In 4.2, setFeatureCompatibilityVersion command is a chain of operations including the the FCV document update operation and the collMod operations for unique index upgrade. Each of these individual operation within setFCV has its own writeConcern without dependancy on the others. It is required to define a clear dependancy between setFCV and the other operations within setFCV.

One example is, setFCV with user supplied writeConcern timeout should propagate this timeout to the writeConcern of collMod operation for unique index upgrade.