os.sched_getaffinity() not supported on MacOSX Blocks #411

Summary

os.sched_getaffinity() is stated in its docs to only be supported on some unix platforms, MacOSX seems to not be one of these

This is used in buildstream/_project.py +577 for defining output.base_variables['max-jobs']

Steps to reproduce

run tests/artifactcache/config.py::test_artifact_cache_precedence[empty_config] on a mac with ./setup.py --addopts "--integration"

What is the current bug behavior?

The test fails with Attribute error: module 'os' has no attribute 'sched_getaffinity'

What is the expected correct behavior?

The test should pass

Relevant logs and/or screenshots

Possible fixes

Either some alternative function is needed or a conditional statement from MacOSX and any other affected unix platforms

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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