RATIS-2167. Add default value in TermIndex

[jianghuazhu]

What changes were proposed in this pull request?

In the Ratis project and other projects, the default value of TermIndex is used. Here we create a default value for TermIndex.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-2167

How was this patch tested?

Make sure UT passes.

[jianghuazhu]

@szetszwo , can you help review this PR?
Thanks

[szetszwo]

@jianghuazhu , thanks a lot for working on this!

Let's call it INITIAL_VALUE, move it to the top and add some javadoc.

BTW, could you also add javadoc for EMPTY_ARRAY?

+++ b/ratis-server-api/src/main/java/org/apache/ratis/server/protocol/TermIndex.java
@@ -19,12 +19,25 @@ package org.apache.ratis.server.protocol;
 
 import org.apache.ratis.proto.RaftProtos.LogEntryProto;
 import org.apache.ratis.proto.RaftProtos.TermIndexProto;
+import org.apache.ratis.server.raftlog.RaftLog;
 
 import java.util.Comparator;
 import java.util.Optional;
 
 /** The term and the log index defined in the Raft consensus algorithm. */
 public interface TermIndex extends Comparable<TermIndex> {
+  /**
+   * The initial value.
+   * When a new Raft group starts,
+   * all the servers has term 0 and index -1 (= {@link RaftLog#INVALID_LOG_INDEX}).
+   * Note that term is incremented during leader election
+   * and index is incremented when writing to the {@link RaftLog}.
+   * The least term and index possibly written to the {@link RaftLog}
+   * are respectively 1 and 0 (= {@link RaftLog#LEAST_VALID_LOG_INDEX}).
+   */
+  TermIndex INITIAL_VALUE = valueOf(0, RaftLog.INVALID_LOG_INDEX);
+
+  /** An empty {@link TermIndex} array. */
   TermIndex[] EMPTY_ARRAY = {};
 
   /** @return the term. */

[szetszwo]

+1 the change looks good.

[jianghuazhu]

Thanks @szetszwo for the comment and review.
Here are some CI run failures that don't seem to be related to this PR.
Here are my tests for my personal branch:
https://github.com/jianghuazhu/ratis/actions/runs/11165969820

[szetszwo]

@jianghuazhu , agree that the failures were not related to this.

[jianghuazhu]

Thanks @szetszwo .
I created a related issue in the Ozone project, HDDS-11529.
I will resolve it when the next new version of Apache Ratis is imported.