add future support to track current merge

src/main/java/org/apache/lucene/index/TrackingConcurrentMergeScheduler.java

@@ -86,8 +86,10 @@ public class TrackingConcurrentMergeScheduler extends ConcurrentMergeScheduler {
             logger.trace("merge [{}] starting...", merge.info.name);
         }
         try {
+            TrackingMergeScheduler.setCurrentMerge(merge);
             super.doMerge(merge);
         } finally {
+            TrackingMergeScheduler.removeCurrentMerge();
             long took = System.currentTimeMillis() - time;
 
             currentMerges.dec();

src/main/java/org/apache/lucene/index/TrackingMergeScheduler.java

@@ -0,0 +1,24 @@
+package org.apache.lucene.index;
+
+import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
+
+import java.util.concurrent.ConcurrentMap;
+
+/**
+ */
+public class TrackingMergeScheduler {
+
+    private static final ConcurrentMap<Thread, MergePolicy.OneMerge> merges = ConcurrentCollections.newConcurrentMap();
+
+    public static void setCurrentMerge(MergePolicy.OneMerge merge) {
+        merges.put(Thread.currentThread(), merge);
+    }
+
+    public static void removeCurrentMerge() {
+        merges.remove(Thread.currentThread());
+    }
+
+    public static MergePolicy.OneMerge getCurrentMerge() {
+        return merges.get(Thread.currentThread());
+    }
+}

src/main/java/org/apache/lucene/index/TrackingSerialMergeScheduler.java

@@ -93,8 +93,10 @@ public class TrackingSerialMergeScheduler extends MergeScheduler {
             currentMergesNumDocs.inc(totalNumDocs);
             currentMergesSizeInBytes.inc(totalSizeInBytes);
             try {
+                TrackingMergeScheduler.setCurrentMerge(merge);
                 writer.merge(merge);
             } finally {
+                TrackingMergeScheduler.removeCurrentMerge();
                 long took = System.currentTimeMillis() - time;
 
                 currentMerges.dec();