Fix serialization of empty field capabilities response (#33263)

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).

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

@@ -166,14 +166,14 @@ public class FieldCapabilities implements Writeable, ToXContentObject {
     }
 
     /**
-     * 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 final class FieldCapabilitiesRequest extends ActionRequest implements Ind
     }
 
     /**
-     *
      * The list of indices to lookup
      */
     public FieldCapabilitiesRequest indices(String... indices) {

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

@@ -35,6 +35,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.stream.Collectors;
 
 /**
@@ -56,15 +57,15 @@ public class FieldCapabilitiesResponse extends ActionResponse implements ToXCont
 
     private FieldCapabilitiesResponse(Map<String, Map<String, FieldCapabilities>> responseMap,
                                       List<FieldCapabilitiesIndexResponse> indexResponses) {
-        this.responseMap = responseMap;
-        this.indexResponses = indexResponses;
+        this.responseMap = Objects.requireNonNull(responseMap);
+        this.indexResponses = Objects.requireNonNull(indexResponses);
     }
 
     /**
      * Used for serialization
      */
     FieldCapabilitiesResponse() {
-        this.responseMap = Collections.emptyMap();
+        this(Collections.emptyMap(), Collections.emptyList());
     }
 
     /**
@@ -81,6 +82,7 @@ public class FieldCapabilitiesResponse extends ActionResponse implements ToXCont
     List<FieldCapabilitiesIndexResponse> getIndexResponses() {
         return indexResponses;
     }
+
     /**
      *
      * Get the field capabilities per type for the provided {@code field}.

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

@@ -90,7 +90,7 @@ public class TransportFieldCapabilitiesAction extends HandledTransportAction<Fie
             }
         };
         if (totalNumRequest == 0) {
-            listener.onResponse(new FieldCapabilitiesResponse());
+            listener.onResponse(new FieldCapabilitiesResponse(Collections.emptyMap()));
         } else {
             ActionListener<FieldCapabilitiesIndexResponse> innerListener = new ActionListener<FieldCapabilitiesIndexResponse>() {
                 @Override

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 abstract class AbstractClient extends AbstractComponent implements Client
     }
 
     @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/action/fieldcaps/FieldCapabilitiesResponseTests.java

@@ -28,11 +28,15 @@ import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.test.AbstractStreamableXContentTestCase;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.function.Predicate;
 
+import static com.carrotsearch.randomizedtesting.RandomizedTest.randomAsciiLettersOfLength;
+
 
 public class FieldCapabilitiesResponseTests extends AbstractStreamableXContentTestCase<FieldCapabilitiesResponse> {
 
@@ -48,22 +52,46 @@ public class FieldCapabilitiesResponseTests extends AbstractStreamableXContentTe
 
     @Override
     protected FieldCapabilitiesResponse createTestInstance() {
-        Map<String, Map<String, FieldCapabilities>> responses = new HashMap<>();
+        if (randomBoolean()) {
+            // merged responses
+            Map<String, Map<String, FieldCapabilities>> responses = new HashMap<>();
+
+            String[] fields = generateRandomStringArray(5, 10, false, true);
+            assertNotNull(fields);
+
+            for (String field : fields) {
+                Map<String, FieldCapabilities> typesToCapabilities = new HashMap<>();
+                String[] types = generateRandomStringArray(5, 10, false, false);
+                assertNotNull(types);
+
+                for (String type : types) {
+                    typesToCapabilities.put(type, FieldCapabilitiesTests.randomFieldCaps(field));
+                }
+                responses.put(field, typesToCapabilities);
+            }
+            return new FieldCapabilitiesResponse(responses);
+        } else {
+            // non-merged responses
+            List<FieldCapabilitiesIndexResponse> responses = new ArrayList<>();
+            int numResponse = randomIntBetween(0, 10);
+            for (int i = 0; i < numResponse; i++) {
+                responses.add(createRandomIndexResponse());
+            }
+            return new FieldCapabilitiesResponse(responses);
+        }
+    }
+
+
+    private FieldCapabilitiesIndexResponse createRandomIndexResponse() {
+        Map<String, FieldCapabilities> responses = new HashMap<>();
 
         String[] fields = generateRandomStringArray(5, 10, false, true);
         assertNotNull(fields);
 
         for (String field : fields) {
-            Map<String, FieldCapabilities> typesToCapabilities = new HashMap<>();
-            String[] types = generateRandomStringArray(5, 10, false, false);
-            assertNotNull(types);
-
-            for (String type : types) {
-                typesToCapabilities.put(type, FieldCapabilitiesTests.randomFieldCaps(field));
-            }
-            responses.put(field, typesToCapabilities);
+            responses.put(field, FieldCapabilitiesTests.randomFieldCaps(field));
         }
-        return new FieldCapabilitiesResponse(responses);
+        return new FieldCapabilitiesIndexResponse(randomAsciiLettersOfLength(10), responses);
     }
 
     @Override
@@ -138,6 +166,11 @@ public class FieldCapabilitiesResponseTests extends AbstractStreamableXContentTe
             "}").replaceAll("\\s+", ""), generatedResponse);
     }
 
+    public void testEmptyResponse() throws IOException {
+        FieldCapabilitiesResponse testInstance = new FieldCapabilitiesResponse();
+        assertSerialization(testInstance);
+    }
+
     private static FieldCapabilitiesResponse createSimpleResponse() {
         Map<String, FieldCapabilities> titleCapabilities = new HashMap<>();
         titleCapabilities.put("text", new FieldCapabilities("title", "text", true, false));