REST API: Consistent get field mapping response

If a get field mapping request is issued, and all but the field can be
found, the response should return an empty JSON object instead of a 404.

Closes #4738

rest-api-spec/test/indices.get_field_mapping/20_missing_field.yaml

@@ -1,5 +1,5 @@
 ---
-"Raise 404 when field doesn't exist":
+"Return empty object if field doesn't exist, but type and index do":
 
   - do:
         indices.create:
@@ -13,9 +13,9 @@
                       analyzer: whitespace
 
   - do:
-      catch: missing
       indices.get_field_mapping:
         index: test_index
         type: test_type
-        field: not_text
+        field: not_existent
   
+  - match: { '': {}}

src/main/java/org/elasticsearch/action/admin/indices/mapping/get/GetFieldMappingsResponse.java

@@ -21,6 +21,7 @@ package org.elasticsearch.action.admin.indices.mapping.get;
 
 import com.google.common.collect.ImmutableMap;
 import org.elasticsearch.action.ActionResponse;
+import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -88,6 +89,8 @@ public class GetFieldMappingsResponse extends ActionResponse implements ToXConte
     }
 
     public static class FieldMappingMetaData implements ToXContent {
+        public static final FieldMappingMetaData NULL = new FieldMappingMetaData("", BytesArray.EMPTY);
+
         private String fullName;
         private BytesReference source;
 

src/main/java/org/elasticsearch/action/admin/indices/mapping/get/TransportGetFieldMappingsAction.java

@@ -92,6 +92,7 @@ public class TransportGetFieldMappingsAction extends TransportClusterInfoAction<
             return ImmutableMap.of();
         }
 
+        boolean isProbablySingleFieldRequest = concreteIndices.length == 1 && types.length == 1 && fields.length == 1;
         ImmutableMap.Builder<String, ImmutableMap<String, ImmutableMap<String, FieldMappingMetaData>>> indexMapBuilder = ImmutableMap.builder();
         Sets.SetView<String> intersection = Sets.intersection(Sets.newHashSet(concreteIndices), indicesService.indices());
         for (String index : intersection) {
@@ -114,7 +115,7 @@ public class TransportGetFieldMappingsAction extends TransportClusterInfoAction<
             MapBuilder<String, ImmutableMap<String, FieldMappingMetaData>> typeMappings = new MapBuilder<String, ImmutableMap<String, FieldMappingMetaData>>();
             for (String type : typeIntersection) {
                 DocumentMapper documentMapper = indexService.mapperService().documentMapper(type);
-                ImmutableMap<String, FieldMappingMetaData> fieldMapping = findFieldMappingsByType(documentMapper, fields, includeDefaults);
+                ImmutableMap<String, FieldMappingMetaData> fieldMapping = findFieldMappingsByType(documentMapper, fields, includeDefaults, isProbablySingleFieldRequest);
                 if (!fieldMapping.isEmpty()) {
                     typeMappings.put(type, fieldMapping);
                 }
@@ -170,7 +171,7 @@ public class TransportGetFieldMappingsAction extends TransportClusterInfoAction<
     };
 
     private ImmutableMap<String, FieldMappingMetaData> findFieldMappingsByType(DocumentMapper documentMapper, String[] fields,
-                                                                               boolean includeDefaults) throws ElasticsearchException {
+                                                                               boolean includeDefaults, boolean isProbablySingleFieldRequest) throws ElasticsearchException {
         MapBuilder<String, FieldMappingMetaData> fieldMappings = new MapBuilder<String, FieldMappingMetaData>();
         ImmutableList<FieldMapper> allFieldMappers = documentMapper.mappers().mappers();
         for (String field : fields) {
@@ -215,6 +216,8 @@ public class TransportGetFieldMappingsAction extends TransportClusterInfoAction<
                 FieldMapper fieldMapper = documentMapper.mappers().smartNameFieldMapper(field);
                 if (fieldMapper != null) {
                     addFieldMapper(field, fieldMapper, fieldMappings, includeDefaults);
+                } else if (isProbablySingleFieldRequest) {
+                    fieldMappings.put(field, FieldMappingMetaData.NULL);
                 }
             }
         }

src/main/java/org/elasticsearch/rest/action/admin/indices/mapping/get/RestGetFieldMappingAction.java

@@ -35,10 +35,12 @@ import org.elasticsearch.rest.*;
 import org.elasticsearch.rest.action.support.RestXContentBuilder;
 
 import java.io.IOException;
+import java.util.Map;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
 import static org.elasticsearch.rest.RestStatus.NOT_FOUND;
 import static org.elasticsearch.rest.RestStatus.OK;
+import static org.elasticsearch.rest.action.support.RestXContentBuilder.emptyBuilder;
 
 /**
  *
@@ -70,14 +72,21 @@ public class RestGetFieldMappingAction extends BaseRestHandler {
             @Override
             public void onResponse(GetFieldMappingsResponse response) {
                 try {
-                    XContentBuilder builder = RestXContentBuilder.restContentBuilder(request);
-                    builder.startObject();
-
                     ImmutableMap<String, ImmutableMap<String, ImmutableMap<String, FieldMappingMetaData>>> mappingsByIndex = response.mappings();
+
+                    boolean isPossibleSingleFieldRequest = indices.length == 1 && types.length == 1 && fields.length == 1;
+                    if (isPossibleSingleFieldRequest && isFieldMappingMissingField(mappingsByIndex)) {
+                        channel.sendResponse(new XContentRestResponse(request, OK, emptyBuilder(request)));
+                        return;
+                    }
+
                     RestStatus status = OK;
                     if (mappingsByIndex.isEmpty() && fields.length > 0) {
                         status = NOT_FOUND;
                     }
+
+                    XContentBuilder builder = RestXContentBuilder.restContentBuilder(request);
+                    builder.startObject();
                     response.toXContent(builder, ToXContent.EMPTY_PARAMS);
                     builder.endObject();
                     channel.sendResponse(new XContentRestResponse(request, status, builder));
@@ -97,4 +106,25 @@ public class RestGetFieldMappingAction extends BaseRestHandler {
         });
     }
 
+    /**
+     *
+     * Helper method to find out if the only included fieldmapping metadata is typed NULL, which means
+     * that type and index exist, but the field did not
+     */
+    private boolean isFieldMappingMissingField(ImmutableMap<String, ImmutableMap<String, ImmutableMap<String, FieldMappingMetaData>>> mappingsByIndex) throws IOException {
+        if (mappingsByIndex.size() != 1) {
+            return false;
+        }
+
+        for (ImmutableMap<String, ImmutableMap<String, FieldMappingMetaData>> value : mappingsByIndex.values()) {
+            for (ImmutableMap<String, FieldMappingMetaData> fieldValue : value.values()) {
+                for (Map.Entry<String, FieldMappingMetaData> fieldMappingMetaDataEntry : fieldValue.entrySet()) {
+                    if (fieldMappingMetaDataEntry.getValue() == FieldMappingMetaData.NULL) {
+                        return true;
+                    }
+                }
+            }
+        }
+        return false;
+    }
 }