For language: python builds on the arm64, s390x, and ppc64le archs we get a permission error for the cache when pip tries to build a wheel (specifically numpy and coverage in our case, but probably any wheel):
Building wheels for collected packages: numpy, coverage
WARNING: Building wheel for numpy failed: [Errno 13] Permission denied: '/home/travis/.cache/pip/wheels/04'
WARNING: Building wheel for coverage failed: [Errno 13] Permission denied: '/home/travis/.cache/pip/wheels/43'
Failed to build numpy coverage
ERROR: Could not build wheels for numpy which use PEP 517 and cannot be installed directly
The command "pip install --upgrade -r requirements.txt" failed and exited with 1 during .
Your build has been stopped.
https://travis-ci.com/taoliu/MACS/jobs/289170123#L169
The current work-around is to append --no-cache-dir to the pip invocation :-/