Add FTR tests for stream/buffer error responses (#188937)

## Summary

Fix https://github.com/elastic/kibana/issues/56305

packages/core/http/core-http-router-server-internal/src/response_adapter.ts

@@ -115,7 +115,7 @@ export class HapiResponseAdapter {
     return response;
   }
 
-  private toError(kibanaResponse: KibanaResponse<ResponseError | Buffer | stream.Readable>) {
+  private toError(kibanaResponse: KibanaResponse<ResponseError>) {
     const { payload } = kibanaResponse;
 
     // Special case for when we are proxying requests and want to enable streaming back error responses opaquely.
@@ -153,7 +153,12 @@ function getErrorMessage(payload?: ResponseError): string {
   if (!payload) {
     throw new Error('expected error message to be provided');
   }
-  if (typeof payload === 'string') return payload;
+  if (typeof payload === 'string') {
+    return payload;
+  }
+  if (isStreamOrBuffer(payload)) {
+    throw new Error(`can't resolve error message from stream or buffer`);
+  }
   // for ES response errors include nested error reason message. it doesn't contain sensitive data.
   if (isElasticsearchResponseError(payload)) {
     return `[${payload.message}]: ${
@@ -164,6 +169,10 @@ function getErrorMessage(payload?: ResponseError): string {
   return getErrorMessage(payload.message);
 }
 
+function isStreamOrBuffer(payload: ResponseError): payload is stream.Stream | Buffer {
+  return Buffer.isBuffer(payload) || stream.isReadable(payload as stream.Readable);
+}
+
 function getErrorAttributes(payload?: ResponseError): ResponseErrorAttributes | undefined {
   return typeof payload === 'object' && 'attributes' in payload ? payload.attributes : undefined;
 }

packages/core/http/core-http-server/src/router/response.ts

@@ -39,6 +39,8 @@ export type ResponseErrorAttributes = Record<string, any>;
  */
 export type ResponseError =
   | string
+  | Buffer
+  | Stream
   | Error
   | {
       message: string | Error;

packages/core/http/core-http-server/src/router/response_factory.ts

@@ -6,7 +6,6 @@
  * Side Public License, v 1.
  */
 
-import type { Stream } from 'stream';
 import type {
   CustomHttpResponseOptions,
   HttpResponseOptions,
@@ -139,7 +138,7 @@ export interface KibanaErrorResponseFactory {
    * Creates an error response with defined status code and payload.
    * @param options - {@link CustomHttpResponseOptions} configures HTTP response headers, error message and other error details to pass to the client
    */
-  customError(options: CustomHttpResponseOptions<ResponseError | Buffer | Stream>): IKibanaResponse;
+  customError(options: CustomHttpResponseOptions<ResponseError>): IKibanaResponse;
 }
 
 /**

test/plugin_functional/plugins/core_http/server/plugin.ts

@@ -6,6 +6,7 @@
  * Side Public License, v 1.
  */
 
+import { Readable } from 'stream';
 import type { Plugin, CoreSetup } from '@kbn/core/server';
 
 export class CoreHttpPlugin implements Plugin {
@@ -87,6 +88,32 @@ export class CoreHttpPlugin implements Plugin {
           },
         });
       });
+
+    router.get(
+      {
+        path: '/api/core_http/error_stream',
+        validate: false,
+      },
+      async (ctx, req, res) => {
+        return res.customError({
+          body: Readable.from(['error stream'], { objectMode: false }),
+          statusCode: 501,
+        });
+      }
+    );
+
+    router.get(
+      {
+        path: '/api/core_http/error_buffer',
+        validate: false,
+      },
+      async (ctx, req, res) => {
+        return res.customError({
+          body: Buffer.from('error buffer', 'utf8'),
+          statusCode: 501,
+        });
+      }
+    );
   }
 
   public start() {}

test/plugin_functional/test_suites/core_plugins/error_response.ts

@@ -0,0 +1,38 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import expect from '@kbn/expect';
+import '@kbn/core-provider-plugin/types';
+import { PluginFunctionalProviderContext } from '../../services';
+
+export default function ({ getService }: PluginFunctionalProviderContext) {
+  const supertest = getService('supertest');
+
+  // routes defined in the `core_http` test plugin
+  describe('Custom errors', () => {
+    it('can serve an error response from stream', async () => {
+      await supertest
+        .get('/api/core_http/error_stream')
+        .expect(501)
+        .then((response) => {
+          const res = response.body.toString();
+          expect(res).to.eql('error stream');
+        });
+    });
+
+    it('can serve an error response from buffer', async () => {
+      await supertest
+        .get('/api/core_http/error_buffer')
+        .expect(501)
+        .then((response) => {
+          const res = response.body.toString();
+          expect(res).to.eql('error buffer');
+        });
+    });
+  });
+}

test/plugin_functional/test_suites/core_plugins/index.ts

@@ -26,5 +26,6 @@ export default function ({ loadTestFile }: PluginFunctionalProviderContext) {
     loadTestFile(require.resolve('./http'));
     loadTestFile(require.resolve('./http_versioned'));
     loadTestFile(require.resolve('./dynamic_contract_resolving'));
+    loadTestFile(require.resolve('./error_response'));
   });
 }