The ensurePrimaryShard helper function is currently used to create a database on a specific primary shard (if not existing) or changing a db's primary shard (if existing).

Inspecting a sample of tests invoking it, it turns out that this function is used in the following ways:

• 83% of current usages: calling enableSharding + ensurePrimaryShard to simply create the db on a primary shard. This is technically equivalent to calling enableSharding (creating an empty database on a random primary shard) + movePrimary (in this case, moving an empty database).
  • This can be avoided by just calling enableSharding with the primaryShard argument.

• 10% of current usages: calling ensurePrimaryShard to create a db on a specific primary shard.
  • Also this can be avoided by just calling enableSharding with the primaryShard argument.

• 7% of current usages: used to actually change the primary shard of a non-empty database.
  • This can be avoided by directly calling movePrimary.

Purpose of this ticket is to get rid of ensurePrimaryShard for the following reasons:

• Bring to an end the unnecessary pattern of creating an empty database and then moving it.
• Mark all the points in the code that really need movePrimary.

This is a precondition to deprecate the movePrimary command once moveCollection will be available, so there will not be too many tests to adapt in the future.