Address review feedback for feature flag isolation

Deduplicate the plugin/module feature flag registration loops and
distinguish the error source (Plugin vs Module). Treat
FEATURE_FLAG_INVALID as a warning in frontend-defaults.

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

.changeset/feature-flag-invalid-warning.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-defaults': patch
+---
+
+Invalid feature flag declarations are now treated as warnings rather than errors, letting the app load normally.

packages/frontend-app-api/src/wiring/apiFactories.test.ts

@@ -177,6 +177,8 @@ describe('registerFeatureFlagDeclarationsInHolder', () => {
       code: 'FEATURE_FLAG_INVALID',
       context: { pluginId: 'my-plugin', flagName: 'mod/invalid' },
     });
+    expect(errors[0].message).toContain("Module for plugin 'my-plugin'");
+    expect(errors[0].message).toContain("'mod/invalid'");
   });
 
   it('should isolate non-validation errors thrown by registerFlag', () => {

packages/frontend-app-api/src/wiring/apiFactories.ts

@@ -183,28 +183,22 @@ function registerFeatureFlagDeclarations(
   collector: ErrorCollector,
 ) {
   for (const feature of features) {
+    let pluginId: string | undefined;
+    let flags: Array<{ name: string; description?: string }> | undefined;
+    let source: string | undefined;
+
     if (OpaqueFrontendPlugin.isType(feature)) {
-      const pluginId = feature.id;
-      for (const flag of OpaqueFrontendPlugin.toInternal(feature)
-        .featureFlags) {
-        try {
-          featureFlagApi.registerFlag({
-            name: flag.name,
-            description: flag.description,
-            pluginId,
-          });
-        } catch (error) {
-          collector.report({
-            code: 'FEATURE_FLAG_INVALID',
-            message: `Plugin '${pluginId}' declared invalid feature flag '${flag.name}': ${error}`,
-            context: { pluginId, flagName: flag.name, error: error as Error },
-          });
-        }
-      }
+      pluginId = feature.id;
+      flags = OpaqueFrontendPlugin.toInternal(feature).featureFlags;
+      source = 'Plugin';
+    } else if (isInternalFrontendModule(feature)) {
+      pluginId = feature.pluginId;
+      flags = toInternalFrontendModule(feature).featureFlags;
+      source = 'Module for plugin';
     }
-    if (isInternalFrontendModule(feature)) {
-      const pluginId = feature.pluginId;
-      for (const flag of toInternalFrontendModule(feature).featureFlags) {
+
+    if (pluginId && flags && source) {
+      for (const flag of flags) {
         try {
           featureFlagApi.registerFlag({
             name: flag.name,
@@ -214,7 +208,7 @@ function registerFeatureFlagDeclarations(
         } catch (error) {
           collector.report({
             code: 'FEATURE_FLAG_INVALID',
-            message: `Plugin '${pluginId}' declared invalid feature flag '${flag.name}': ${error}`,
+            message: `${source} '${pluginId}' declared invalid feature flag '${flag.name}': ${error}`,
             context: { pluginId, flagName: flag.name, error: error as Error },
           });
         }

packages/frontend-defaults/src/maybeCreateErrorPage.tsx

@@ -26,6 +26,7 @@ const DEFAULT_WARNING_CODES: Array<keyof AppErrorTypes> = [
   'EXTENSION_OUTPUT_IGNORED',
   'EXTENSION_BOOTSTRAP_PREDICATE_IGNORED',
   'EXTENSION_BOOTSTRAP_API_UNAVAILABLE',
+  'FEATURE_FLAG_INVALID',
 ];
 
 function AppErrorItem(props: { error: AppError }): JSX.Element {