feat(event-handler): throw error when middleware does not await next() (#4511)

Co-authored-by: Andrea Amorosi <[email protected]>

Author: svozza

packages/event-handler/src/rest/utils.ts

@@ -172,9 +172,22 @@ export const composeMiddleware = (middleware: Middleware[]): Middleware => {
       }
 
       const middlewareFn = middleware[i];
-      const middlewareResult = await middlewareFn(params, reqCtx, () =>
-        dispatch(i + 1)
-      );
+      let nextPromise: Promise<void> | null = null;
+      let nextAwaited = false;
+      const nextFn = async () => {
+        nextPromise = dispatch(i + 1);
+        const result = await nextPromise;
+        nextAwaited = true;
+        return result;
+      };
+
+      const middlewareResult = await middlewareFn(params, reqCtx, nextFn);
+
+      if (nextPromise && !nextAwaited && i < middleware.length - 1) {
+        throw new Error(
+          'Middleware called next() without awaiting. This may lead to unexpected behavior.'
+        );
+      }
 
       if (middlewareResult !== undefined) {
         result = middlewareResult;

packages/event-handler/tests/unit/rest/Router/middleware.test.ts

@@ -180,6 +180,32 @@ describe('Class: Router - Middleware', () => {
       expect(body.message).toContain('next() called multiple times');
     });
 
+    it('should throw error if middleware does not await next()', async () => {
+      // Prepare
+      vi.stubEnv('POWERTOOLS_DEV', 'true');
+      const app = new Router();
+
+      app.use(async (_params, _reqCtx, next) => {
+        await next();
+      });
+
+      app.use(async (_params, _reqCtx, next) => {
+        next();
+      });
+
+      // Act
+      const result = await app.resolve(
+        createTestEvent('/test', 'OPTIONS'),
+        context
+      );
+
+      // Assess
+      const body = JSON.parse(result.body);
+      expect(body.message).toEqual(
+        'Middleware called next() without awaiting. This may lead to unexpected behavior.'
+      );
+    });
+
     it('handles errors thrown in middleware before next()', async () => {
       // Prepare
       const app = new Router();