feat: read assets inside an edge function

packages/adapter-vercel/files/edge.js

@@ -2,16 +2,53 @@ import { Server } from 'SERVER';
 import { manifest } from 'MANIFEST';
 
 const server = new Server(manifest);
+
+/**
+ * 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({
-	env: /** @type {Record<string, string>} */ (process.env)
+	env: /** @type {Record<string, string>} */ (process.env),
+	read: (file) => {
+		const controller = new AbortController();
+		const signal = controller.signal;
+
+		return new ReadableStream({
+			async start(controller) {
+				try {
+					const response = await fetch(`${origin}/${file}`, { signal });
+					const reader = /** @type {ReadableStream} */ (response.body).getReader();
+
+					while (true) {
+						const { done, value } = await reader.read();
+						if (done) break;
+						controller.enqueue(value);
+					}
+
+					controller.close();
+				} catch (error) {
+					controller.error(error);
+				}
+			},
+			cancel(reason) {
+				controller.abort(reason);
+			}
+		});
+	}
 });
 
 /**
  * @param {Request} request
  * @param {import('../index.js').RequestContext} context
  */
 export default async (request, context) => {
-	await initialized;
+	if (!origin) {
+		origin = new URL(request.url).origin;
+		await initialized;
+	}
 
 	return server.respond(request, {
 		getClientAddress() {

packages/adapter-vercel/index.js

@@ -385,18 +385,7 @@ const plugin = function (defaults = {}) {
 		},
 
 		supports: {
-			// reading from the filesystem only works in serverless functions
-			read: ({ config, route }) => {
-				const runtime = config.runtime ?? defaults.runtime;
-
-				if (runtime === 'edge') {
-					throw new Error(
-						`${name}: Cannot use \`read\` from \`$app/server\` in route \`${route.id}\` configured with \`runtime: 'edge'\``
-					);
-				}
-
-				return true;
-			}
+			read: () => true
 		}
 	};
 };

sites/kit.svelte.dev/src/routes/delete-me/+server.js

@@ -0,0 +1,14 @@
+import { read } from '$app/server';
+import file from './file.txt?url';
+
+export const config = {
+	runtime: 'edge'
+};
+
+export const prerender = false;
+
+export async function GET() {
+	const text = await read(file).text();
+
+	return new Response(`${text} at ${new Date()}`);
+}

sites/kit.svelte.dev/src/routes/delete-me/file.txt

@@ -0,0 +1 @@
+Hello! This file was read inside an edge function