feat: add support for read in edge environments (#13859)

closes #12446
closes #11674

---------

Co-authored-by: Chew Tee Ming <[email protected]>
Co-authored-by: Simon Holthausen <[email protected]>

Author: 3 people

.changeset/moody-birds-help.md

@@ -0,0 +1,6 @@
+---
+'@sveltejs/adapter-netlify': minor
+'@sveltejs/adapter-vercel': minor
+---
+
+feat: add support for `read` imported from `$app/server` in edge functions

.changeset/old-moons-lick.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-cloudflare': minor
+---
+
+feat: add support for `read` imported from `$app/server`

.changeset/weak-pillows-doubt.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: support asynchronous `read` implementations from adapters

eslint.config.js

@@ -33,6 +33,7 @@ export default [
 		},
 		ignores: [
 			'packages/adapter-cloudflare/test/apps/**/*',
+			'packages/adapter-netlify/test/apps/**/*',
 			'packages/adapter-node/rollup.config.js',
 			'packages/adapter-node/tests/smoke.spec_disabled.js',
 			'packages/adapter-static/test/apps/**/*',

package.json

@@ -37,10 +37,13 @@
 	},
 	"pnpm": {
 		"onlyBuiltDependencies": [
+			"@parcel/watcher",
 			"esbuild",
+			"netlify-cli",
 			"rolldown",
 			"sharp",
 			"svelte-preprocess",
+			"unix-dgram",
 			"workerd"
 		]
 	}

packages/adapter-cloudflare/index.js

@@ -1,13 +1,17 @@
+import { VERSION } from '@sveltejs/kit';
 import { copyFileSync, existsSync, writeFileSync } from 'node:fs';
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { is_building_for_cloudflare_pages } from './utils.js';
 import { getPlatformProxy, unstable_readConfig } from 'wrangler';
 
+const name = '@sveltejs/adapter-cloudflare';
+const [kit_major, kit_minor] = VERSION.split('.');
+
 /** @type {import('./index.js').default} */
 export default function (options = {}) {
 	return {
-		name: '@sveltejs/adapter-cloudflare',
+		name,
 		async adapt(builder) {
 			if (existsSync('_routes.json')) {
 				throw new Error(
@@ -162,6 +166,18 @@ export default function (options = {}) {
 					return prerender ? emulated.prerender_platform : emulated.platform;
 				}
 			};
+		},
+		supports: {
+			read: ({ route }) => {
+				// TODO bump peer dep in next adapter major to simplify this
+				if (kit_major === '2' && kit_minor < '25') {
+					throw new Error(
+						`${name}: Cannot use \`read\` from \`$app/server\` in route \`${route.id}\` when using SvelteKit < 2.25.0`
+					);
+				}
+
+				return true;
+			}
 		}
 	};
 }

packages/adapter-cloudflare/package.json

@@ -33,14 +33,14 @@
 		"ambient.d.ts"
 	],
 	"scripts": {
-		"build": "esbuild src/worker.js --bundle --outfile=files/worker.js --external:SERVER --external:MANIFEST --format=esm",
+		"build": "esbuild src/worker.js --bundle --outfile=files/worker.js --external:SERVER --external:MANIFEST --external:cloudflare:workers --format=esm",
 		"lint": "prettier --check .",
 		"format": "pnpm lint --write",
 		"check": "tsc --skipLibCheck",
-		"prepublishOnly": "pnpm build",
 		"test:unit": "vitest run",
 		"test:e2e": "pnpm build && pnpm -r --workspace-concurrency 1 --filter=\"./test/**\" test",
-		"test": "pnpm test:unit && pnpm test:e2e"
+		"test": "pnpm test:unit && pnpm test:e2e",
+		"prepublishOnly": "pnpm build"
 	},
 	"dependencies": {
 		"@cloudflare/workers-types": "^4.20250507.0",

packages/adapter-cloudflare/src/worker.js

@@ -1,5 +1,6 @@
 import { Server } from 'SERVER';
 import { manifest, prerendered, base_path } from 'MANIFEST';
+import { env } from 'cloudflare:workers';
 import * as Cache from 'worktop/cfw.cache';
 
 const server = new Server(manifest);
@@ -9,6 +10,27 @@ const app_path = `/${manifest.appPath}`;
 const immutable = `${app_path}/immutable/`;
 const version_file = `${app_path}/version.json`;
 
+/**
+ * We don't know the origin until we receive a request, but
+ * that's guaranteed to happen before we call `read`
+ * @type {string}
+ */
+let origin;
+
+const initialized = server.init({
+	// @ts-expect-error env contains environment variables and bindings
+	env,
+	read: async (file) => {
+		const response = await /** @type {{ ASSETS: { fetch: typeof fetch } }} */ (env).ASSETS.fetch(
+			`${origin}/${file}`
+		);
+		if (!response.ok) {
+			throw new Error(`Failed to fetch ${file}: ${response.status} ${response.statusText}`);
+		}
+		return response.body;
+	}
+});
+
 export default {
 	/**
 	 * @param {Request} req
@@ -17,10 +39,10 @@ export default {
 	 * @returns {Promise<Response>}
 	 */
 	async fetch(req, env, ctx) {
-		await server.init({
-			// @ts-expect-error env contains environment variables and bindings
-			env
-		});
+		if (!origin) {
+			origin = new URL(req.url).origin;
+			await initialized;
+		}
 
 		// skip cache if "cache-control: no-cache" in request
 		let pragma = req.headers.get('cache-control') || '';

packages/adapter-cloudflare/test/apps/workers/src/routes/read/+server.js

@@ -0,0 +1,6 @@
+import { read } from '$app/server';
+import file from './file.txt?url';
+
+export function GET() {
+	return read(file);
+}

packages/adapter-cloudflare/test/apps/workers/src/routes/read/file.txt

@@ -0,0 +1 @@
+Hello! This file is read by `read` from `$app/server`.