I recently tried to test a change on chromeos-autotest.hot which depended on some new code in chromite prod branch. Despite this code being already pushed to prod, the update_autotest script did not call build_externals and therefore the code was not available on the test instance.

This looks like it is due to some logic in update_autotest which only calls build_externals in case you have recently touched the utils/external_packages file.

pkg_head=`git log -1 --pretty=tformat:%h utils/external_packages.py`

This logic seems quite wrong, and I don't think it matches what we do in deploy_production (where we call build_externals on every server...)