backend-app-api: keep track of the provided services

Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

packages/backend-app-api/src/wiring/BackendInitializer.test.ts

@@ -23,7 +23,7 @@ import {
   createExtensionPoint,
 } from '@backstage/backend-plugin-api';
 import { BackendInitializer } from './BackendInitializer';
-import { ServiceRegistry } from './ServiceRegistry';
+
 import {
   lifecycleServiceFactory,
   loggerServiceFactory,
@@ -65,7 +65,7 @@ describe('BackendInitializer', () => {
     const rootFactory = jest.fn();
     const pluginFactory = jest.fn();
 
-    const registry = new ServiceRegistry([
+    const services = [
       createServiceFactory({
         service: rootRef,
         deps: {},
@@ -82,9 +82,9 @@ describe('BackendInitializer', () => {
         deps: {},
         factory: () => new MockLogger(),
       })(),
-    ]);
+    ];
 
-    const init = new BackendInitializer(registry);
+    const init = new BackendInitializer(services);
     await init.start();
 
     expect(rootFactory).toHaveBeenCalled();
@@ -97,7 +97,7 @@ describe('BackendInitializer', () => {
     const extensionPoint = createExtensionPoint<{ values: string[] }>({
       id: 'a',
     });
-    const init = new BackendInitializer(new ServiceRegistry(baseFactories));
+    const init = new BackendInitializer(baseFactories);
 
     init.add(
       createBackendModule({
@@ -151,7 +151,7 @@ describe('BackendInitializer', () => {
   });
 
   it('should forward errors when plugins fail to start', async () => {
-    const init = new BackendInitializer(new ServiceRegistry([]));
+    const init = new BackendInitializer([]);
     init.add(
       createBackendPlugin({
         pluginId: 'test',
@@ -171,7 +171,7 @@ describe('BackendInitializer', () => {
   });
 
   it('should forward errors when modules fail to start', async () => {
-    const init = new BackendInitializer(new ServiceRegistry([]));
+    const init = new BackendInitializer([]);
     init.add(
       createBackendModule({
         pluginId: 'test',
@@ -192,7 +192,7 @@ describe('BackendInitializer', () => {
   });
 
   it('should reject duplicate plugins', async () => {
-    const init = new BackendInitializer(new ServiceRegistry([]));
+    const init = new BackendInitializer([]);
     init.add(
       createBackendPlugin({
         pluginId: 'test',
@@ -221,7 +221,7 @@ describe('BackendInitializer', () => {
   });
 
   it('should reject duplicate modules', async () => {
-    const init = new BackendInitializer(new ServiceRegistry([]));
+    const init = new BackendInitializer([]);
     init.add(
       createBackendModule({
         pluginId: 'test',
@@ -254,16 +254,14 @@ describe('BackendInitializer', () => {
   it('should reject modules with circular dependencies', async () => {
     const extA = createExtensionPoint<string>({ id: 'a' });
     const extB = createExtensionPoint<string>({ id: 'b' });
-    const init = new BackendInitializer(
-      new ServiceRegistry([
-        rootLifecycleServiceFactory(),
-        createServiceFactory({
-          service: coreServices.rootLogger,
-          deps: {},
-          factory: () => new MockLogger(),
-        })(),
-      ]),
-    );
+    const init = new BackendInitializer([
+      rootLifecycleServiceFactory(),
+      createServiceFactory({
+        service: coreServices.rootLogger,
+        deps: {},
+        factory: () => new MockLogger(),
+      })(),
+    ]);
     init.add(
       createBackendModule({
         pluginId: 'test',
@@ -296,7 +294,7 @@ describe('BackendInitializer', () => {
   });
 
   it('should reject modules that depend on extension points other plugins', async () => {
-    const init = new BackendInitializer(new ServiceRegistry(baseFactories));
+    const init = new BackendInitializer(baseFactories);
     const extA = createExtensionPoint<string>({ id: 'a' });
     init.add(
       createBackendPlugin({

