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bpo-33725: multiprocessing uses spawn by default on macOS #13603

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May 28, 2019
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bpo-33725: multiprocessing uses spawn by default on macOS #13603

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7 changes: 6 additions & 1 deletion Doc/library/multiprocessing.rst
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Expand Up @@ -102,7 +102,7 @@ to start a process. These *start methods* are
will not be inherited. Starting a process using this method is
rather slow compared to using *fork* or *forkserver*.

Available on Unix and Windows. The default on Windows.
Available on Unix and Windows. The default on Windows and macOS.

*fork*
The parent process uses :func:`os.fork` to fork the Python
Expand All @@ -124,6 +124,11 @@ to start a process. These *start methods* are
Available on Unix platforms which support passing file descriptors
over Unix pipes.

.. versionchanged:: 3.8

On macOS, *spawn* start method is now the default: *fork* start method is no
longer reliable on macOS, see :issue:`33725`.

.. versionchanged:: 3.4
*spawn* added on all unix platforms, and *forkserver* added for
some unix platforms.
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10 changes: 10 additions & 0 deletions Doc/whatsnew/3.8.rst
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Expand Up @@ -469,6 +469,16 @@ access the ``madvise()`` system call.
(Contributed by Zackery Spytz in :issue:`32941`.)


multiprocessing
---------------

Added new :mod:`multiprocessing.shared_memory` module.
(Contributed Davin Potts in :issue:`35813`.)

On macOS, the *spawn* start method is now used by default.
(Contributed by Victor Stinner in :issue:`33725`.)


os
--

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7 changes: 6 additions & 1 deletion Lib/multiprocessing/context.py
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Expand Up @@ -309,7 +309,12 @@ def _check_available(self):
'spawn': SpawnContext(),
'forkserver': ForkServerContext(),
}
_default_context = DefaultContext(_concrete_contexts['fork'])
if sys.platform == 'darwin':
# bpo-33725: running arbitrary code after fork() is no longer reliable
# on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
_default_context = DefaultContext(_concrete_contexts['spawn'])
else:
_default_context = DefaultContext(_concrete_contexts['fork'])

else:

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2019-05-28-01-17-42.bpo-33725.fFZoDG.rst
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@@ -0,0 +1,2 @@
On macOS, the :mod:`multiprocessing` module now uses *spawn* start method by
default.