backend-plugin-api: allow modules to register extension points

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

.changeset/gorgeous-dragons-peel.md

@@ -0,0 +1,6 @@
+---
+'@backstage/backend-plugin-api': patch
+'@backstage/backend-app-api': patch
+---
+
+Allow modules to register extension points.

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

@@ -161,7 +161,7 @@ export class BackendInitializer {
       for (const r of feature.getRegistrations()) {
         const provides = new Set<ExtensionPoint<unknown>>();
 
-        if (r.type === 'plugin') {
+        if (r.type === 'plugin' || r.type === 'module') {
           for (const [extRef, extImpl] of r.extensionPoints) {
             if (this.#extensionPoints.has(extRef)) {
               throw new Error(

packages/backend-plugin-api/api-report.md

@@ -31,6 +31,11 @@ export interface BackendModuleConfig {
 
 // @public
 export interface BackendModuleRegistrationPoints {
+  // (undocumented)
+  registerExtensionPoint<TExtensionPoint>(
+    ref: ExtensionPoint<TExtensionPoint>,
+    impl: TExtensionPoint,
+  ): void;
   // (undocumented)
   registerInit<
     Deps extends {

packages/backend-plugin-api/src/wiring/factories.test.ts

@@ -149,6 +149,7 @@ describe('createBackendModule', () => {
         type: 'module',
         pluginId: 'x',
         moduleId: 'y',
+        extensionPoints: [],
         init: {
           deps: expect.any(Object),
           func: expect.any(Function),

packages/backend-plugin-api/src/wiring/factories.ts

@@ -159,13 +159,14 @@ export function createBackendPlugin<TOptions extends [options?: object] = []>(
  */
 export interface BackendModuleConfig {
   /**
-   * The ID of this plugin.
+   * Should exactly match the `id` of the plugin that the module extends.
    *
    * @see {@link https://backstage.io/docs/backend-system/architecture/naming-patterns | Recommended naming patterns}
    */
   pluginId: string;
+
   /**
-   * Should exactly match the `id` of the plugin that the module extends.
+   * The ID of this module, used to identify the module and ensure that it is not installed twice.
    */
   moduleId: string;
   register(reg: BackendModuleRegistrationPoints): void;
@@ -194,10 +195,20 @@ export function createBackendModule<TOptions extends [options?: object] = []>(
         if (registrations) {
           return registrations;
         }
+        const extensionPoints: InternalBackendPluginRegistration['extensionPoints'] =
+          [];
         let init: InternalBackendModuleRegistration['init'] | undefined =
           undefined;
 
         c.register({
+          registerExtensionPoint(ext, impl) {
+            if (init) {
+              throw new Error(
+                'registerExtensionPoint called after registerInit',
+              );
+            }
+            extensionPoints.push([ext, impl]);
+          },
           registerInit(regInit) {
             if (init) {
               throw new Error('registerInit must only be called once');
@@ -220,6 +231,7 @@ export function createBackendModule<TOptions extends [options?: object] = []>(
             type: 'module',
             pluginId: c.pluginId,
             moduleId: c.moduleId,
+            extensionPoints,
             init,
           },
         ];

packages/backend-plugin-api/src/wiring/types.ts

@@ -59,6 +59,10 @@ export interface BackendPluginRegistrationPoints {
  * @public
  */
 export interface BackendModuleRegistrationPoints {
+  registerExtensionPoint<TExtensionPoint>(
+    ref: ExtensionPoint<TExtensionPoint>,
+    impl: TExtensionPoint,
+  ): void;
   registerInit<Deps extends { [name in string]: unknown }>(options: {
     deps: {
       [name in keyof Deps]: ServiceRef<Deps[name]> | ExtensionPoint<Deps[name]>;
@@ -105,6 +109,7 @@ export interface InternalBackendModuleRegistration {
   pluginId: string;
   moduleId: string;
   type: 'module';
+  extensionPoints: Array<readonly [ExtensionPoint<unknown>, unknown]>;
   init: {
     deps: Record<string, ServiceRef<unknown> | ExtensionPoint<unknown>>;
     func(deps: Record<string, unknown>): Promise<void>;