In getBuildConfig tag.extra values are effectively inherited. However, there is no way for a tag to unset a value that an ancestor has set. You can set it to None, but that is not the same as unset.

This matters because we have code that distinguishes when values are unset. One case is the code that checks for mock.new_chroot. It is only when this value is unset that neither --old-chroot nor --new-chroot is passed. Legacy builders with an older mock might not be able to handle these options.

There could easily be other examples.