gh-140814: Fix freeze_support() setting start method as side effect (GH-144608)
freeze_support() called get_start_method() without allow_none=True,
which locked in the default start method context. This caused a
subsequent set_start_method() call to raise "context has already been
set". Use allow_none=True and accept None as a matching value, since
spawn.freeze_support() independently detects spawned child processes.
Test that freeze_support() does not lock in the default start method,
which would prevent a subsequent set_start_method() call.
(cherry picked from commit ee5318025b)
Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
gh-80334: fix multiprocessing.freeze_support for other spawn platforms (GH-134462)
Doc/library/multiprocessing.rst: freeze_support: Change to specify spawn method instead of platform
Have multiprocessing.freeze_support() enable on spawn, not just win32.
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(cherry picked from commit 80284b5c5e)
Co-authored-by: Eddy Mulyono <eddymul@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>
Change the default multiprocessing start method away from fork to forkserver or spawn on the remaining platforms where it was fork. See the issue for context. This makes the default far more thread safe (other than for people spawning threads at import time... - don't do that!).
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Tools such as ruff can ignore "imported but unused" warnings if a
line ends with "# noqa: F401". It avoids the temptation to remove
an import which is used effectively.
This starts the process. Users who don't specify their own start method
and use the default on platforms where it is 'fork' will see a
DeprecationWarning upon multiprocessing.Pool() construction or upon
multiprocessing.Process.start() or concurrent.futures.ProcessPool use.
See the related issue and documentation within this change for details.
