In mock 1.1.36 was introduced new option in mock config as fix for https://bugzilla.redhat.com/show_bug.cgi?id=1052932 this option is missing in Fedora Koji configuration. Which results in failing builds. E.g: https://koji.fedoraproject.org/koji/taskinfo?taskID=6894360
attachment 0001-create-var-lock.patch
This won't help us in koji. ;(
koji generates it's own mock configs, and after talking with other releng folks we think it should be fixed in the packageset (by being made by filesystem or systemd or something).
Feel free to bring it up to the releng list if you have additional info...