improve logging of failure when merging

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

@@ -32,7 +32,7 @@ import java.io.IOException;
  */
 public class TrackingConcurrentMergeScheduler extends ConcurrentMergeScheduler {
 
-    private final ESLogger logger;
+    protected final ESLogger logger;
 
     private final MeanMetric totalMerges = new MeanMetric();
     private final CounterMetric totalMergesNumDocs = new CounterMetric();

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

@@ -29,7 +29,7 @@ import java.io.IOException;
 // LUCENE MONITOR - Copied from SerialMergeScheduler
 public class TrackingSerialMergeScheduler extends MergeScheduler {
 
-    private final ESLogger logger;
+    protected final ESLogger logger;
 
     private final MeanMetric totalMerges = new MeanMetric();
     private final CounterMetric totalMergesNumDocs = new CounterMetric();

modules/elasticsearch/src/main/java/org/elasticsearch/index/merge/scheduler/ConcurrentMergeSchedulerProvider.java

@@ -97,7 +97,12 @@ public class ConcurrentMergeSchedulerProvider extends AbstractIndexShardComponen
                 // since we do it outside of a lock in the RobinEngine
                 return;
             }
-            super.merge(writer);
+            try {
+                super.merge(writer);
+            } catch (IOException e) {
+                logger.warn("failed to merge", e);
+                throw e;
+            }
         }
 
         @Override protected MergeThread getMergeThread(IndexWriter writer, MergePolicy.OneMerge merge) throws IOException {
@@ -106,6 +111,11 @@ public class ConcurrentMergeSchedulerProvider extends AbstractIndexShardComponen
             return thread;
         }
 
+        @Override protected void handleMergeException(Throwable exc) {
+            logger.warn("failed to merge", exc);
+            super.handleMergeException(exc);
+        }
+
         @Override public void close() {
             super.close();
             provider.schedulers.remove(this);

modules/elasticsearch/src/main/java/org/elasticsearch/index/merge/scheduler/SerialMergeSchedulerProvider.java

@@ -87,7 +87,12 @@ public class SerialMergeSchedulerProvider extends AbstractIndexShardComponent im
                 // since we do it outside of a lock in the RobinEngine
                 return;
             }
-            super.merge(writer);
+            try {
+                super.merge(writer);
+            } catch (IOException e) {
+                logger.warn("failed to merge", e);
+                throw e;
+            }
         }
 
         @Override public void close() {