add trailing slash redirect middleware to adapter-node (#8857)

* add trailing slash redirect middleware

* add changeset

* skip middleware where possible

* dont add ? to URLs

* minor tweak

---------

Co-authored-by: Rich Harris <hello@rich-harris.dev>

.changeset/warm-bananas-sneeze.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-node': patch
+---
+
+fix: correctly redirect trailing slashes for `adapter-node`

packages/adapter-node/ambient.d.ts

@@ -8,7 +8,9 @@ declare module 'HANDLER' {
 
 declare module 'MANIFEST' {
 	import { SSRManifest } from '@sveltejs/kit';
+
 	export const manifest: SSRManifest;
+	export const prerendered: Set<string>;
 }
 
 declare module 'SERVER' {

packages/adapter-node/index.js

@@ -1,5 +1,5 @@
-import { readFileSync, writeFileSync } from 'fs';
-import { fileURLToPath } from 'url';
+import { readFileSync, writeFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
 import { rollup } from 'rollup';
 import { nodeResolve } from '@rollup/plugin-node-resolve';
 import commonjs from '@rollup/plugin-commonjs';
@@ -39,7 +39,8 @@ export default function (opts = {}) {
 
 			writeFileSync(
 				`${tmp}/manifest.js`,
-				`export const manifest = ${builder.generateManifest({ relativePath: './' })};`
+				`export const manifest = ${builder.generateManifest({ relativePath: './' })};\n\n` +
+					`export const prerendered = new Set(${JSON.stringify(builder.prerendered.paths)});\n`
 			);
 
 			const pkg = JSON.parse(readFileSync('package.json', 'utf8'));

packages/adapter-node/src/handler.js

@@ -1,11 +1,11 @@
 import './shims';
-import fs from 'fs';
-import path from 'path';
+import fs from 'node:fs';
+import path from 'node:path';
 import sirv from 'sirv';
-import { fileURLToPath } from 'url';
+import { fileURLToPath } from 'node:url';
 import { getRequest, setResponse } from '@sveltejs/kit/node';
 import { Server } from 'SERVER';
-import { manifest } from 'MANIFEST';
+import { manifest, prerendered } from 'MANIFEST';
 import { env } from 'ENV';
 
 /* global ENV_PREFIX */
@@ -44,8 +44,39 @@ function serve(path, client = false) {
 	);
 }
 
+// required because the static file server ignores trailing slashes
+/** @returns {import('polka').Middleware} */
+function serve_prerendered() {
+	const handler = serve(path.join(dir, 'prerendered'));
+
+	return (req, res, next) => {
+		let pathname = req.path;
+
+		try {
+			pathname = decodeURIComponent(pathname);
+		} catch {
+			// ignore invalid URI
+		}
+
+		if (prerendered.has(pathname)) {
+			return handler(req, res, next);
+		}
+
+		// remove or add trailing slash as appropriate
+		let location = pathname.at(-1) === '/' ? pathname.slice(0, -1) : pathname + '/';
+		if (prerendered.has(location)) {
+			const search = req.url.split('?')[1];
+			if (search) location += `?${search}`;
+			res.writeHead(308, { location }).end();
+		} else {
+			next();
+		}
+	};
+}
+
 /** @type {import('polka').Middleware} */
 const ssr = async (req, res) => {
+	/** @type {Request | undefined} */
 	let request;
 
 	try {
@@ -139,7 +170,7 @@ export const handler = sequence(
 	[
 		serve(path.join(dir, 'client'), true),
 		serve(path.join(dir, 'static')),
-		serve(path.join(dir, 'prerendered')),
+		serve_prerendered(),
 		ssr
 	].filter(Boolean)
 );