See https://github.com/etcd-io/etcd/issues/14370.
When run in a single-voter configuration, prior to this PR
raft would emit `HardState`s that would emit a proposed `Entry`
simultaneously in `CommittedEntries` and `Entries`.
To be correct, this requires users of the raft library to enforce an
ordering between appending to the log and notifying the client about
`CommittedEntries` also present in `Entries`. This was easy to miss.
Walk back this behavior to arrive at a simpler contract: what's
emitted in `CommittedEntries` is truly committed, i.e. present
in stable storage on a quorum of voters.
This in turn pessimizes the single-voter case: rather than fully
handling an `Entry` in just one `Ready`, now two are required,
and in particular one has to do extra work to save on allocations.
We accept this as a good tradeoff, since raft primarily serves
multi-voter configurations.
Signed-off-by: Tobias Grieger <tobias.b.grieger@gmail.com>
Show-cases the current behavior and changes made in future commits for [^1].
The test demonstrates that a single-voter raft instance will emit an
entry as committed while it still needs to be appended to the log.
[^1]: https://github.com/etcd-io/etcd/issues/14370
Signed-off-by: Tobias Grieger <tobias.b.grieger@gmail.com>
