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bpo-33966, multiprocessing: Pool wait for worker start #7966

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bpo-33966, multiprocessing: Pool wait for worker start #7966

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19 changes: 17 additions & 2 deletions Lib/multiprocessing/pool.py
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Original file line number Diff line number Diff line change
Expand Up @@ -91,7 +91,7 @@ def __repr__(self):


def worker(inqueue, outqueue, initializer=None, initargs=(), maxtasks=None,
wrap_exception=False):
wrap_exception=False, event=None):
if (maxtasks is not None) and not (isinstance(maxtasks, int)
and maxtasks >= 1):
raise AssertionError("Maxtasks {!r} is not valid".format(maxtasks))
Expand All @@ -104,6 +104,9 @@ def worker(inqueue, outqueue, initializer=None, initargs=(), maxtasks=None,
if initializer is not None:
initializer(*initargs)

if event is not None:
event.set()

completed = 0
while maxtasks is None or (maxtasks and completed < maxtasks):
try:
Expand Down Expand Up @@ -228,19 +231,31 @@ def _repopulate_pool(self):
"""Bring the number of pool processes up to the specified number,
for use after reaping workers which have exited.
"""
events = []
for i in range(self._processes - len(self._pool)):
event = self._ctx.Event()
events.append(event)
w = self.Process(target=worker,
args=(self._inqueue, self._outqueue,
self._initializer,
self._initargs, self._maxtasksperchild,
self._wrap_exception)
self._wrap_exception,
event)
)
self._pool.append(w)
w.name = w.name.replace('Process', 'PoolWorker')
w.daemon = True
w.start()
util.debug('added worker')

# bpo-33966: On Windows, if the worker is terminated too quickly,
# handles created by DupHandle() on reduction.dump() remain open in
# the parent process, causing a handles leak. Wait until all new
# workers started to make sure that the workers closed handles
# inherited by DupHandle.
for event in events:
event.wait()

def _maintain_pool(self):
"""Clean up any exited workers and start replacements for them.
"""
Expand Down
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2018-06-27-14-26-32.bpo-33966.aLZJuF.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
multiprocess: On Windows, if a worker is terminated too quickly, handles
created by DupHandle() on reduction.dump() remain open in the parent process,
causing a handles leak. Pool now waits using events until all new workers
started to make sure that the workers closed handles inherited by DupHandle.