backend-app-api: validate extension point deps

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

.changeset/quick-cups-float.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-app-api': patch
+---
+
+Add validation to make sure that extension points do not cross plugin boundaries.

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

@@ -294,4 +294,36 @@ describe('BackendInitializer', () => {
       "Circular dependency detected for modules of plugin 'test', 'modA' -> 'modB' -> 'modA'",
     );
   });
+
+  it('should reject modules that depend on extension points other plugins', async () => {
+    const init = new BackendInitializer(new ServiceRegistry(baseFactories));
+    const extA = createExtensionPoint<string>({ id: 'a' });
+    init.add(
+      createBackendPlugin({
+        pluginId: 'testA',
+        register(reg) {
+          reg.registerExtensionPoint(extA, 'a');
+          reg.registerInit({
+            deps: {},
+            async init() {},
+          });
+        },
+      })(),
+    );
+    init.add(
+      createBackendModule({
+        pluginId: 'testB',
+        moduleId: 'mod',
+        register(reg) {
+          reg.registerInit({
+            deps: { ext: extA },
+            async init() {},
+          });
+        },
+      })(),
+    );
+    await expect(init.start()).rejects.toThrow(
+      "Extension point registered for plugin 'testA' may not be used by module for plugin 'testB'",
+    );
+  });
 });

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

@@ -42,7 +42,10 @@ export interface BackendRegisterInit {
 export class BackendInitializer {
   #startPromise?: Promise<void>;
   #features = new Array<InternalBackendFeature>();
-  #extensionPoints = new Map<ExtensionPoint<unknown>, unknown>();
+  #extensionPoints = new Map<
+    ExtensionPoint<unknown>,
+    { impl: unknown; pluginId: string }
+  >();
   #serviceHolder: EnumerableServiceHolder;
 
   constructor(serviceHolder: EnumerableServiceHolder) {
@@ -57,11 +60,14 @@ export class BackendInitializer {
     const missingRefs = new Set<ServiceOrExtensionPoint>();
 
     for (const [name, ref] of Object.entries(deps)) {
-      const extensionPoint = this.#extensionPoints.get(
-        ref as ExtensionPoint<unknown>,
-      );
-      if (extensionPoint) {
-        result.set(name, extensionPoint);
+      const ep = this.#extensionPoints.get(ref as ExtensionPoint<unknown>);
+      if (ep) {
+        if (ep.pluginId !== pluginId) {
+          throw new Error(
+            `Extension point registered for plugin '${ep.pluginId}' may not be used by module for plugin '${pluginId}'`,
+          );
+        }
+        result.set(name, ep.impl);
       } else {
         const impl = await this.#serviceHolder.get(
           ref as ServiceRef<unknown>,
@@ -169,7 +175,10 @@ export class BackendInitializer {
                 `ExtensionPoint with ID '${extRef.id}' is already registered`,
               );
             }
-            this.#extensionPoints.set(extRef, extImpl);
+            this.#extensionPoints.set(extRef, {
+              impl: extImpl,
+              pluginId: r.pluginId,
+            });
             provides.add(extRef);
           }
         }