backend-app-api: throw error if duplicate service implementations are provided

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/backend-defaults/src/CreateBackend.test.ts

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed 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 {
+  coreServices,
+  createServiceFactory,
+} from '@backstage/backend-plugin-api';
+import { createBackend } from './CreateBackend';
+
+describe('createBackend', () => {
+  it('should not throw when overriding a default service implementation', () => {
+    expect(() =>
+      createBackend({
+        services: [
+          createServiceFactory({
+            service: coreServices.rootLifecycle,
+            deps: {},
+            factory: async () => ({ addShutdownHook: () => {} }),
+          }),
+        ],
+      }),
+    ).not.toThrow();
+  });
+
+  it('should throw on duplicate service implementations', () => {
+    expect(() =>
+      createBackend({
+        services: [
+          createServiceFactory({
+            service: coreServices.rootLifecycle,
+            deps: {},
+            factory: async () => ({ addShutdownHook: () => {} }),
+          }),
+          createServiceFactory({
+            service: coreServices.rootLifecycle,
+            deps: {},
+            factory: async () => ({ addShutdownHook: () => {} }),
+          }),
+        ],
+      }),
+    ).toThrow(
+      'Duplicate service implementations provided for core.rootLifecycle',
+    );
+  });
+});

packages/backend-defaults/src/CreateBackend.ts

@@ -35,20 +35,20 @@ import {
 import { ServiceFactory } from '@backstage/backend-plugin-api';
 
 export const defaultServiceFactories = [
-  cacheFactory,
-  configFactory,
-  databaseFactory,
-  discoveryFactory,
-  loggerFactory,
-  rootLoggerFactory,
-  permissionsFactory,
-  schedulerFactory,
-  tokenManagerFactory,
-  urlReaderFactory,
-  httpRouterFactory,
-  rootHttpRouterFactory,
-  lifecycleFactory,
-  rootLifecycleFactory,
+  cacheFactory(),
+  configFactory(),
+  databaseFactory(),
+  discoveryFactory(),
+  loggerFactory(),
+  rootLoggerFactory(),
+  permissionsFactory(),
+  schedulerFactory(),
+  tokenManagerFactory(),
+  urlReaderFactory(),
+  httpRouterFactory(),
+  rootHttpRouterFactory(),
+  lifecycleFactory(),
+  rootLifecycleFactory(),
 ];
 
 /**
@@ -62,7 +62,15 @@ export interface CreateBackendOptions {
  * @public
  */
 export function createBackend(options?: CreateBackendOptions): Backend {
+  const providedServices = (options?.services ?? []).map(sf =>
+    typeof sf === 'function' ? sf() : sf,
+  );
+  const providedIds = new Set(providedServices.map(sf => sf.service.id));
+  const neededDefaultFactories = defaultServiceFactories.filter(
+    sf => !providedIds.has(sf.service.id),
+  );
+
   return createSpecializedBackend({
-    services: [...defaultServiceFactories, ...(options?.services ?? [])],
+    services: [...neededDefaultFactories, ...providedServices],
   });
 }