feat: async SSR (#14447)

* chore: use catalog version for svelte dev dependency

* WIP async SSR

* bump

* partial fix

* bump

* catalog version

* WIP

* tidy up

* WIP

* construct relative paths from event store

* wait really typescript?

* revert

* fix

* changeset

* fix

* use pkg.imports

Author: Rich-Harris

.changeset/full-bats-itch.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: experimental async SSR

packages/kit/package.json

@@ -87,6 +87,12 @@
 		"generate:version": "node scripts/generate-version.js",
 		"generate:types": "node scripts/generate-dts.js"
 	},
+	"imports": {
+		"#app/paths": {
+			"browser": "./src/runtime/app/paths/client.js",
+			"default": "./src/runtime/app/paths/server.js"
+		}
+	},
 	"exports": {
 		"./package.json": "./package.json",
 		".": {

packages/kit/scripts/generate-dts.js

@@ -12,7 +12,7 @@ await createBundle({
 		'$app/environment': 'src/runtime/app/environment/types.d.ts',
 		'$app/forms': 'src/runtime/app/forms.js',
 		'$app/navigation': 'src/runtime/app/navigation.js',
-		'$app/paths': 'src/runtime/app/paths/types.d.ts',
+		'$app/paths': 'src/runtime/app/paths/public.d.ts',
 		'$app/server': 'src/runtime/app/server/index.js',
 		'$app/state': 'src/runtime/app/state/index.js',
 		'$app/stores': 'src/runtime/app/stores.js'

packages/kit/src/core/sync/write_server.js

@@ -31,12 +31,13 @@ const server_template = ({
 }) => `
 import root from '../root.${isSvelte5Plus() ? 'js' : 'svelte'}';
 import { set_building, set_prerendering } from '__sveltekit/environment';
-import { set_assets } from '__sveltekit/paths';
+import { set_assets } from '$app/paths/internal/server';
 import { set_manifest, set_read_implementation } from '__sveltekit/server';
 import { set_private_env, set_public_env } from '${runtime_directory}/shared-server.js';
 
 export const options = {
 	app_template_contains_nonce: ${template.includes('%sveltekit.nonce%')},
+	async: ${s(!!config.compilerOptions?.experimental?.async)},
 	csp: ${s(config.kit.csp)},
 	csrf_check_origin: ${s(config.kit.csrf.checkOrigin && !config.kit.csrf.trustedOrigins.includes('*'))},
 	csrf_trusted_origins: ${s(config.kit.csrf.trustedOrigins)},

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

@@ -522,7 +522,7 @@ export async function dev(vite, vite_config, svelte_config, get_remotes) {
 				);
 				set_fix_stack_trace(fix_stack_trace);
 
-				const { set_assets } = await vite.ssrLoadModule('__sveltekit/paths');
+				const { set_assets } = await vite.ssrLoadModule('$app/paths/internal/server');
 				set_assets(assets);
 
 				const server = new Server(manifest);

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

@@ -36,7 +36,6 @@ import {
 	env_static_public,
 	service_worker,
 	sveltekit_environment,
-	sveltekit_paths,
 	sveltekit_server
 } from './module_ids.js';
 import { import_peer } from '../../utils/import.js';
@@ -318,26 +317,34 @@ async function kit({ svelte_config }) {
 						// because they for example use esbuild.build with `platform: 'browser'`
 						'esm-env',
 						// This forces `$app/*` modules to be bundled, since they depend on
-						// virtual modules like `__sveltekit/paths` (this isn't a valid bare
+						// virtual modules like `__sveltekit/environment` (this isn't a valid bare
 						// import, but it works with vite-node's externalization logic, which
 						// uses basic concatenation)
 						'@sveltejs/kit/src/runtime'
 					]
 				}
 			};
 
