Make sure that the server builder propagates init errors properly

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

.changeset/rich-pillows-cough.md

@@ -0,0 +1,12 @@
+---
+'@backstage/backend-common': patch
+---
+
+Make sure that server builder `start()` propagates errors (such as failing to bind to the required port) properly and doesn't resolve the promise prematurely.
+
+After this change, the backend logger will be able to actually capture the error as it happens:
+
+```
+2021-11-11T10:54:21.334Z backstage info Initializing http server
+2021-11-11T10:54:21.335Z backstage error listen EADDRINUSE: address already in use :::7000 code=EADDRINUSE errno=-48 syscall=listen address=:: port=7000
+```

packages/backend-common/src/service/lib/ServiceBuilderImpl.ts

@@ -176,25 +176,29 @@ export class ServiceBuilderImpl implements ServiceBuilder {
       : createHttpServer(app, logger);
 
     return new Promise((resolve, reject) => {
-      app.on('error', e => {
-        logger.error(`Failed to start up on port ${port}, ${e}`);
+      function handleStartupError(e: unknown) {
+        server.close();
         reject(e);
+      }
+
+      app.on('error', handleStartupError);
+      server.on('error', handleStartupError);
+
+      server.listen(port, host, () => {
+        app.off('error', handleStartupError);
+        server.off('error', handleStartupError);
+
+        const stoppableServer = stoppable(server, 0);
+
+        useHotCleanup(this.module, () =>
+          stoppableServer.stop((e: any) => {
+            if (e) console.error(e);
+          }),
+        );
+
+        logger.info(`Listening on ${host}:${port}`);
+        resolve(stoppableServer);
       });
-
-      const stoppableServer = stoppable(
-        server.listen(port, host, () => {
-          logger.info(`Listening on ${host}:${port}`);
-        }),
-        0,
-      );
-
-      useHotCleanup(this.module, () =>
-        stoppableServer.stop((e: any) => {
-          if (e) console.error(e);
-        }),
-      );
-
-      resolve(stoppableServer);
     });
   }
 

packages/backend/src/index.ts

@@ -140,7 +140,7 @@ async function main() {
 
   await service.start().catch(err => {
     logger.error(err);
-    process.exit(1);
+    throw err;
   });
 }