feat: add ability to set custom errorHandler

Signed-off-by: Radoslaw Wielonski <radoslaw.wielonski@nordcloud.com>

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

@@ -14,7 +14,8 @@
  * limitations under the License.
  */
 
-import { applyCspDirectives } from './ServiceBuilderImpl';
+import { NextFunction, Request, Response } from 'express';
+import { applyCspDirectives, ServiceBuilderImpl } from './ServiceBuilderImpl';
 
 describe('ServiceBuilderImpl', () => {
   describe('applyCspDirectives', () => {
@@ -33,4 +34,22 @@ describe('ServiceBuilderImpl', () => {
       expect(result!['upgrade-insecure-requests']).toBeUndefined();
     });
   });
+
+  describe('setCustomErrorHandler', () => {
+    it('adds custom error handler', () => {
+      const serviceBuilder = new ServiceBuilderImpl(module);
+      const customErrorHandler = (
+        error: Error,
+        req: Request,
+        res: Response,
+        next: NextFunction,
+      ) => {};
+      serviceBuilder.setErrorHandler(customErrorHandler);
+      expect(serviceBuilder.errorHandler).toEqual(customErrorHandler);
+    });
+    it('use default error handler', () => {
+      const serviceBuilder = new ServiceBuilderImpl(module);
+      expect(serviceBuilder.errorHandler).toBeUndefined();
+    });
+  });
 });

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

@@ -17,7 +17,7 @@
 import { Config } from '@backstage/config';
 import compression from 'compression';
 import cors from 'cors';
-import express, { Router } from 'express';
+import express, { Router, ErrorRequestHandler } from 'express';
 import helmet from 'helmet';
 import * as http from 'http';
 import stoppable from 'stoppable';
@@ -25,7 +25,7 @@ import { Logger } from 'winston';
 import { useHotCleanup } from '../../hot';
 import { getRootLogger } from '../../logging';
 import {
-  errorHandler,
+  errorHandler as defaultErrorHandler,
   notFoundHandler,
   requestLoggingHandler as defaultRequestLoggingHandler,
 } from '../../middleware';
@@ -66,6 +66,7 @@ export class ServiceBuilderImpl implements ServiceBuilder {
   private httpsSettings: HttpsSettings | undefined;
   private routers: [string, Router][];
   private requestLoggingHandler: RequestLoggingHandlerFactory | undefined;
+  private errorHandler: ErrorRequestHandler | undefined;
   // Reference to the module where builder is created - needed for hot module
   // reloading
   private module: NodeModule;
@@ -152,6 +153,11 @@ export class ServiceBuilderImpl implements ServiceBuilder {
     return this;
   }
 
+  setErrorHandler(errorHandler: ErrorRequestHandler) {
+    this.errorHandler = errorHandler;
+    return this;
+  }
+
   async start(): Promise<http.Server> {
     const app = express();
     const { port, host, logger, corsOptions, httpsSettings, helmetOptions } =
@@ -169,7 +175,7 @@ export class ServiceBuilderImpl implements ServiceBuilder {
       app.use(root, route);
     }
     app.use(notFoundHandler());
-    app.use(errorHandler());
+    app.use(this.errorHandler ?? defaultErrorHandler());
 
     const server: http.Server = httpsSettings
       ? await createHttpsServer(app, httpsSettings, logger)

packages/backend-common/src/service/types.ts

@@ -16,7 +16,7 @@
 
 import { Config } from '@backstage/config';
 import cors from 'cors';
-import { Router, RequestHandler } from 'express';
+import { Router, RequestHandler, ErrorRequestHandler } from 'express';
 import { Server } from 'http';
 import { Logger } from 'winston';
 
@@ -98,6 +98,15 @@ export type ServiceBuilder = {
     requestLoggingHandler: RequestLoggingHandlerFactory,
   ): ServiceBuilder;
 
+  /**
+   * Set the error handler
+   *
+   * If no handler is given the default one is used
+   *
+   * @param errorHandler - an error handler
+   */
+  setErrorHandler(errorHandler: ErrorRequestHandler): ServiceBuilder;
+
   /**
    * Starts the server using the given settings.
    */