RATIS-2237 Add getFollowerMatchIndices interface for DivisionInfo

ratis-server-api/src/main/java/org/apache/ratis/server/DivisionInfo.java

@@ -80,4 +80,10 @@ default boolean isAlive() {
    *         otherwise, return null.
    */
   long[] getFollowerNextIndices();
+
+  /**
+   * @return an array of match indices of the followers if this server division is the leader;
+   *         otherwise, return null.
+   */
+  long[] getFollowerMatchIndices();
 }

ratis-server/src/main/java/org/apache/ratis/server/impl/LeaderStateImpl.java

@@ -1214,6 +1214,10 @@ long[] getFollowerNextIndices() {
     return getLogAppenders().mapToLong(s -> s.getFollower().getNextIndex()).toArray();
   }
 
+  long[] getFollowerMatchIndices() {
+    return getLogAppenders().mapToLong(s -> s.getFollower().getMatchIndex()).toArray();
+  }
+
   static Map<RaftPeerId, RaftPeer> newMap(Collection<RaftPeer> peers, String str) {
     Objects.requireNonNull(peers, () -> str + " == null");
     final Map<RaftPeerId, RaftPeer> map = new HashMap<>();

ratis-server/src/main/java/org/apache/ratis/server/impl/RaftServerImpl.java

@@ -206,6 +206,14 @@ public long[] getFollowerNextIndices() {
           .map(LeaderStateImpl::getFollowerNextIndices)
           .orElse(null);
     }
+
+    @Override
+    public long[] getFollowerMatchIndices() {
+      return role.getLeaderState()
+          .filter(leader -> isLeader())
+          .map(LeaderStateImpl::getFollowerMatchIndices)
+          .orElse(null);
+    }
   }
 
   private final RaftServerProxy proxy;