Aggregations: Fixed bug in top_hits aggregation to not fail with NPE when shard results are empty.

The top_hits aggregation returned an empty InternalTopHits instance with no fields set when there were no result, causing reduce and serialization errors down the road. This is fixed by setting all required fields when a there are no results.

Closes #6346

src/main/java/org/elasticsearch/search/aggregations/bucket/tophits/InternalTopHits.java

@@ -73,7 +73,7 @@ public class InternalTopHits extends InternalAggregation implements TopHits, ToX
     public InternalTopHits(String name, InternalSearchHits searchHits) {
         this.name = name;
         this.searchHits = searchHits;
-        this.topDocs = new TopDocs(0, Lucene.EMPTY_SCORE_DOCS, 0);
+        this.topDocs = Lucene.EMPTY_TOP_DOCS;
     }
 
 

src/main/java/org/elasticsearch/search/aggregations/bucket/tophits/TopHitsAggregator.java

@@ -23,6 +23,7 @@ import org.apache.lucene.index.AtomicReaderContext;
 import org.apache.lucene.search.*;
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.common.lease.Releasables;
+import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.ScorerAware;
 import org.elasticsearch.common.util.LongObjectPagedHashMap;
 import org.elasticsearch.search.aggregations.*;
@@ -31,6 +32,7 @@ import org.elasticsearch.search.aggregations.support.AggregationContext;
 import org.elasticsearch.search.fetch.FetchPhase;
 import org.elasticsearch.search.fetch.FetchSearchResult;
 import org.elasticsearch.search.internal.InternalSearchHit;
+import org.elasticsearch.search.internal.InternalSearchHits;
 
 import java.io.IOException;
 
@@ -94,7 +96,7 @@ public class TopHitsAggregator extends BucketsAggregator implements ScorerAware
 
     @Override
     public InternalAggregation buildEmptyAggregation() {
-        return new InternalTopHits();
+        return new InternalTopHits(name, topHitsContext.size(), topHitsContext.sort(), Lucene.EMPTY_TOP_DOCS, InternalSearchHits.empty());
     }
 
     @Override

src/main/java/org/elasticsearch/search/aggregations/bucket/tophits/TopHitsBuilder.java

@@ -368,7 +368,7 @@ public class TopHitsBuilder extends AbstractAggregationBuilder {
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         builder.startObject(name).field(type);
-        sourceBuilder.toXContent(builder, params);
+        sourceBuilder().toXContent(builder, params);
         return builder.endObject();
     }
 

src/main/java/org/elasticsearch/search/internal/InternalSearchHits.java

@@ -90,6 +90,10 @@ public class InternalSearchHits implements SearchHits {
         return cache.get().reset();
     }
 
+    public static InternalSearchHits empty() {
+        // We shouldn't use static final instance, since that could directly be returned by native transport clients
+        return new InternalSearchHits(EMPTY, 0, 0);
+    }
 
     public static final InternalSearchHit[] EMPTY = new InternalSearchHit[0];
 

src/main/java/org/elasticsearch/search/internal/InternalSearchResponse.java

@@ -41,7 +41,7 @@ import static org.elasticsearch.search.internal.InternalSearchHits.readSearchHit
 public class InternalSearchResponse implements Streamable, ToXContent {
 
     public static InternalSearchResponse empty() {
-        return new InternalSearchResponse(new InternalSearchHits(new InternalSearchHit[0], 0, 0), null, null, null, false);
+        return new InternalSearchResponse(InternalSearchHits.empty(), null, null, null, false);
     }
 
     private InternalSearchHits hits;

src/test/java/org/elasticsearch/search/aggregations/bucket/TopHitsTests.java

@@ -64,6 +64,7 @@ public class TopHitsTests extends ElasticsearchIntegrationTest {
     @Override
     public void setupSuiteScopeCluster() throws Exception {
         createIndex("idx");
+        createIndex("empty");
         List<IndexRequestBuilder> builders = new ArrayList<>();
         for (int i = 0; i < 50; i++) {
             builders.add(client().prepareIndex("idx", "type", Integer.toString(i)).setSource(jsonBuilder()
@@ -363,4 +364,17 @@ public class TopHitsTests extends ElasticsearchIntegrationTest {
         }
     }
 
+    @Test
+    public void testEmptyIndex() throws Exception {
+        SearchResponse response = client().prepareSearch("empty").setTypes("type")
+                .addAggregation(topHits("hits"))
+                .get();
+        assertSearchResponse(response);
+
+        TopHits hits = response.getAggregations().get("hits");
+        assertThat(hits, notNullValue());
+        assertThat(hits.getName(), equalTo("hits"));
+        assertThat(hits.getHits().totalHits(), equalTo(0l));
+    }
+
 }