At the beginning of the rollback process, the server stops all in-progess index builds. This is done in both rollback algorithms - Recover to a Timestamp and rollbackViaRefetch.

In order to resume an index build without restarting the index build from the beginning, the Recover to a Timestamp algorithm needs additional information from the IndexBuildsCoordinator on the current state of each in-progress index build. This information will be the same information as what is currently written to disk on a clean shutdown for index builds.

The Recover to a Timestamp rollback algorithm should be able to report the state of each interrupted index build before it starts the actual rollback process. We will use this information to change how rollbacks work for interrupted index builds in SERVER-48419.