ReshardingTest#_recipientShards and ReshardingTest#_donorShards both use the ShardingTest#shard{n} member variable, which is a replset connection to the shard node. However, it does not refresh it / make sure it gets reconnected again after shutdownAndRestartPrimaryOnShard is called. It should be doing to somethign similar with ShardingTest#restartShardRS to ensure that connections are ready to use without getting network errors.

ReshardingTest#killAndRestartPrimaryOnShard also have a similar issue.