inner_hits: Return an empty _source for nested inner hit when filtering on a field that doesn't exist.

Before this change the search request would fail with an error indicating that it couldn't detect xcontent type based on the string: `null`

core/src/main/java/org/elasticsearch/search/fetch/subphase/FetchSourceSubPhase.java

@@ -20,21 +20,16 @@
 package org.elasticsearch.search.fetch.subphase;
 
 import org.elasticsearch.ElasticsearchException;
-import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.xcontent.XContentBuilder;
-import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.search.SearchHit;
 import org.elasticsearch.search.fetch.FetchSubPhase;
 import org.elasticsearch.search.internal.SearchContext;
 import org.elasticsearch.search.lookup.SourceLookup;
 
 import java.io.IOException;
-import java.io.UncheckedIOException;
 import java.util.Map;
 
-import static org.elasticsearch.common.xcontent.XContentFactory.contentBuilder;
-
 public final class FetchSourceSubPhase implements FetchSubPhase {
 
     @Override
@@ -65,7 +60,17 @@ public final class FetchSourceSubPhase implements FetchSubPhase {
             final int initialCapacity = nestedHit ? 1024 : Math.min(1024, source.internalSourceRef().length());
             BytesStreamOutput streamOutput = new BytesStreamOutput(initialCapacity);
             XContentBuilder builder = new XContentBuilder(source.sourceContentType().xContent(), streamOutput);
-            builder.value(value);
+            if (value != null) {
+                builder.value(value);
+            } else {
+                // This happens if the source filtering could not find the specified in the _source.
+                // Just doing `builder.value(null)` is valid, but the xcontent validation can't detect what format
+                // it is. In certain cases, for example response serialization we fail if no xcontent type can't be
+                // detected. So instead we just return an empty top level object. Also this is in inline with what was
+                // being return in this situation in 5.x and earlier.
+                builder.startObject();
+                builder.endObject();
+            }
             hitContext.hit().sourceRef(builder.bytes());
         } catch (IOException e) {
             throw new ElasticsearchException("Error filtering source", e);

core/src/test/java/org/elasticsearch/search/fetch/subphase/InnerHitsIT.java

@@ -56,7 +56,6 @@ import java.util.function.Function;
 
 import static org.elasticsearch.action.support.WriteRequest.RefreshPolicy.IMMEDIATE;
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
-import static org.elasticsearch.common.xcontent.support.XContentMapValues.extractValue;
 import static org.elasticsearch.index.query.QueryBuilders.boolQuery;
 import static org.elasticsearch.index.query.QueryBuilders.matchAllQuery;
 import static org.elasticsearch.index.query.QueryBuilders.matchQuery;
@@ -635,6 +634,18 @@ public class InnerHitsIT extends ESIntegTestCase {
         assertThat(response.getHits().getAt(0).getInnerHits().get("comments").getAt(0).getSourceAsMap().size(), equalTo(2));
         assertThat(response.getHits().getAt(0).getInnerHits().get("comments").getAt(1).getSourceAsMap().get("message"),
                 equalTo("fox ate rabbit x y z"));
+
+        // Source filter on a field that does not exist inside the nested document and just check that we do not fail and
+        // return an empty _source:
+        response = client().prepareSearch()
+            .setQuery(nestedQuery("comments", matchQuery("comments.message", "away"), ScoreMode.None)
+                .innerHit(new InnerHitBuilder().setFetchSourceContext(new FetchSourceContext(true,
+                    new String[]{"comments.missing_field"}, null))))
+            .get();
+        assertNoFailures(response);
+        assertHitCount(response, 1);
+        assertThat(response.getHits().getAt(0).getInnerHits().get("comments").getTotalHits(), equalTo(1L));
+        assertThat(response.getHits().getAt(0).getInnerHits().get("comments").getAt(0).getSourceAsMap().size(), equalTo(0));
     }
 
     public void testInnerHitsWithIgnoreUnmapped() throws Exception {