fix: improve streaming error propagation and forwarded response logging (#5317)

* Fix: Improve streaming error handling and forwarded response logging

* Fix: fix ESLint error  Strings must use singlequote  quotes

* fix: preserve and log forwarded stream errors

* chore: narrow forwarded stream error fix scope

* fix: make forwardFetchResponse awaitable and forward upstream error text

* Restore original happy path handling

* Remove redundant checks in forwardFetchResponse function

* Don't send anything on parsing error end

---------

Co-authored-by: Cohee <18619528+Cohee1207@users.noreply.github.com>

tests/util.test.js

@@ -1,6 +1,31 @@
-import { describe, test, expect } from '@jest/globals';
+import { afterEach, describe, test, expect, jest } from '@jest/globals';
+import { once } from 'node:events';
+import { PassThrough } from 'node:stream';
+import { Response } from 'node-fetch';
 import { CHAT_COMPLETION_SOURCES } from '../src/constants';
-import { flattenSchema } from '../src/util';
+import { flattenSchema, forwardFetchResponse } from '../src/util';
+
+function createMockExpressResponse() {
+    const response = new PassThrough();
+    response.statusCode = 200;
+    response.statusMessage = '';
+
+    return response;
+}
+
+async function collectResponseBody(response) {
+    const chunks = [];
+
+    response.on('data', chunk => chunks.push(Buffer.from(chunk)));
+
+    await once(response, 'finish');
+
+    return Buffer.concat(chunks).toString('utf8');
+}
+
+afterEach(() => {
+    jest.restoreAllMocks();
+});
 
 describe('flattenSchema', () => {
     test('should return the schema if it is not an object', () => {
@@ -105,3 +130,37 @@ describe('flattenSchema', () => {
         expect(flattenSchema(schema, 'some-other-api')).toEqual(expected);
     });
 });
+
+describe('forwardFetchResponse', () => {
+    test('should log JSON error bodies and return the original body for non-2xx streaming responses', async () => {
+        const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => undefined);
+        const body = JSON.stringify({ error: { message: 'Forbidden by upstream policy' }, detail: 'policy_denied' });
+        const response = createMockExpressResponse();
+        const bodyPromise = collectResponseBody(response);
+
+        await forwardFetchResponse(new Response(body, {
+            status: 403,
+            statusText: 'Forbidden',
+        }), response);
+
+        expect(await bodyPromise).toBe(body);
+        expect(response.statusCode).toBe(403);
+        expect(warnSpy).toHaveBeenCalledWith(`Streaming request failed with status 403 Forbidden: ${body}`);
+    });
+
+    test('should log plain text error bodies and return the original body for non-2xx streaming responses', async () => {
+        const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => undefined);
+        const body = 'Plain text upstream failure';
+        const response = createMockExpressResponse();
+        const bodyPromise = collectResponseBody(response);
+
+        await forwardFetchResponse(new Response(body, {
+            status: 502,
+            statusText: 'Bad Gateway',
+        }), response);
+
+        expect(await bodyPromise).toBe(body);
+        expect(response.statusCode).toBe(502);
+        expect(warnSpy).toHaveBeenCalledWith(`Streaming request failed with status 502 Bad Gateway: ${body}`);
+    });
+});