feat: Cloudflare Pages _routes.json specification (#6441)

When a SvelteKit project is deployed to Cloudflare Pages, the server-side code is supported through Pages Functions (currently in beta). Unfortunately, by using Functions, each request must go through the server-side Functions code even if it doesn't need to. For instance, an immutable asset request to https://kit.svelte.dev/_app/immutable/assets/_layout-ab34ca4f.css would first have to route through Functions.

This is problematic since the request would "count" as a request to Functions even though only a static asset was served. Further, there is a slight amount of added latency.

This change exposes a set of includes and excludes based on static files generated.

.changeset/afraid-gifts-act.md

@@ -0,0 +1,6 @@
+---
+'@sveltejs/adapter-cloudflare': patch
+'@sveltejs/kit': patch
+---
+
+Support Cloudflare Pages \_routes.json specification

packages/adapter-cloudflare/index.d.ts

@@ -2,3 +2,10 @@ import { Adapter } from '@sveltejs/kit';
 import './ambient.js';
 
 export default function plugin(): Adapter;
+
+export interface RoutesJSONSpec {
+	version: 1;
+	description: string;
+	include: string[];
+	exclude: string[];
+}

packages/adapter-cloudflare/index.js

@@ -1,4 +1,4 @@
-import { writeFileSync } from 'fs';
+import { writeFile } from 'fs/promises';
 import { posix } from 'path';
 import { fileURLToPath } from 'url';
 import * as esbuild from 'esbuild';
@@ -23,18 +23,31 @@ export default function () {
 			builder.rimraf(tmp);
 			builder.mkdirp(tmp);
 
-			builder.writeClient(dest);
+			const written_files = builder.writeClient(dest);
 			builder.writePrerendered(dest);
 
 			const relativePath = posix.relative(tmp, builder.getServerDirectory());
 
-			writeFileSync(
-				`${tmp}/manifest.js`,
-				`export const manifest = ${builder.generateManifest({
-					relativePath
-				})};\n\nexport const prerendered = new Set(${JSON.stringify(builder.prerendered.paths)});\n`
+			builder.log.info(
+				`adapter-cloudfare is writing its own _headers file. If you have your own, you should duplicate the headers contained in: ${dest}/_headers`
 			);
 
+			await Promise.all([
+				writeFile(
+					`${tmp}/manifest.js`,
+					`export const manifest = ${builder.generateManifest({
+						relativePath
+					})};\n\nexport const prerendered = new Set(${JSON.stringify(
+						builder.prerendered.paths
+					)});\n`
+				),
+				writeFile(
+					`${dest}/_routes.json`,
+					JSON.stringify(getRoutesJSON(builder.config.kit.appDir, written_files))
+				),
+				writeFile(`${dest}/_headers`, getAutogeneratedHeaders(builder.config.kit.appDir))
+			]);
+
 			builder.copy(`${files}/worker.js`, `${tmp}/_worker.js`, {
 				replace: {
 					SERVER: `${relativePath}/index.js`,
@@ -55,3 +68,43 @@ export default function () {
 		}
 	};
 }
+
+/**
+ * @param {string} app_dir
+ * @param {string[]} assets
+ * @returns {import('.').RoutesJSONSpec}
+ */
+function getRoutesJSON(app_dir, assets) {
+	return {
+		version: 1,
+		description: 'Generated by @sveltejs/adapter-cloudflare',
+		include: ['/*'],
+		exclude: [
+			`/${app_dir}/immutable/*`,
+			...assets
+				// We're being conservative by not excluding all assets in
+				// /static just yet. If there are any upstream auth rules to
+				// protect certain things (e.g. a PDF that requires auth),
+				// then we wouldn't want to prevent those requests from going
+				// to the user functions worker.
+				// We do want to show an example of a _routes.json that
+				// excludes more than just /_app/immutable/*, and favicons
+				// are a reasonable choice
+				.filter((filePath) => filePath.includes('favicon'))
+		]
+	};
+}
+
+/**
+ * @param {string} app_dir
+ * @returns {string}
+ */
+function getAutogeneratedHeaders(app_dir) {
+	return `
+# === START AUTOGENERATED SVELTE IMMUTABLE HEADERS ===
+/${app_dir}/immutable/*
+	Cache-Control: public, immutable, max-age=31536000
+	X-Robots-Tag: noindex
+# === END AUTOGENERATED SVELTE IMMUTABLE HEADERS ===	
+	`.trim();
+}