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gh-90622: Do not spawn ProcessPool workers on demand via fork method. (GH-91598) (#92495)

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Do not spawn ProcessPool workers on demand when they spawn via fork.

This avoids potential deadlocks in the child processes due to forking from
a multithreaded process.
(cherry picked from commit ebb37fc3fd)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
This commit is contained in:
Miss Islington (bot) 2022-05-08 10:14:14 -07:00 • committed by GitHub
parent 5917e71017
commit 4270b7927d
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3 changed files with 49 additions and 11 deletions
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12
Lib/test/test_concurrent_futures.py
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@ -497,10 +497,16 @@ class ProcessPoolShutdownTest(ExecutorShutdownTest):
lock.acquire()
mp_context = self.get_context()
if mp_context.get_start_method(allow_none=False) == "fork":
# fork pre-spawns, not on demand.
expected_num_processes = self.worker_count
else:
expected_num_processes = 3
sem = mp_context.Semaphore(0)
for _ in range(3):
self.executor.submit(acquire_lock, sem)
self.assertEqual(len(self.executor._processes), 3)
self.assertEqual(len(self.executor._processes), expected_num_processes)
for _ in range(3):
sem.release()
processes = self.executor._processes
@ -1021,6 +1027,8 @@ class ProcessPoolExecutorTest(ExecutorTest):
def test_idle_process_reuse_one(self):
executor = self.executor
assert executor._max_workers >= 4
if self.get_context().get_start_method(allow_none=False) == "fork":
raise unittest.SkipTest("Incompatible with the fork start method.")
executor.submit(mul, 21, 2).result()
executor.submit(mul, 6, 7).result()
executor.submit(mul, 3, 14).result()
@ -1029,6 +1037,8 @@ class ProcessPoolExecutorTest(ExecutorTest):
def test_idle_process_reuse_multiple(self):
executor = self.executor
assert executor._max_workers <= 5
if self.get_context().get_start_method(allow_none=False) == "fork":
raise unittest.SkipTest("Incompatible with the fork start method.")
executor.submit(mul, 12, 7).result()
executor.submit(mul, 33, 25)
executor.submit(mul, 25, 26).result()