Re: [Buildroot] [PATCH] support/testing/run-tests: unbreak on Debian testing/unstable

[Thomas Perale via buildroot <buildroot@buildroot.org>]

In reply of:
> Commit 3d2141bcee("support/testing/run-tests: specify multiprocessing
> method") added a call to multiprocessing.set_start_method('fork') as a
> workaround for python 3.14, which changed the default start method to
> forkserver - Which is incompatible with the nose2 setup.
> 
> multiprocessing.set_start_method() is only supposed to be called a maximum
> of 1 time per process and throws a RuntimeError if called more than that
> (even with the same arguments):
> 
> >>> import multiprocessing
> >>> multiprocessing.set_start_method('fork')
> >>> multiprocessing.set_start_method('fork')
> Traceback (most recent call last):
>   File "<python-input-2>", line 1, in <module>
>     multiprocessing.set_start_method('fork')
>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
>   File "/usr/lib/python3.13/multiprocessing/context.py", line 247, in set_start_method
>     raise RuntimeError('context has already been set')
> 
> Debian included a similar patch in python3-nose2 0.51.1-2 (currently in
> testing/unstable) which adds its own call to set_start_method():
> 
> https://salsa.debian.org/python-team/packages/nose2/-/blob/debian/0.15.1-2/debian/patches/0004-plugins-mp-set-context-to-fork-for-Python-3.14-mp-AP.patch?ref_type=tags
> 
> Which comes from:
> https://github.com/nose-devs/nose2/pull/644
> 
> As discussed in the upstream PR, this is not a correct fix is wrong and
> breaks various use cases.  An issue has been opened to get this fixed in the
> Debian packaging at:
> 
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1129350
> 
> But until that is done, rework the patch to:
> 
> - Only override set_start_method() if needed to limit impact
> - Monkey patch set_start_method() so additional calls are ignored
> 
> To unbreak run-test on affected Debian systems and add some documentation to
> make it clear why this is done.
> 
> Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Applied to 2025.02.x & 2025.11.x. Thanks

> ---
>  support/testing/run-tests | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/support/testing/run-tests b/support/testing/run-tests
> index 1c4d2b4a85..2bbe7b140f 100755
> --- a/support/testing/run-tests
> +++ b/support/testing/run-tests
> @@ -142,5 +142,15 @@ def main():
>  
>  
>  if __name__ == "__main__":
> -    multiprocessing.set_start_method("fork")
> +    # python 3.14 changed default start method from fork to
> +    # fork-server, which is not compatible with the nose2 setup
> +    if multiprocessing.get_start_method() != 'fork':
> +        multiprocessing.set_start_method("fork")
> +        # set_start_method throws a RuntimeError if called more than
> +        # once.
> +        # Debian python3-nose2 0.15.1-2 includes a patch adding
> +        # another set_start_method() call, so monkey patch it out to
> +        # get rid of this
> +        multiprocessing.set_start_method = lambda *args: None
> +
>      sys.exit(main())
> -- 
> 2.47.3
> 
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