In C2C replication, we would like to only replicate majority committed data so that we can avoid handling rollbacks of the source cluster. This is now done by using majority readConcern in collection scans in the cloning phase. However, listDatabases/listCollections/listIndexes don't support reading majority committed catalog as the catalog is not versioned. This makes it very hard for an external process to detect when the result returned by these list commands rolls back. For mongosync, we are planning to use $indexStats instead of listIndexes to get the index info.

This work is to support read concern level “majority” for $indexStats (or support it for some new $listIndexes aggregation stage which targets all shards owning chunks) by reading from a majority committed snapshot of the _mdb_catalog without actually introducing versioned catalog.

And it should work for both replica sets and sharded clusters.