Downgrading the Raft version protocol is not a supported operation.
Checking for a downgrade is hard since this information is not stored in
any persistent place. When a server re-joins a cluster with a prior Raft
version, the Serf tag is updated so Nomad can't tell that the version
changed.
Mixed version clusters must be supported to allow for zero-downtime
rolling upgrades. During this it's expected that the cluster will have
mixed Raft versions. Enforcing consistency strong version consistency
would disrupt this flow.
The approach taken here is to store the Raft version on disk. When the
server starts the `raft_protocol` value is written to the file
`data_dir/raft/version`. If that file already exists, its content is
checked against the current `raft_protocol` value to detect downgrades
and prevent the server from starting.
Any other types of errors are ignore to prevent disruptions that are
outside the control of operators. The only option in cases of an invalid
or corrupt file would be to delete it, making this check useless. So
just overwrite its content with the new version and provide guidance on
how to check that their cluster is an expected state.
