Prior to SERVER-114271, the shardsvrAddShard command sent a request from the shardsvr to the configsvr as part of the refresh of the balancer settings. This request had the side effect of ensuring that the shard could connect to the config server prior to committing the add shard. Now that this has been removed, it is possible for a shard to be added to a cluster when that shard cannot actually contact the config server (for example, due to misconfigured SSL parameters).

In FCV 8.3+, this problem was fixed by adding the ShardsvrCheckCanConnectToConfigServerCommand which asks the shard to ping the config server prior to writing the shard identity. However, this cannot be used on prior versions because it is a newly introduced command.

To preserve these checks without the balancer refresh or new command, we should have the shardsvrAddShard command send a hello to the config server. This will allow older versions to check connectivity without triggering the invariant in the balancer configuration.