+			const define = {
+				__SVELTEKIT_APP_DIR__: s(kit.appDir),
+				__SVELTEKIT_EMBEDDED__: s(kit.embedded),
+				__SVELTEKIT_EXPERIMENTAL__REMOTE_FUNCTIONS__: s(kit.experimental.remoteFunctions),
+				__SVELTEKIT_PATHS_ASSETS__: s(kit.paths.assets),
+				__SVELTEKIT_PATHS_BASE__: s(kit.paths.base),
+				__SVELTEKIT_PATHS_RELATIVE__: s(kit.paths.relative),
+				__SVELTEKIT_CLIENT_ROUTING__: s(kit.router.resolution === 'client'),
+				__SVELTEKIT_SERVER_TRACING_ENABLED__: s(kit.experimental.tracing.server)
+			};
+
 			if (is_build) {
 				if (!new_config.build) new_config.build = {};
 				new_config.build.ssr = !secondary_build_started;
 
 				new_config.define = {
+					...define,
 					__SVELTEKIT_ADAPTER_NAME__: s(kit.adapter?.name),
 					__SVELTEKIT_APP_VERSION_FILE__: s(`${kit.appDir}/version.json`),
 					__SVELTEKIT_APP_VERSION_POLL_INTERVAL__: s(kit.version.pollInterval),
-					__SVELTEKIT_EMBEDDED__: s(kit.embedded),
-					__SVELTEKIT_EXPERIMENTAL__REMOTE_FUNCTIONS__: s(kit.experimental.remoteFunctions),
-					__SVELTEKIT_CLIENT_ROUTING__: s(kit.router.resolution === 'client'),
-					__SVELTEKIT_SERVER_TRACING_ENABLED__: s(kit.experimental.tracing.server),
 					__SVELTEKIT_PAYLOAD__: new_config.build.ssr
 						? '{}'
 						: `globalThis.__sveltekit_${version_hash}`
@@ -348,11 +355,8 @@ async function kit({ svelte_config }) {
 				}
 			} else {
 				new_config.define = {
+					...define,
 					__SVELTEKIT_APP_VERSION_POLL_INTERVAL__: '0',
-					__SVELTEKIT_EMBEDDED__: s(kit.embedded),
-					__SVELTEKIT_EXPERIMENTAL__REMOTE_FUNCTIONS__: s(kit.experimental.remoteFunctions),
-					__SVELTEKIT_CLIENT_ROUTING__: s(kit.router.resolution === 'client'),
-					__SVELTEKIT_SERVER_TRACING_ENABLED__: s(kit.experimental.tracing.server),
 					__SVELTEKIT_PAYLOAD__: 'globalThis.__sveltekit_dev'
 				};
 
@@ -460,48 +464,6 @@ async function kit({ svelte_config }) {
 				case service_worker:
 					return create_service_worker_module(svelte_config);
 
-				// for internal use only. it's published as $app/paths externally
-				// we use this alias so that we won't collide with user aliases
-				case sveltekit_paths: {
-					const { assets, base } = svelte_config.kit.paths;
-
-					// use the values defined in `global`, but fall back to hard-coded values
-					// for the sake of things like Vitest which may import this module
-					// outside the context of a page
-					if (browser) {
-						return dedent`
-							export const base = ${global}?.base ?? ${s(base)};
-							export const assets = ${global}?.assets ?? ${assets ? s(assets) : 'base'};
-							export const app_dir = ${s(kit.appDir)};
-						`;
-					}
-
-					return dedent`
-						export let base = ${s(base)};
-						export let assets = ${assets ? s(assets) : 'base'};
-						export const app_dir = ${s(kit.appDir)};
-
-						export const relative = ${svelte_config.kit.paths.relative};
-
-						const initial = { base, assets };
-
-						export function override(paths) {
-							base = paths.base;
-							assets = paths.assets;
-						}
-
-						export function reset() {
-							base = initial.base;
-							assets = initial.assets;
-						}
-
-						/** @param {string} path */
-						export function set_assets(path) {
-							assets = initial.assets = path;
-						}
-					`;
-				}
-
 				case sveltekit_environment: {
 					const { version } = svelte_config.kit;
 

