ignore processors emitting entities that are their own children

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/smart-zoos-wash.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-catalog-backend': patch
+---
+
+Ignore attempts at emitting the current entity as a child of itself.

plugins/catalog-backend/src/processing/DefaultCatalogProcessingOrchestrator.test.ts

@@ -258,13 +258,22 @@ describe('DefaultCatalogProcessingOrchestrator', () => {
       },
     };
 
+    const child: Entity = {
+      apiVersion: '1',
+      kind: 'Component',
+      metadata: {
+        name: 'Test2',
+        namespace: 'test1',
+      },
+    };
+
     it('enforces catalog rules', async () => {
       const integrations = ScmIntegrations.fromConfig(new ConfigReader({}));
       const processor: jest.Mocked<CatalogProcessor> = {
         getProcessorName: jest.fn(),
         validateEntityKind: jest.fn(async () => true),
         readLocation: jest.fn(async (_l, _o, emit) => {
-          emit(processingResult.entity({ type: 't', target: 't' }, entity));
+          emit(processingResult.entity({ type: 't', target: 't' }, child));
           return true;
         }),
       };
@@ -300,7 +309,7 @@ describe('DefaultCatalogProcessingOrchestrator', () => {
         getProcessorName: jest.fn(),
         validateEntityKind: jest.fn(async () => true),
         readLocation: jest.fn(async (_l, _o, emit) => {
-          emit(processingResult.entity({ type: 't', target: 't' }, entity));
+          emit(processingResult.entity({ type: 't', target: 't' }, child));
           return true;
         }),
       };

plugins/catalog-backend/src/processing/ProcessorOutputCollector.ts

@@ -19,6 +19,7 @@ import {
   ANNOTATION_LOCATION,
   ANNOTATION_ORIGIN_LOCATION,
   stringifyLocationRef,
+  stringifyEntityRef,
 } from '@backstage/catalog-model';
 import { assertError } from '@backstage/errors';
 import { Logger } from 'winston';
@@ -89,6 +90,17 @@ export class ProcessorOutputCollector {
         return;
       }
 
+      // The processor contract says you should return the "trunk" (current)
+      // entity, not emit it. But it happens that this is misunderstood or
+      // accidentally forgotten. This can lead to circular references which at
+      // best is wasteful, so we try to be helpful by ignoring such emitted
+      // entities.
+      if (
+        stringifyEntityRef(entity) === stringifyEntityRef(this.parentEntity)
+      ) {
+        return;
+      }
+
       // Note that at this point, we have only validated the envelope part of
       // the entity data. Annotations are not part of that, so we have to be
       // defensive. If the annotations were malformed (e.g. were not a valid