make defaultRoute accessible in NP Config

src/core/server/http/http_config.ts

@@ -38,7 +38,15 @@ export const config = {
           validate: match(validBasePathRegex, "must start with a slash, don't end with one"),
         })
       ),
-      defaultRoute: schema.maybe(schema.string()),
+      defaultRoute: schema.maybe(
+        schema.string({
+          validate(value) {
+            if (!value.startsWith('/')) {
+              return 'must start with a slash';
+            }
+          },
+        })
+      ),
       cors: schema.conditional(
         schema.contextRef('dev'),
         true,

src/core/server/legacy/config/legacy_object_to_config_adapter.ts

@@ -62,6 +62,7 @@ export class LegacyObjectToConfigAdapter extends ObjectToConfigAdapter {
     return {
       autoListen: configValue.autoListen,
       basePath: configValue.basePath,
+      defaultRoute: configValue.defaultRoute,
       cors: configValue.cors,
       host: configValue.host,
       maxPayload: configValue.maxPayloadBytes,

src/legacy/server/http/integration_tests/default_route_provider_config.test.ts

@@ -0,0 +1,53 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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.
+ */
+import * as kbnTestServer from '../../../../test_utils/kbn_server';
+// eslint-disable-next-line @kbn/eslint/no-restricted-paths
+import { Root } from '../../../../core/server/root';
+
+describe('default route provider', () => {
+  let root: Root;
+
+  afterEach(async () => await root.shutdown());
+
+  it('redirects to the configured default route', async function() {
+    root = kbnTestServer.createRoot({
+      server: {
+        defaultRoute: '/app/some/default/route',
+      },
+    });
+
+    await root.setup();
+    await root.start();
+
+    const kbnServer = kbnTestServer.getKbnServer(root);
+
+    kbnServer.server.decorate('request', 'getSavedObjectsClient', function() {
+      return {
+        get: (type: string, id: string) => ({ attributes: {} }),
+      };
+    });
+
+    const { status, header } = await kbnTestServer.request.get(root, '/');
+
+    expect(status).toEqual(302);
+    expect(header).toMatchObject({
+      location: '/app/some/default/route',
+    });
+  });
+});

src/legacy/server/saved_objects/saved_objects_mixin.js

@@ -55,7 +55,7 @@ function getImportableAndExportableTypes({ kbnServer, visibleTypes }) {
   );
 }
 
-export async function savedObjectsMixin(kbnServer, server) {
+export function savedObjectsMixin(kbnServer, server) {
   const migrator = kbnServer.newPlatform.__internals.kibanaMigrator;
   const mappings = migrator.getActiveMappings();
   const allTypes = Object.keys(getRootPropertiesObjects(mappings));