Always use the spawn start method for multiprocessing for wpt commands

[jgraham]

No description provided.

[foolip]

@jgraham I've tried this locally and it doesn't fix #29024 if that was the hope?

[jgraham]

I mean the hope was that we end up with consistent usage of multiprocesssing and in particular always stick to the safest context type. I thought it might affect #29024 but I"m not really surprised it doesn't given that this should be the default on macOS anyway.

[foolip]

https://docs.python.org/3/library/multiprocessing.html#multiprocessing.set_start_method says "it should be protected inside the if __name__ == '__main__' clause of the main module" which would require putting it in the root wpt script. I don't know why the documentation says that, and guess that calling it from a method called from that clause is fine, but 🤷

[foolip]

This should be possible to avoid if we put it in the root wpt right?

[jgraham]

I think so. But the reason I noticed this is that we have unit tests that call wpt.main instead of going via the command line. So I think we'd run the risk of those tests running under an unsupported configuration if we so this in a pre-main location.

I tried a test and I can't see that calling this in main vs if __name__ == "__main__" makes any difference to the behaviour; in each case the subprocess gets the same default spawn method. It seems like the important thing is really that it's called at the main entrypoint to the code; as long as we're confident everything goes through main it's all fine.

[foolip]

Yeah, probably the Python documentation says that as a slightly inaccurate way of saying "please make sure to call it always and early".