ElasticsearchMergePolicy extend from MergePolicyWrapper (elastic#29476)

The skeleton of ElasticsearchMergePolicy is quite similar to
MergePolicyWrapper. This commit therefore makes ElasticsearchMergePolicy
inherited from MergePolicyWrapper instead of MergePolicy.

Author: dnhatn

server/src/main/java/org/elasticsearch/index/shard/ElasticsearchMergePolicy.java

@@ -22,7 +22,7 @@
 import org.apache.logging.log4j.Logger;
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.MergePolicy;
-import org.apache.lucene.index.MergeTrigger;
+import org.apache.lucene.index.MergePolicyWrapper;
 import org.apache.lucene.index.SegmentCommitInfo;
 import org.apache.lucene.index.SegmentInfos;
 import org.elasticsearch.Version;
@@ -44,12 +44,10 @@
  * For now, this {@link MergePolicy} takes care of moving versions that used to
  * be stored as payloads to numeric doc values.
  */
-public final class ElasticsearchMergePolicy extends MergePolicy {
+public final class ElasticsearchMergePolicy extends MergePolicyWrapper {
 
     private static Logger logger = Loggers.getLogger(ElasticsearchMergePolicy.class);
 
-    private final MergePolicy delegate;
-
     // True if the next merge request should do segment upgrades:
     private volatile boolean upgradeInProgress;
 
@@ -60,13 +58,7 @@ public final class ElasticsearchMergePolicy extends MergePolicy {
 
     /** @param delegate the merge policy to wrap */
     public ElasticsearchMergePolicy(MergePolicy delegate) {
-        this.delegate = delegate;
-    }
-
-    @Override
-    public MergeSpecification findMerges(MergeTrigger mergeTrigger,
-        SegmentInfos segmentInfos, IndexWriter writer) throws IOException {
-        return delegate.findMerges(mergeTrigger, segmentInfos, writer);
+        super(delegate);
     }
 
     private boolean shouldUpgrade(SegmentCommitInfo info) {
@@ -130,18 +122,7 @@ public MergeSpecification findForcedMerges(SegmentInfos segmentInfos,
             // has a chance to decide what to do (e.g. collapse the segments to satisfy maxSegmentCount)
         }
 
-        return delegate.findForcedMerges(segmentInfos, maxSegmentCount, segmentsToMerge, writer);
-    }
-
-    @Override
-    public MergeSpecification findForcedDeletesMerges(SegmentInfos segmentInfos, IndexWriter writer)
-        throws IOException {
-        return delegate.findForcedDeletesMerges(segmentInfos, writer);
-    }
-
-    @Override
-    public boolean useCompoundFile(SegmentInfos segments, SegmentCommitInfo newSegment, IndexWriter writer) throws IOException {
-        return delegate.useCompoundFile(segments, newSegment, writer);
+        return super.findForcedMerges(segmentInfos, maxSegmentCount, segmentsToMerge, writer);
     }
 
     /**
@@ -154,10 +135,4 @@ public void setUpgradeInProgress(boolean upgrade, boolean onlyAncientSegments) {
         this.upgradeInProgress = upgrade;
         this.upgradeOnlyAncientSegments = onlyAncientSegments;
     }
-
-    @Override
-    public String toString() {
-        return getClass().getSimpleName() + "(" + delegate + ")";
-    }
-
 }