Alexandros Frantzis and Alan Griffiths of Canonical added the EmergencyCleanup interface on Wednesday. What this new Mir display server interface does is allow handlers to be defined that will be called in case of Mir hitting fatal errors. The hope is that if these handlers are called if Mir is going to crash or run into other serious problems, they'll be able to return the system to a usable state in order to provide a clean user experience.
"Introduce a way for components to add emergency cleanup handlers. Such handlers are called in case of fatal errors, to ensure we leave the system in an usable/clean state. This MP introduces the interfaces, a default implementation and change our existing socket file clean-up mechanism to use the new infrastructure." More information on this emergency infrastructure added to Mir can be found via this Bzr revision.