Currently, both ReplicaSetFixture and ReplSetTest set up replica sets by initiating a single-node replica set, starting additional nodes if necessary, and then performing a reconfig to add them. Running the rsInitiate command with a single node will use “latest” FCV. This will cause IncompatibleServerVersion errors if additional "last-stable" binary version nodes are subsequently added to the set, since such nodes cannot set their FCV to "latest".

To get around this problem, if the randomBinVersion and mixed_bin_version flags are set to true in ReplSetTest and ReplicaSetFixture respectively, we will set the FCV of the first node to “last-stable” before adding new members to the set.