This phase include the following steps:

• validate host as a shard
  • this step is the same as ShardingCatalogManager::_validateHostAsShard but without returning a ShardType
• block the user writes
  • this is needed in case of the given shard is not the first shard (to support the RS -> sharded cluster transition)
  • this is needed in order to stabilize the RS before checking the existing data on it
• check existing data on RS
  • if we prohibit non-empty RSs (guarded by a feature flag) check if no DB presents
  • otherwise check for clashing DB names

The changes include the followings:

• changes in the coordinator
• changes in ShardingCatalogManager in order to enable clean changes in the coordinator
• unify global functions used in the coordinator to use asserts instead of Status