backend-app-api: add test for circular module dependency check

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

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

@@ -250,4 +250,48 @@ describe('BackendInitializer', () => {
       "Module 'mod' for plugin 'test' is already registered",
     );
   });
+
+  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(),
+        })(),
+      ]),
+    );
+    init.add(
+      createBackendModule({
+        pluginId: 'test',
+        moduleId: 'modA',
+        register(reg) {
+          reg.registerExtensionPoint(extA, 'a');
+          reg.registerInit({
+            deps: { ext: extB },
+            async init() {},
+          });
+        },
+      })(),
+    );
+    init.add(
+      createBackendModule({
+        pluginId: 'test',
+        moduleId: 'modB',
+        register(reg) {
+          reg.registerExtensionPoint(extB, 'b');
+          reg.registerInit({
+            deps: { ext: extA },
+            async init() {},
+          });
+        },
+      })(),
+    );
+    await expect(init.start()).rejects.toThrow(
+      "Circular dependency detected for modules of plugin 'test', 'modA' -> 'modB' -> 'modA'",
+    );
+  });
 });

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

@@ -26,7 +26,7 @@ import { EnumerableServiceHolder, ServiceOrExtensionPoint } from './types';
 // Direct internal import to avoid duplication
 // eslint-disable-next-line @backstage/no-forbidden-package-imports
 import { InternalBackendFeature } from '@backstage/backend-plugin-api/src/wiring/types';
-import { ForwardedError, InputError } from '@backstage/errors';
+import { ForwardedError, ConflictError } from '@backstage/errors';
 import { featureDiscoveryServiceRef } from '@backstage/backend-plugin-api/alpha';
 import { DependencyTree } from '../lib/DependencyTree';
 
@@ -225,7 +225,7 @@ export class BackendInitializer {
           );
           const circular = tree.detectCircularDependency();
           if (circular) {
-            throw new InputError(
+            throw new ConflictError(
               `Circular dependency detected for modules of plugin '${pluginId}', '${circular.join(
                 "' -> '",
               )}'`,