FMX opened a new pull request, #1519:
URL: https://github.com/apache/ratis/pull/1519
## What changes were proposed in this pull request?
This patch fixes leader-side AppendEntries success handling for heartbeat
requests.
A follower returns its local `nextIndex` for successful heartbeat
AppendEntries replies. That value describes the follower's local log tail, but
it does not prove that all entries
up to `nextIndex - 1` match the leader's log. If the follower has a
divergent uncommitted tail from an old leader, trusting the heartbeat reply
`nextIndex` may incorrectly advance
the leader-side `matchIndex`.
Once `matchIndex` is polluted, later `INCONSISTENCY` handling can use
`matchIndex + 1` as a lower bound and prevent `nextIndex` from backing off to
the real common prefix.
This patch preserves the AppendEntries request metadata together with the
reply. For successful heartbeat replies, the leader advances only to
`request.previousLog.index + 1`. For
successful append replies, the existing behavior is preserved and
`reply.nextIndex` is still used.
Unit tests are added for:
1. Heartbeat `SUCCESS` with `previousLog` advances only to
`previousLog.index + 1`.
2. Heartbeat `SUCCESS` without `previousLog` does not advance `matchIndex`.
3. Append `SUCCESS` still advances using `reply.nextIndex`.
## What is the link to the Apache JIRA
https://issues.apache.org/jira/browse/RATIS-2605
## How was this patch tested?
```bash
./mvnw -pl ratis-server -am -Dtest=TestLogAppenderDefault test
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