When we upgrade a cluster to use Raft, or perform manual Raft recovery
procedure (which also creates a fresh group 0 cluster, using the same
algorithm as during upgrade), we start with a non-empty group 0 state
machine; in particular, the schema tables are non-empty.
In this case we need to ensure that nodes which join group 0 receive the
group 0 state. Right now this is not the case. In previous releases,
where group 0 consisted only of schema, and schema pulls were also done
outside Raft, those nodes received schema through this outside
mechanism. In 91f609d065 we disabled
schema pulls outside Raft; we're also extending group 0 with other
things, like topology-specific state.
To solve this, we force snapshot transfers by setting the initial
snapshot index on the first group 0 server to `1` instead of `0`. During
replication, Raft will see that the joining servers are behind,
triggering snapshot transfer and forcing them to pull group 0 state.
It's unnecessary to do this for cluster which bootstraps with Raft
enabled right away but it also doesn't hurt, so we keep the logic simple
and don't introduce branches based on that.
Extend Raft upgrade tests with a node bootstrap step at the end to
prevent regressions (without this patch, the step would hang - node
would never join, waiting for schema).
Fixes: #14066Closes#14336
ff386e7a44