xgitlab.cels.anl.gov is now deprecated. If you're a project owner and wish to continue to host this project in CELS gitlab, see https://help.cels.anl.gov/docs/migrating-from-legacy-to-gce/gitlab/ for instructions on how to have your project migrated to the new gitlab server.
This server will be retired on December 15, 2022.
New tests pending, but the optimistic mode backfiller does appear to be working properly. Old behavior is preserved and may be enabled by setting the mode to pessimistic.