fix: ensure redirect used in packages uses the same Redirect instance (#13843)

* add failing test

* format

* thank you Dominik G!

* better comment

* changeset

* avoid specifying cjs test dep as a workspace

Author: eltigerchino

.changeset/old-eels-shop.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: avoid externalising packages that depend on `@sveltejs/kit` so that libraries can also use `redirect` and `error` helpers

packages/kit/package.json

@@ -29,7 +29,8 @@
 		"mrmime": "^2.0.0",
 		"sade": "^1.8.1",
 		"set-cookie-parser": "^2.6.0",
-		"sirv": "^3.0.0"
+		"sirv": "^3.0.0",
+		"vitefu": "^1.0.6"
 	},
 	"devDependencies": {
 		"@playwright/test": "catalog:",

packages/kit/src/exports/vite/index.js

@@ -36,6 +36,7 @@ import {
 } from './module_ids.js';
 import { import_peer } from '../../utils/import.js';
 import { compact } from '../../utils/array.js';
+import { crawlFrameworkPkgs } from 'vitefu';
 
 const cwd = process.cwd();
 
@@ -227,7 +228,7 @@ async function kit({ svelte_config }) {
 		 * Build the SvelteKit-provided Vite config to be merged with the user's vite.config.js file.
 		 * @see https://vitejs.dev/guide/api-plugin.html#config
 		 */
-		config(config, config_env) {
+		async config(config, config_env) {
 			initial_config = config;
 			vite_config_env = config_env;
 			is_build = config_env.command === 'build';
@@ -250,6 +251,20 @@ async function kit({ svelte_config }) {
 
 			const generated = path.posix.join(kit.outDir, 'generated');
 
+			const packages_depending_on_svelte_kit = (
+				await crawlFrameworkPkgs({
+					root: cwd,
+					isBuild: is_build,
+					viteUserConfig: config,
+					isSemiFrameworkPkgByJson: (pkg_json) => {
+						return (
+							!!pkg_json.dependencies?.['@sveltejs/kit'] ||
+							!!pkg_json.peerDependencies?.['@sveltejs/kit']
+						);
+					}
+				})
+			).ssr.noExternal;
+
 			// dev and preview config can be shared
 			/** @type {import('vite').UserConfig} */
 			const new_config = {
@@ -306,7 +321,11 @@ async function kit({ svelte_config }) {
 						//    when it is detected to keep our virtual modules working.
 						//    See https://github.com/sveltejs/kit/pull/9172
 						//    and https://vitest.dev/config/#deps-registernodeloader
-						'@sveltejs/kit'
+						'@sveltejs/kit',
+						// We need to bundle any packages depending on @sveltejs/kit so that
+						// everyone uses the same instances of classes such as `Redirect`
+						// which we use in `instanceof` checks
+						...packages_depending_on_svelte_kit
 					]
 				}
 			};

packages/kit/test/apps/basics/_test_dependencies/redirect-pkg/index.d.ts

@@ -0,0 +1 @@
+export function authenticate(redirectUrl: string): never;

packages/kit/test/apps/basics/_test_dependencies/redirect-pkg/index.js

@@ -0,0 +1,9 @@
+import { redirect } from '@sveltejs/kit';
+
+/**
+ * @param {string} redirectUrl
+ * @returns {never}
+ */
+export function authenticate(redirectUrl) {
+	redirect(302, redirectUrl);
+}

packages/kit/test/apps/basics/_test_dependencies/redirect-pkg/package.json

@@ -0,0 +1,15 @@
+{
+	"name": "redirect-pkg",
+	"private": true,
+	"version": "1.0.0",
+	"main": "index.js",
+	"types": "index.d.ts",
+	"files": [
+		"index.js",
+		"index.d.ts"
+	],
+	"type": "module",
+	"peerDependencies": {
+		"@sveltejs/kit": "workspace:*"
+	}
+}

packages/kit/test/apps/basics/package.json

@@ -22,6 +22,7 @@
 		"cross-env": "^7.0.3",
 		"svelte": "^5.23.1",
 		"svelte-check": "^4.1.1",
+		"redirect-pkg": "file:./_test_dependencies/redirect-pkg",
 		"typescript": "^5.5.4",
 		"vite": "^6.2.7"
 	},

packages/kit/test/apps/basics/src/routes/redirect/package/+page.server.js

@@ -0,0 +1,5 @@
+import { authenticate } from 'redirect-pkg';
+
+export function load() {
+	authenticate('/redirect/c');
+}

packages/kit/test/apps/basics/test/cross-platform/test.js

@@ -616,6 +616,11 @@ test.describe('Redirects', () => {
 		span = page.locator('#cookie-value');
 		expect(await span.innerText()).toContain('undefined');
 	});
+
+	test('works when used from another package', async ({ page }) => {
+		await page.goto('/redirect/package');
+		expect(await page.textContent('h1')).toBe('c');
+	});
 });
 
 test.describe('Routing', () => {