The automated rollback system has been designed, but not implemented yet.
- Set up an evergreen instance, it is at UL-X
- publish a (broken) update level (example: adding a plugin without a required dependency) => instance upgrades to UL-X
- after restart, evergreen client detects Jenkins cannot restart, and rolls back to UL-X.
- Jenkins is accessible again
- a correct UL is published, Jenkins can upgrade to it