fix: ensure rendering starts off synchronously (#14517)

We have to invoke .then eagerly here in order to kick off rendering: it's only starting on access, and `await maybe_promise` would eagerly access the .then property but call its function only after a tick, which is too late for the paths.reset() below and for any eager getRequestEvent() calls during rendering without AsyncLocalStorage available.

Fixes https://github.com/sveltejs/svelte/issues/16788

Author: dummdidumm

.changeset/flat-ducks-attack.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: ensure rendering starts off synchronously

packages/kit/src/exports/internal/event.js

@@ -45,7 +45,17 @@ export function getRequestEvent() {
 export function get_request_store() {
 	const result = try_get_request_store();
 	if (!result) {
-		throw new Error('Could not get the request store. This is an internal error.');
+		let message = 'Could not get the request store.';
+
+		if (als) {
+			message += ' This is an internal error.';
+		} else {
+			message +=
+				' In environments without `AsyncLocalStorage`, the request store (used by e.g. remote functions) must be accessed synchronously, not after an `await`.' +
+				' If it was accessed synchronously then this is an internal error.';
+		}
+
+		throw new Error(message);
 	}
 	return result;
 }

packages/kit/src/runtime/server/page/render.js

@@ -205,7 +205,16 @@ export async function render_response({
 				// portable as possible, but reset afterwards
 				if (paths.relative) paths.override({ base, assets });
 
-				const rendered = options.root.render(props, render_opts);
+				const maybe_promise = options.root.render(props, render_opts);
+				// We have to invoke .then eagerly here in order to kick off rendering: it's only starting on access,
+				// and `await maybe_promise` would eagerly access the .then property but call its function only after a tick, which is too late
+				// for the paths.reset() below and for any eager getRequestEvent() calls during rendering without AsyncLocalStorage available.
+				const rendered =
+					options.async && 'then' in maybe_promise
+						? /** @type {ReturnType<typeof options.root.render> & Promise<any>} */ (
+								maybe_promise
+							).then((r) => r)
+						: maybe_promise;
 
 				// TODO 3.0 remove options.async
 				if (options.async) {