We only run the cleanup code once per request. After its run, any new objects added onto the cleanup list for this request are wrong and cannot be satisifed. If we don't have this guard, a potential code change could ask for a ReviewDb to be opened after cleanup was done, leaking the database connection. Change-Id: I854f6790b36cad4ad07356a7239c8648dc3937e4 Signed-off-by: Shawn O. Pearce <sop@google.com>