fix: redirect trailing slash normalization relatively instead of against root

.changeset/honest-pandas-itch.md

@@ -0,0 +1,6 @@
+---
+'@sveltejs/adapter-node': patch
+'@sveltejs/kit': patch
+---
+
+fix: redirect trailing slash normalization relatively instead of against root

packages/adapter-node/src/handler.js

@@ -5,7 +5,12 @@ import process from 'node:process';
 import sirv from 'sirv';
 import { fileURLToPath } from 'node:url';
 import { parse as polka_url_parser } from '@polka/url';
-import { getRequest, setResponse, createReadableStream } from '@sveltejs/kit/node';
+import {
+	getRequest,
+	setResponse,
+	createReadableStream,
+	relative_pathname
+} from '@sveltejs/kit/node';
 import { Server } from 'SERVER';
 import { manifest, prerendered, base } from 'MANIFEST';
 import { env } from 'ENV';
@@ -81,8 +86,11 @@ function serve_prerendered() {
 		}
 
 		// remove or add trailing slash as appropriate
-		let location = pathname.at(-1) === '/' ? pathname.slice(0, -1) : pathname + '/';
-		if (prerendered.has(location)) {
+		const inverted_trailing_slash =
+			pathname.at(-1) === '/' ? pathname.slice(0, -1) : pathname + '/';
+		if (prerendered.has(inverted_trailing_slash)) {
+			// ensure preservation of (possibly invisible) path prefixes
+			let location = relative_pathname(pathname, inverted_trailing_slash);
 			if (query) location += search;
 			res.writeHead(308, { location }).end();
 		} else {

packages/kit/package.json

@@ -22,6 +22,7 @@
 		"cookie": "^0.6.0",
 		"devalue": "^5.1.0",
 		"esm-env": "^1.2.2",
+		"get-relative-path": "^1.0.2",
 		"import-meta-resolve": "^4.1.0",
 		"kleur": "^4.1.5",
 		"magic-string": "^0.30.5",

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

@@ -222,3 +222,5 @@ export async function setResponse(res, response) {
 export function createReadableStream(file) {
 	return /** @type {ReadableStream} */ (Readable.toWeb(createReadStream(file)));
 }
+
+export { relative_pathname } from '../../utils/url.js';

packages/kit/src/runtime/server/respond.js

@@ -11,7 +11,13 @@ import {
 	method_not_allowed,
 	redirect_response
 } from './utils.js';
-import { decode_pathname, decode_params, disable_search, normalize_path } from '../../utils/url.js';
+import {
+	decode_pathname,
+	decode_params,
+	disable_search,
+	normalize_path,
+	relative_pathname
+} from '../../utils/url.js';
 import { exec } from '../../utils/routing.js';
 import { redirect_json_response, render_data } from './data/index.js';
 import { add_cookies_to_headers, get_cookies } from './cookie.js';
@@ -321,9 +327,8 @@ export async function respond(request, options, manifest, state) {
 						headers: {
 							'x-sveltekit-normalize': '1',
 							location:
-								// ensure paths starting with '//' are not treated as protocol-relative
-								(normalized.startsWith('//') ? url.origin + normalized : normalized) +
-								(url.search === '?' ? '' : url.search)
+								// ensure preservation of (possibly invisible) path prefixes
+								relative_pathname(url.pathname, normalized) + (url.search === '?' ? '' : url.search)
 						}
 					});
 				}

packages/kit/src/utils/url.js

@@ -1,4 +1,5 @@
 import { BROWSER, DEV } from 'esm-env';
+import getRelativePath from 'get-relative-path';
 
 /**
  * Matches a URI scheme. See https://www.rfc-editor.org/rfc/rfc3986#section-3.1
@@ -77,6 +78,18 @@ export function decode_uri(uri) {
 	}
 }
 
+/**
+ * Calculates the relative path between two URL pathnames.
+ * Note that `relative` from `node:path` works with file system paths which are subtly diffent.
+ * For example: it ignores trailing slashes, which are significant in URLs.
+ * @param {string} from
+ * @param {string} to
+ */
+export function relative_pathname(from, to) {
+	// TODO inline
+	return getRelativePath(from, to);
+}
+
 /**
  * Returns everything up to the first `#` in a URL
  * @param {{href: string}} url_like

packages/kit/types/index.d.ts

@@ -2145,6 +2145,12 @@ declare module '@sveltejs/kit/node' {
 	 * @since 2.4.0
 	 */
 	export function createReadableStream(file: string): ReadableStream;
+	/**
+	 * Calculates the relative path between two URL pathnames.
+	 * Note that `relative` from `node:path` works with file system paths which are subtly diffent.
+	 * For example: it ignores trailing slashes, which are significant in URLs.
+	 * */
+	export function relative_pathname(from: string, to: string): string;
 
 	export {};
 }