set_cluster_param.js tests that the following two actions are serialized:

• SetClusterParameterCoordinator's propagation of the new parameter value to all shards
• addShard

To check that, the tests uses the 'failCommand' failpoint with an 'errorCode' PrimarySteppedDown, which the coordinator will retry on. However, the coordinator only reties the _shardsvrSetClusterParameter command a fixed amount of attempts. Once the attempts are exhausted, the coordinator will release the lock that was ensuring the serialization, and retry again later.

This is fine, but the test can fail because it can issue the addShard command at a moment where the coordinator is not holding the lock after having exhausted retries.