Fix prerendered paths for adapter-node

Sharing internal utilities through `@sveltejs/kit/node` feels... odd

Author: HoldYourWaffle

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/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/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 {};
 }