Fix serialization of empty field capabilities response

When no response are required (no indices match the requested patterns) the
empty response throws an NPE in the transport serialization (writeTo).

Author: jimczi

server/src/main/java/org/elasticsearch/action/fieldcaps/FieldCapabilities.java

@@ -166,14 +166,14 @@ public String getName() {
     }
 
     /**
-     * Whether this field is indexed for search on all indices.
+     * Whether this field can be aggregated on all indices.
      */
     public boolean isAggregatable() {
         return isAggregatable;
     }
 
     /**
-     * Whether this field can be aggregated on all indices.
+     * Whether this field is indexed for search on all indices.
      */
     public boolean isSearchable() {
         return isSearchable;

server/src/main/java/org/elasticsearch/action/fieldcaps/FieldCapabilitiesRequest.java

@@ -111,7 +111,6 @@ public String[] fields() {
     }
 
     /**
-     *
      * The list of indices to lookup
      */
     public FieldCapabilitiesRequest indices(String... indices) {

server/src/main/java/org/elasticsearch/action/fieldcaps/FieldCapabilitiesResponse.java

@@ -65,6 +65,7 @@ private FieldCapabilitiesResponse(Map<String, Map<String, FieldCapabilities>> re
      */
     FieldCapabilitiesResponse() {
         this.responseMap = Collections.emptyMap();
+        this.indexResponses = Collections.emptyList();
     }
 
     /**

server/src/main/java/org/elasticsearch/client/Client.java

@@ -455,7 +455,7 @@ public interface Client extends ElasticsearchClient, Releasable {
     /**
      * Builder for the field capabilities request.
      */
-    FieldCapabilitiesRequestBuilder prepareFieldCaps();
+    FieldCapabilitiesRequestBuilder prepareFieldCaps(String... indices);
 
     /**
      * An action that returns the field capabilities from the provided request

server/src/main/java/org/elasticsearch/client/support/AbstractClient.java

@@ -651,8 +651,8 @@ public ActionFuture<FieldCapabilitiesResponse> fieldCaps(FieldCapabilitiesReques
     }
 
     @Override
-    public FieldCapabilitiesRequestBuilder prepareFieldCaps() {
-        return new FieldCapabilitiesRequestBuilder(this, FieldCapabilitiesAction.INSTANCE);
+    public FieldCapabilitiesRequestBuilder prepareFieldCaps(String... indices) {
+        return new FieldCapabilitiesRequestBuilder(this, FieldCapabilitiesAction.INSTANCE, indices);
     }
 
     static class Admin implements AdminClient {

server/src/test/java/org/elasticsearch/search/fieldcaps/FieldCapabilitiesIT.java

@@ -21,6 +21,7 @@
 
 import org.elasticsearch.action.fieldcaps.FieldCapabilities;
 import org.elasticsearch.action.fieldcaps.FieldCapabilitiesResponse;
+import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.plugins.MapperPlugin;
@@ -148,4 +149,11 @@ public void testFieldAliasFilteringWithWildcard() {
         assertEquals(1, response.get().size());
         assertTrue(response.get().containsKey("distance"));
     }
+
+    public void testEmptyIndexPattern() {
+        FieldCapabilitiesResponse response = client().prepareFieldCaps("empty_index_pattern*")
+            .setFields("*")
+            .execute().actionGet();
+        assertEquals(0, response.get().size());
+    }
 }