parent/child: Allow adding additional child types that point to an existing parent type

From 2.0 adding child types to existing types was forbidden because the`_parent` field stores the join between parent and child at index time.
This is to protect from the fact that types that weren't a parent before become a parent while previously indexed documents would not have a join field.
 This would break the parent/child queries.

The restriction was a bit too strict in the sense that also if a type was a parent type the restriction would forbid adding child types that point to a parent type (so child points already point to it).
This change make sure that the restriction only applies if that type isn't a parent type already.

Closes #17956

core/src/main/java/org/elasticsearch/cluster/metadata/MetaDataMappingService.java

@@ -281,8 +281,11 @@ public class MetaDataMappingService extends AbstractComponent {
                         // Also the order of the mappings may be backwards.
                         if (newMapper.parentFieldMapper().active()) {
                             for (ObjectCursor<MappingMetaData> mapping : indexMetaData.getMappings().values()) {
-                                if (newMapper.parentFieldMapper().type().equals(mapping.value.type())) {
-                                    throw new IllegalArgumentException("can't add a _parent field that points to an already existing type");
+                                String parentType = newMapper.parentFieldMapper().type();
+                                if (parentType.equals(mapping.value.type()) &&
+                                        indexService.mapperService().getParentTypes().contains(parentType) == false) {
+                                    throw new IllegalArgumentException("can't add a _parent field that points to an " +
+                                            "already existing type, that isn't already a parent");
                                 }
                             }
                         }

core/src/test/java/org/elasticsearch/cluster/metadata/MetaDataMappingServiceTests.java

@@ -0,0 +1,66 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.cluster.metadata;
+
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.IndexService;
+import org.elasticsearch.index.mapper.DocumentMapper;
+import org.elasticsearch.test.ESSingleNodeTestCase;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
+
+public class MetaDataMappingServiceTests extends ESSingleNodeTestCase {
+
+    // Tests _parent meta field logic, because part of the validation is in MetaDataMappingService
+    public void testAddChildTypePointingToAlreadyExistingType() throws Exception {
+        createIndex("test", Settings.EMPTY, "type", "field", "type=keyword");
+
+        // Shouldn't be able the add the _parent field pointing to an already existing type, which isn't a parent type
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> client().admin()
+                .indices()
+                .preparePutMapping("test")
+                .setType("child")
+                .setSource("_parent", "type=type")
+                .get());
+        assertThat(e.getMessage(),
+                equalTo("can't add a _parent field that points to an already existing type, that isn't already a parent"));
+    }
+
+    // Tests _parent meta field logic, because part of the validation is in MetaDataMappingService
+    public void testAddExtraChildTypePointingToAlreadyParentExistingType() throws Exception {
+        IndexService indexService = createIndex("test", client().admin().indices().prepareCreate("test")
+                .addMapping("parent")
+                .addMapping("child1", "_parent", "type=parent")
+        );
+
+        // adding the extra child type that points to an already existing parent type is allowed:
+        client().admin()
+                .indices()
+                .preparePutMapping("test")
+                .setType("child2")
+                .setSource("_parent", "type=parent")
+                .get();
+
+        DocumentMapper documentMapper = indexService.mapperService().documentMapper("child2");
+        assertThat(documentMapper.parentFieldMapper().type(), equalTo("parent"));
+        assertThat(documentMapper.parentFieldMapper().active(), is(true));
+    }
+
+}

core/src/test/java/org/elasticsearch/search/child/ChildQuerySearchIT.java

@@ -1321,29 +1321,6 @@ public class ChildQuerySearchIT extends ESIntegTestCase {
         }
     }
 
-    public void testAddParentFieldAfterIndexingParentDocButBeforeIndexingChildDoc() throws Exception {
-        assertAcked(prepareCreate("test")
-                .setSettings(Settings.builder()
-                        .put(indexSettings())
-                        .put("index.refresh_interval", -1)));
-        ensureGreen();
-
-        String parentId = "p1";
-        client().prepareIndex("test", "parent", parentId).setSource("p_field", "1").get();
-        refresh();
-
-        try {
-            assertAcked(client().admin()
-                    .indices()
-                    .preparePutMapping("test")
-                    .setType("child")
-                    .setSource("_parent", "type=parent"));
-            fail("Shouldn't be able the add the _parent field pointing to an already existing parent type");
-        } catch (IllegalArgumentException e) {
-            assertThat(e.getMessage(), equalTo("can't add a _parent field that points to an already existing type"));
-        }
-    }
-
     public void testParentChildCaching() throws Exception {
         assertAcked(prepareCreate("test")
                 .setSettings(