Fix isAheadOf logic for ReplicationCheckpoint comparison (opensearch-…

…project#4112)

Signed-off-by: Suraj Singh <surajrider@gmail.com>

server/src/main/java/org/opensearch/indices/replication/checkpoint/ReplicationCheckpoint.java

@@ -125,10 +125,13 @@ public int hashCode() {
  }
 
  /**
- * Checks if other is aheadof current replication point by comparing segmentInfosVersion. Returns true for null
+ * Checks if current replication checkpoint is AheadOf `other` replication checkpoint point by first comparing
+ * primaryTerm followed by segmentInfosVersion. Returns true when `other` is null.
  */
  public boolean isAheadOf(@Nullable ReplicationCheckpoint other) {
- return other == null || segmentInfosVersion > other.getSegmentInfosVersion() || primaryTerm > other.getPrimaryTerm();
+ return other == null
+ || primaryTerm > other.getPrimaryTerm()
+ || (primaryTerm == other.getPrimaryTerm() && segmentInfosVersion > other.getSegmentInfosVersion());
  }
 
  @Override