fix: support custom servers using HTTP/2 in production

.changeset/shy-needles-warn.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: support HTTP/2 in dev and production. Revert the changes from [#12907](https://github.com/sveltejs/kit/pull/12907) to downgrade HTTP/2 to TLS as now being unnecessary

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

@@ -106,11 +106,25 @@ function get_raw_body(req, body_size_limit) {
 // TODO 3.0 make the signature synchronous?
 // eslint-disable-next-line @typescript-eslint/require-await
 export async function getRequest({ request, base, bodySizeLimit }) {
+	let headers = /** @type {Record<string, string>} */ (request.headers);
+	if (request.httpVersionMajor >= 2) {
+		// the Request constructor rejects headers with ':' in the name
+		headers = Object.assign({}, headers);
+		// https://www.rfc-editor.org/rfc/rfc9113.html#section-8.3.1-2.3.5
+		if (headers[':authority']) {
+			headers.host = headers[':authority'];
+		}
+		delete headers[':authority'];
+		delete headers[':method'];
+		delete headers[':path'];
+		delete headers[':scheme'];
+	}
+
 	return new Request(base + request.url, {
 		// @ts-expect-error
 		duplex: 'half',
 		method: request.method,
-		headers: /** @type {Record<string, string>} */ (request.headers),
+		headers: Object.entries(headers),
 		body:
 			request.method === 'GET' || request.method === 'HEAD'
 				? undefined

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

@@ -349,22 +349,6 @@ async function kit({ svelte_config }) {
 		 * Stores the final config.
 		 */
 		configResolved(config) {
-			// we search for this plugin by name because we can't detect it
-			// since it doesn't directly modify the https config unlike the mkcert plugin
-			const vite_basic_ssl = config.plugins.find(({ name }) => name === 'vite:basic-ssl');
-
-			// by default, when enabling HTTPS in Vite, it also enables HTTP/2
-			// however, undici has not yet enabled HTTP/2 by default: https://github.com/nodejs/undici/issues/2750
-			// we set a no-op proxy config to force Vite to downgrade to TLS-only
-			// see https://vitejs.dev/config/#server-https
-			if ((config.server.https || vite_basic_ssl) && !config.server.proxy) {
-				config.server.proxy = {};
-			}
-
-			if ((config.preview.https || vite_basic_ssl) && !config.preview.proxy) {
-				config.preview.proxy = {};
-			}
-
 			vite_config = config;
 		}
 	};

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

@@ -185,7 +185,7 @@ export async function preview(vite, vite_config, svelte_config) {
 
 		// SSR
 		vite.middlewares.use(async (req, res) => {
-			const host = req.headers['host'];
+			const host = req.headers[':authority'] || req.headers.host;
 
 			const request = await getRequest({
 				base: `${protocol}://${host}`,