Improvements from review

Author: parkertimmins

test/framework/src/main/java/org/elasticsearch/datageneration/datasource/DefaultObjectGenerationHandler.java

@@ -20,6 +20,14 @@
 import static org.elasticsearch.test.ESTestCase.randomRealisticUnicodeOfCodepointLengthBetween;
 
 public class DefaultObjectGenerationHandler implements DataSourceHandler {
+
+    /**
+     * Field names will not be generated which start with `_reserved_`. Handlers can safely
+     * create field names starting with this prefix without the concern of randomly generated
+     * fields having the same name.
+     */
+    public static final String RESERVED_FIELD_NAME_PREFIX = "_reserved_";
+
     @Override
     public DataSourceResponse.ChildFieldGenerator handle(DataSourceRequest.ChildFieldGenerator request) {
         return new DataSourceResponse.ChildFieldGenerator() {
@@ -57,6 +65,9 @@ public String generateFieldName() {
                     if (fieldName.indexOf('.') != -1) {
                         continue;
                     }
+                    if (fieldName.startsWith(RESERVED_FIELD_NAME_PREFIX)) {
+                        continue;
+                    }
 
                     return fieldName;
                 }

test/framework/src/main/java/org/elasticsearch/datageneration/datasource/MultifieldAddonHandler.java

@@ -18,7 +18,7 @@
 
 public class MultifieldAddonHandler implements DataSourceHandler {
 
-    private static final String PLACEHOLDER_NAME = "_an_improbable_placeholder_name";
+    private static final String PLACEHOLDER = DefaultObjectGenerationHandler.RESERVED_FIELD_NAME_PREFIX + "multifield";
     private static final float DEFAULT_CHANCE_OF_CHILD_FIELD = 0.5f;
     private final Map<FieldType, List<FieldType>> subfieldTypes;
     private final float chanceOfChildField;
@@ -47,7 +47,7 @@ public DataSourceResponse.LeafMappingParametersGenerator handle(DataSourceReques
 
         // Need to delegate creation of the same type of field to other handlers. So skip request
         // if it's for the placeholder name used when creating the child and parent fields.
-        if (request.fieldName().equals(PLACEHOLDER_NAME)) {
+        if (request.fieldName().equals(PLACEHOLDER)) {
             return null;
         }
 
@@ -76,9 +76,7 @@ public DataSourceResponse.LeafMappingParametersGenerator handle(DataSourceReques
 
     private static Map<String, Object> getChildMappingForType(FieldType type, DataSourceRequest.LeafMappingParametersGenerator request) {
         Map<String, Object> mapping = getMappingForType(type, request);
-        if (type == FieldType.KEYWORD) {
-            mapping.remove("copy_to");
-        }
+        mapping.remove("copy_to");
         return mapping;
     }
 
@@ -87,7 +85,7 @@ private static Map<String, Object> getMappingForType(FieldType type, DataSourceR
             .get(
                 new DataSourceRequest.LeafMappingParametersGenerator(
                     request.dataSource(),
-                    PLACEHOLDER_NAME,
+                    PLACEHOLDER,
                     type.toString(),
                     request.eligibleCopyToFields(),
                     request.dynamicMapping()