I'm not sure what changed recently, but now "mock --init -r fedora-rawhide-x86_64" in el6 distro fails on yum's error: ... Error: xz compression not available As 'pyliblzma' is EPEL only, I think best approach is require 'pyliblzma' by mock, not by yum (that way it is done in Fedora now).
*** This bug has been marked as a duplicate of bug 1099676 ***