add feature discovery service

Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

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

@@ -27,6 +27,7 @@ import { EnumerableServiceHolder, ServiceOrExtensionPoint } from './types';
 // eslint-disable-next-line @backstage/no-forbidden-package-imports
 import { InternalBackendFeature } from '@backstage/backend-plugin-api/src/wiring/types';
 import { ForwardedError } from '@backstage/errors';
+import { featureDiscoveryServiceRef } from '@backstage/backend-plugin-api/alpha';
 
 export interface BackendRegisterInit {
   consumes: Set<ServiceOrExtensionPoint>;
@@ -87,6 +88,10 @@ export class BackendInitializer {
     if (this.#startPromise) {
       throw new Error('feature can not be added after the backend has started');
     }
+    this.#addFeature(feature);
+  }
+
+  #addFeature(feature: BackendFeature) {
     if (feature.$$type !== '@backstage/BackendFeature') {
       throw new Error(
         `Failed to add feature, invalid type '${feature.$$type}'`,
@@ -129,6 +134,18 @@ export class BackendInitializer {
   }
 
   async #doStart(): Promise<void> {
+    const featureDiscovery = await this.#serviceHolder.get(
+      featureDiscoveryServiceRef,
+      'root',
+    );
+
+    if (featureDiscovery) {
+      const { features } = await featureDiscovery.getBackendFeatures();
+      for (const plugin of features) {
+        this.#addFeature(plugin);
+      }
+    }
+
     // Initialize all root scoped services
     for (const ref of this.#serviceHolder.getServiceRefs()) {
       if (ref.scope === 'root') {

packages/backend-plugin-api/package.json

@@ -5,13 +5,26 @@
   "main": "src/index.ts",
   "types": "src/index.ts",
   "publishConfig": {
-    "access": "public",
-    "main": "dist/index.cjs.js",
-    "types": "dist/index.d.ts"
+    "access": "public"
   },
   "backstage": {
     "role": "node-library"
   },
+  "exports": {
+    ".": "./src/index.ts",
+    "./alpha": "./src/alpha.ts",
+    "./package.json": "./package.json"
+  },
+  "typesVersions": {
+    "*": {
+      "alpha": [
+        "src/alpha.ts"
+      ],
+      "package.json": [
+        "package.json"
+      ]
+    }
+  },
   "homepage": "https://backstage.io",
   "repository": {
     "type": "git",

packages/backend-plugin-api/src/alpha.ts

@@ -0,0 +1,29 @@
+/*
+ * 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 { createServiceRef } from './services';
+import { BackendFeature } from './wiring';
+
+/** @alpha */
+export interface FeatureDiscoveryService {
+  getBackendFeatures(): Promise<{ features: Array<BackendFeature> }>;
+}
+
+/** @alpha */
+export const featureDiscoveryServiceRef =
+  createServiceRef<FeatureDiscoveryService>({
+    id: 'core.featureDiscovery',
+  });