packages/backend-app-api/src/wiring/BackendInitializer.ts

@@ -30,6 +30,7 @@ import { InternalBackendFeature } from '@backstage/backend-plugin-api/src/wiring
 import { ForwardedError, ConflictError } from '@backstage/errors';
 import { featureDiscoveryServiceRef } from '@backstage/backend-plugin-api/alpha';
 import { DependencyGraph } from '../lib/DependencyGraph';
+import { ServiceRegistry } from './ServiceRegistry';
 
 export interface BackendRegisterInit {
   consumes: Set<ServiceOrExtensionPoint>;
@@ -47,10 +48,12 @@ export class BackendInitializer {
     ExtensionPoint<unknown>,
     { impl: unknown; pluginId: string }
   >();
-  #serviceHolder: EnumerableServiceHolder;
+  #serviceHolder: EnumerableServiceHolder | undefined;
+  #providedServiceFactories = new Array<ServiceFactory>();
+  #defaultApiFactories: ServiceFactory[];
 
-  constructor(serviceHolder: EnumerableServiceHolder) {
-    this.#serviceHolder = serviceHolder;
+  constructor(defaultApiFactories: ServiceFactory[]) {
+    this.#defaultApiFactories = defaultApiFactories;
   }
 
   async #getInitDeps(
@@ -70,7 +73,7 @@ export class BackendInitializer {
         }
         result.set(name, ep.impl);
       } else {
-        const impl = await this.#serviceHolder.get(
+        const impl = await this.#serviceHolder!.get(
           ref as ServiceRef<unknown>,
           pluginId,
         );
@@ -112,7 +115,16 @@ export class BackendInitializer {
           `The ${coreServices.pluginMetadata.id} service cannot be overridden`,
         );
       }
-      this.#serviceHolder.add(feature);
+      if (
+        this.#providedServiceFactories.find(
+          sf => sf.service.id === feature.service.id,
+        )
+      ) {
+        throw new Error(
+          `Duplicate service implementations provided for ${feature.service.id}`,
+        );
+      }
+      this.#providedServiceFactories.push(feature);
     } else if (isInternalBackendFeature(feature)) {
       if (feature.version !== 'v1') {
         throw new Error(
@@ -155,6 +167,8 @@ export class BackendInitializer {
   }
 
   async #doStart(): Promise<void> {
+    this.#serviceHolder = new ServiceRegistry(this.#defaultApiFactories);
+
     const featureDiscovery = await this.#serviceHolder.get(
       featureDiscoveryServiceRef,
       'root',
@@ -333,7 +347,7 @@ export class BackendInitializer {
 
   // Bit of a hacky way to grab the lifecycle services, potentially find a nicer way to do this
   async #getRootLifecycleImpl(): Promise<BackendLifecycleImpl> {
-    const lifecycleService = await this.#serviceHolder.get(
+    const lifecycleService = await this.#serviceHolder!.get(
       coreServices.rootLifecycle,
       'root',
     );
@@ -346,7 +360,7 @@ export class BackendInitializer {
   async #getPluginLifecycleImpl(
     pluginId: string,
   ): Promise<BackendPluginLifecycleImpl> {
-    const lifecycleService = await this.#serviceHolder.get(
+    const lifecycleService = await this.#serviceHolder!.get(
       coreServices.lifecycle,
       pluginId,
     );

packages/backend-app-api/src/wiring/BackstageBackend.ts

@@ -14,18 +14,15 @@
  * limitations under the License.
  */
 
-import { ServiceFactory, BackendFeature } from '@backstage/backend-plugin-api';
+import { BackendFeature, ServiceFactory } from '@backstage/backend-plugin-api';
 import { BackendInitializer } from './BackendInitializer';
-import { ServiceRegistry } from './ServiceRegistry';
 import { Backend } from './types';
 
 export class BackstageBackend implements Backend {
-  #services: ServiceRegistry;
   #initializer: BackendInitializer;
 
-  constructor(apiFactories: ServiceFactory[]) {
-    this.#services = new ServiceRegistry(apiFactories);
-    this.#initializer = new BackendInitializer(this.#services);
+  constructor(defaultServiceFactories: ServiceFactory[]) {
+    this.#initializer = new BackendInitializer(defaultServiceFactories);
   }
 
   add(feature: BackendFeature | (() => BackendFeature)): void {