[breaking] add builder.generateFallback(fallback) API (#8013)

closes #7899

.changeset/shaggy-bikes-wink.md

@@ -0,0 +1,6 @@
+---
+'@sveltejs/adapter-static': patch
+'@sveltejs/kit': patch
+---
+
+[breaking] replace automatic fallback generation with `builder.generateFallback(fallback)`

packages/adapter-static/index.js

@@ -79,7 +79,11 @@ See https://kit.svelte.dev/docs/page-options#prerender for more details`
 			builder.rimraf(pages);
 
 			builder.writeClient(assets);
-			builder.writePrerendered(pages, { fallback });
+			builder.writePrerendered(pages);
+
+			if (fallback) {
+				builder.generateFallback(path.join(pages, fallback));
+			}
 
 			if (precompress) {
 				builder.log.minor('Compressing assets and pages');

packages/kit/src/core/adapt/builder.js

@@ -1,11 +1,14 @@
+import { existsSync, statSync, createReadStream, createWriteStream } from 'node:fs';
+import { pipeline } from 'node:stream';
+import { promisify } from 'node:util';
+import { fork } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
 import glob from 'tiny-glob';
 import zlib from 'zlib';
-import { existsSync, statSync, createReadStream, createWriteStream } from 'fs';
-import { pipeline } from 'stream';
-import { promisify } from 'util';
 import { copy, rimraf, mkdirp } from '../../utils/filesystem.js';
 import { generate_manifest } from '../generate_manifest/index.js';
 import { get_route_segments } from '../../utils/routing.js';
+import { get_env } from '../../exports/vite/utils.js';
 
 const pipe = promisify(pipeline);
 
@@ -104,6 +107,33 @@ export function create_builder({ config, build_data, routes, prerendered, log })
 			}
 		},
 
+		generateFallback(dest) {
+			// do prerendering in a subprocess so any dangling stuff gets killed upon completion
+			const script = fileURLToPath(new URL('../prerender/fallback.js', import.meta.url));
+
+			const manifest_path = `${config.kit.outDir}/output/server/manifest-full.js`;
+
+			const env = get_env(config.kit.env, 'production');
+
+			return new Promise((fulfil, reject) => {
+				const child = fork(
+					script,
+					[dest, manifest_path, JSON.stringify({ ...env.private, ...env.public })],
+					{
+						stdio: 'inherit'
+					}
+				);
+
+				child.on('exit', (code) => {
+					if (code) {
+						reject(new Error(`Could not create a fallback page — failed with code ${code}`));
+					} else {
+						fulfil(undefined);
+					}
+				});
+			});
+		},
+
 		generateManifest: ({ relativePath }) => {
 			return generate_manifest({
 				build_data,
@@ -132,16 +162,17 @@ export function create_builder({ config, build_data, routes, prerendered, log })
 			return [...copy(`${config.kit.outDir}/output/client`, dest)];
 		},
 
-		writePrerendered(dest, { fallback } = {}) {
-			const source = `${config.kit.outDir}/output/prerendered`;
-			const files = [...copy(`${source}/pages`, dest), ...copy(`${source}/dependencies`, dest)];
-
-			if (fallback) {
-				files.push(fallback);
-				copy(`${source}/fallback.html`, `${dest}/${fallback}`);
+		// @ts-expect-error
+		writePrerendered(dest, opts) {
+			// TODO remove for 1.0
+			if (opts?.fallback) {
+				throw new Error(
+					'The fallback option no longer exists — use builder.generateFallback(fallback) instead'
+				);
 			}
 
-			return files;
+			const source = `${config.kit.outDir}/output/prerendered`;
+			return [...copy(`${source}/pages`, dest), ...copy(`${source}/dependencies`, dest)];
 		},
 
 		writeServer(dest) {

packages/kit/src/core/prerender/fallback.js

@@ -0,0 +1,43 @@
+import { readFileSync, writeFileSync } from 'fs';
+import { dirname, join } from 'path';
+import { pathToFileURL } from 'url';
+import { mkdirp } from '../../utils/filesystem.js';
+import { installPolyfills } from '../../exports/node/polyfills.js';
+import { load_config } from '../config/index.js';
+
+const [, , dest, manifest_path, env] = process.argv;
+
+/** @type {import('types').ValidatedKitConfig} */
+const config = (await load_config()).kit;
+
+installPolyfills();
+
+const server_root = join(config.outDir, 'output');
+
+/** @type {import('types').ServerModule} */
+const { Server, override } = await import(pathToFileURL(`${server_root}/server/index.js`).href);
+
+/** @type {import('types').SSRManifest} */
+const manifest = (await import(pathToFileURL(manifest_path).href)).manifest;
+
+override({
+	building: true,
+	paths: config.paths,
+	read: (file) => readFileSync(join(config.files.assets, file))
+});
+
+const server = new Server(manifest);
+await server.init({ env: JSON.parse(env) });
+
+const rendered = await server.respond(new Request(config.prerender.origin + '/[fallback]'), {
+	getClientAddress: () => {
+		throw new Error('Cannot read clientAddress during prerendering');
+	},
+	prerendering: {
+		fallback: true,
+		dependencies: new Map()
+	}
+});
+
+mkdirp(dirname(dest));
+writeFileSync(dest, await rendered.text());

packages/kit/src/core/prerender/prerender.js

@@ -453,18 +453,6 @@ export async function prerender() {
 		);
 	}
 
-	const rendered = await server.respond(new Request(config.prerender.origin + '/[fallback]'), {
-		getClientAddress,
-		prerendering: {
-			fallback: true,
-			dependencies: new Map()
-		}
-	});
-
-	const file = `${config.outDir}/output/prerendered/fallback.html`;
-	mkdirp(dirname(file));
-	writeFileSync(file, await rendered.text());
-
 	output_and_exit({ prerendered, prerender_map });
 }
 

packages/kit/types/index.d.ts

@@ -90,6 +90,11 @@ export interface Builder {
 	 */
 	createEntries(fn: (route: RouteDefinition) => AdapterEntry): Promise<void>;
 
+	/**
+	 * Generate a fallback page for a static webserver to use when no route is matched. Useful for single-page apps.
+	 */
+	generateFallback(dest: string): Promise<void>;
+
 	/**
 	 * Generate a server-side manifest to initialise the SvelteKit [server](https://kit.svelte.dev/docs/types#public-types-server) with.
 	 * @param opts a relative path to the base directory of the app and optionally in which format (esm or cjs) the manifest should be generated
@@ -117,15 +122,9 @@ export interface Builder {
 	/**
 	 * Write prerendered files to `dest`.
 	 * @param dest the destination folder
-	 * @param opts.fallback the name of a file for fallback responses, like `200.html` or `404.html` depending on where the app is deployed
 	 * @returns an array of files written to `dest`
 	 */
-	writePrerendered(
-		dest: string,
-		opts?: {
-			fallback?: string;
-		}
-	): string[];
+	writePrerendered(dest: string): string[];
 	/**
 	 * Write server-side code to `dest`.
 	 * @param dest the destination folder