Following SPM-3729, the shards will store the authoritative metadata durably, which will be used for database placement information. The relevant collection is config.shard.catalog.databases, which won't exist before FCV transition, but will after. FCV downgrade deletes the collection and upgrade creates it - cloning from the CSRS.

In the rare case (bugs or otherwise), if the collection pre-exists with some data before upgrade - it could cause unexpected incorrectness after/during upgrade.

We will like to disallow/fail FCV upgrade if this collection is not empty. Let it be investigated why there is this collection, and clear it up before proceeding with FCV upgrade.

Note:
Moving this ticket out of the project, blocked on being able to safely cancel an ongoing FCV upgrade.