RATIS-2008. Follower should recognize candidate if the candidate is the same peer as the current valid leader

[ivandika3]

What changes were proposed in this pull request?

During pre-vote, some follower reject the PRE_VOTE request from a candidate although the candidate has the same peer ID as the current leader.

2024-01-18 13:44:50,123 [grpc-default-executor-100] INFO org.apache.ratis.server.impl.VoteContext: e46cc30b-13ca-4778-b856-e84b0677493d@group-059247EC8137-FOLLOWER: reject PRE_VOTE from c7e3fa47-df62-4883-8d6e-50c3b6a9b94c: this server is a follower and still has a valid leader c7e3fa47-df62-4883-8d6e-50c3b6a9b94c

It might be a good idea to add another check so that if the candidate has the same peer ID as the follower's current recognized leader, we approve the PRE_VOTE request.

Note: The optimization might be marginal since leader is only valid after min.rpc.timeout (150ms). Afterwards, the follower should recognize the candidate.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-2008

How was this patch tested?

Existing tests.

[szetszwo]

@ivandika3 , this is a good idea!

• We may combine the if-conditions.
• Let's update the rejection message.

@@ -102,8 +102,10 @@ class VoteContext {
     if (info.isFollower()) {
       final RaftPeerId leader = impl.getState().getLeaderId();
       if (leader != null
+          && !leader.equals(candidateId)
           && impl.getRole().getFollowerState().map(FollowerState::isCurrentLeaderValid).orElse(false)) {
-        return reject("this server is a follower and still has a valid leader " + leader);
+        return reject("this server is a follower and still has a valid leader " + leader
+            + " different than the candidate " + candidateId);
       }
     }

[ivandika3]

@szetszwo Thank you for the review. I have updated it accordingly. PTAL.

[szetszwo]

+1 the change looks good.

[ivandika3]

Thank you for the review and merge @szetszwo