fix: race condition when importing AsyncLocalStorage (#17350)

* fix: race condition when importing `AsyncLocalStorage`

* types:

* tweak

* remove a line

* comment

* dumb esbuild

* final final pt2 final

* ugh

Author: elliott-with-the-longest-name-on-github

.changeset/early-showers-marry.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: race condition when importing `AsyncLocalStorage`

packages/svelte/src/internal/server/render-context.js

@@ -2,7 +2,7 @@
 /** @import { AsyncLocalStorage } from 'node:async_hooks' */
 /** @import { RenderContext } from '#server' */
 
-import { deferred } from '../shared/utils.js';
+import { deferred, noop } from '../shared/utils.js';
 import * as e from './errors.js';
 
 /** @type {Promise<void> | null} */
@@ -56,14 +56,26 @@ export async function with_render_context(fn) {
 
 /** @type {AsyncLocalStorage<RenderContext | null> | null} */
 let als = null;
+/** @type {Promise<void> | null} */
+let als_import = null;
 
-export async function init_render_context() {
-	if (als !== null) return;
-	try {
-		// @ts-ignore -- we don't include node types in the production build
-		const { AsyncLocalStorage } = await import('node:async_hooks');
-		als = new AsyncLocalStorage();
-	} catch {}
+/**
+ *
+ * @returns {Promise<void>}
+ */
+export function init_render_context() {
+	// It's important the right side of this assignment can run a maximum of one time
+	// otherwise it's possible for a very, very well-timed race condition to assign to `als`
+	// at the beginning of a render, and then another render to assign to it again, which causes
+	// the first render's second half to use a new instance of `als` which doesn't have its
+	// context anymore.
+	// @ts-ignore -- we don't include node types in the production build
+	als_import ??= import('node:async_hooks')
+		.then((hooks) => {
+			als = new hooks.AsyncLocalStorage();
+		})
+		.then(noop, noop);
+	return als_import;
 }
 
 // this has to be a function because rollup won't treeshake it if it's a constant

packages/svelte/tests/runtime-browser/test.ts

@@ -164,6 +164,7 @@ async function run_test(
 				}
 			],
 			bundle: true,
+			platform: 'node',
 			format: 'iife',
 			globalName: 'test_ssr'
 		});