fix(event-handler): allow http handlers to return duplex streams

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

@@ -1,4 +1,4 @@
-import { Readable, Writable } from 'node:stream';
+import { Duplex, Readable, Writable } from 'node:stream';
 import {
   isRecord,
   isRegExp,
@@ -112,7 +112,7 @@ export const isNodeReadableStream = (value: unknown): value is Readable => {
   return (
     value != null &&
     typeof value === 'object' &&
-    value instanceof Readable &&
+    (value instanceof Readable || value instanceof Duplex) &&
     'readable' in value &&
     'read' in value &&
     typeof value.read === 'function'

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

@@ -1,4 +1,4 @@
-import { Readable } from 'node:stream';
+import { Duplex, PassThrough, Readable } from 'node:stream';
 import context from '@aws-lambda-powertools/testing-utils/context';
 import { describe, expect, it, vi } from 'vitest';
 import { UnauthorizedError } from '../../../../src/rest/errors.js';
@@ -306,4 +306,29 @@ describe('Class: Router - Streaming', () => {
       app.resolveStream(invalidEvent, context, { responseStream })
     ).rejects.toThrow();
   });
+
+  it('handles duplex stream body', async () => {
+    // Prepare
+    const app = new Router();
+    const passThrough = new PassThrough();
+    passThrough.write(Buffer.from('{"message":"duplex stream body"}'));
+    passThrough.end();
+
+    app.get('/test', () => ({
+      statusCode: 200,
+      body: Duplex.from(passThrough),
+    }));
+
+    const responseStream = new MockResponseStream();
+
+    // Act
+    await app.resolveStream(createTestEvent('/test', 'GET'), context, {
+      responseStream,
+    });
+
+    // Assess
+    const { prelude, body } = parseStreamOutput(responseStream.chunks);
+    expect(prelude.statusCode).toBe(200);
+    expect(JSON.parse(body)).toEqual({ message: 'duplex stream body' });
+  });
 });