.submit(mul, i, i))

5
Misc/NEWS.d/next/Library/2022-04-15-18-32-38.gh-issue-90622.WQjFDe.rst Normal file
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@ -0,0 +1,5 @@
In ``concurrent.futures.process.ProcessPoolExecutor`` disallow the "fork"
multiprocessing start method when the new ``max_tasks_per_child`` feature is
used as the mix of threads+fork can hang the child processes. Default to
using the safe "spawn" start method in that circumstance if no
``mp_context`` was supplied.