backend-app-api: earlier handling or unhandled errors

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

.changeset/cuddly-mugs-act.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-app-api': patch
+---
+
+Moved up registration of unhandled rejections and errors listeners to be done as early as possible, avoiding flakiness in backend startups and instead always logging these failures rather than sometimes crashing the process.

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

@@ -258,6 +258,30 @@ export class BackendInitializer {
       createInstanceMetadataServiceFactory(this.#registrations),
     );
 
+    // This makes sure that any uncaught errors or unhandled rejections are
+    // caught and logged, rather than terminating the process. We register these
+    // as early as possible while still using the root logger service, the
+    // tradeoff that if there are any unhandled errors as part of the that
+    // instationation, it will cause the process to crash. If there are multiple
+    // backend instances, each instance will log the error, because we can't
+    // determine which instance the error came from.
+    if (process.env.NODE_ENV !== 'test') {
+      const rootLogger = await this.#serviceRegistry.get(
+        coreServices.rootLogger,
+        'root',
+      );
+      process.on('unhandledRejection', (reason: Error) => {
+        rootLogger
+          ?.child({ type: 'unhandledRejection' })
+          ?.error('Unhandled rejection', reason);
+      });
+      process.on('uncaughtException', error => {
+        rootLogger
+          ?.child({ type: 'uncaughtException' })
+          ?.error('Uncaught exception', error);
+      });
+    }
+
     // Initialize all root scoped services
     await this.#serviceRegistry.initializeEagerServicesWithScope('root');
 
@@ -449,25 +473,6 @@ export class BackendInitializer {
     await lifecycleService.startup();
 
     initLogger.onAllStarted();
-
-    // Once the backend is started, any uncaught errors or unhandled rejections are caught
-    // and logged, in order to avoid crashing the entire backend on local failures.
-    if (process.env.NODE_ENV !== 'test') {
-      const rootLogger = await this.#serviceRegistry.get(
-        coreServices.rootLogger,
-        'root',
-      );
-      process.on('unhandledRejection', (reason: Error) => {
-        rootLogger
-          ?.child({ type: 'unhandledRejection' })
-          ?.error('Unhandled rejection', reason);
-      });
-      process.on('uncaughtException', error => {
-        rootLogger
-          ?.child({ type: 'uncaughtException' })
-          ?.error('Uncaught exception', error);
-      });
-    }
   }
 
   // It's fine to call .stop() multiple times, which for example can happen with manual stop + process exit