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

@@ -9,7 +9,6 @@ export const env_dynamic_public = '\0virtual:env/dynamic/public';
 export const service_worker = '\0virtual:service-worker';
 
 export const sveltekit_environment = '\0virtual:__sveltekit/environment';
-export const sveltekit_paths = '\0virtual:__sveltekit/paths';
 export const sveltekit_server = '\0virtual:__sveltekit/server';
 
 export const app_server = posixify(

packages/kit/src/runtime/app/paths/client.js

@@ -0,0 +1,57 @@
+/** @import { Asset, RouteId, Pathname, ResolvedPathname } from '$app/types' */
+/** @import { ResolveArgs } from './types.js' */
+import { base, assets } from './internal/client.js';
+import { resolve_route } from '../../../utils/routing.js';
+
+/**
+ * Resolve the URL of an asset in your `static` directory, by prefixing it with [`config.kit.paths.assets`](https://svelte.dev/docs/kit/configuration#paths) if configured, or otherwise by prefixing it with the base path.
+ *
+ * During server rendering, the base path is relative and depends on the page currently being rendered.
+ *
+ * @example
+ * ```svelte
+ * <script>
+ * 	import { asset } from '$app/paths';
+ * </script>
+ *
+ * <img alt="a potato" src={asset('/potato.jpg')} />
+ * ```
+ * @since 2.26
+ *
+ * @param {Asset} file
+ * @returns {string}
+ */
+export function asset(file) {
+	return (assets || base) + file;
+}
+
+/**
+ * Resolve a pathname by prefixing it with the base path, if any, or resolve a route ID by populating dynamic segments with parameters.
+ *
+ * During server rendering, the base path is relative and depends on the page currently being rendered.
+ *
+ * @example
+ * ```js
+ * import { resolve } from '$app/paths';
+ *
+ * // using a pathname
+ * const resolved = resolve(`/blog/hello-world`);
+ *
+ * // using a route ID plus parameters
+ * const resolved = resolve('/blog/[slug]', {
+ * 	slug: 'hello-world'
+ * });
+ * ```
+ * @since 2.26
+ *
+ * @template {RouteId | Pathname} T
+ * @param {ResolveArgs<T>} args
+ * @returns {ResolvedPathname}
+ */
+export function resolve(...args) {
+	// The type error is correct here, and if someone doesn't pass params when they should there's a runtime error,
+	// but we don't want to adjust the internal resolve_route function to accept `undefined`, hence the type cast.
+	return base + resolve_route(args[0], /** @type {Record<string, string>} */ (args[1]));
+}
+
+export { base, assets, resolve as resolveRoute };

packages/kit/src/runtime/app/paths/index.js

@@ -1,16 +1 @@
-import { base, assets } from '__sveltekit/paths';
-import { resolve_route } from '../../../utils/routing.js';
-
-/** @type {import('./types.d.ts').asset} */
-export function asset(file) {
-	return (assets || base) + file;
-}
-
-/** @type {import('./types.d.ts').resolve} */
-export function resolve(id, params) {
-	// The type error is correct here, and if someone doesn't pass params when they should there's a runtime error,
-	// but we don't want to adjust the internal resolve_route function to accept `undefined`, hence the type cast.
-	return base + resolve_route(id, /** @type {Record<string, string>} */ (params));
-}
-
-export { base, assets, resolve as resolveRoute };
+export * from '#app/paths';

packages/kit/src/runtime/app/paths/internal/client.js

@@ -0,0 +1,3 @@
+export const base = __SVELTEKIT_PAYLOAD__?.base ?? __SVELTEKIT_PATHS_BASE__;
+export const assets = __SVELTEKIT_PAYLOAD__?.assets ?? base ?? __SVELTEKIT_PATHS_ASSETS__;
+export const app_dir = __SVELTEKIT_APP_DIR__;