fix: generate fallback page before compressing (#8972)

* generate fallback page before compressing - fixes #8971

* remove logging

* lint

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/rich-llamas-knock.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-static': patch
+---
+
+fix: generate fallback page before compressing

.changeset/thirty-ways-dress.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+chore: refactor fallback generation

packages/adapter-static/index.js

@@ -64,7 +64,7 @@ See https://kit.svelte.dev/docs/page-options#prerender for more details`
 			builder.writePrerendered(pages);
 
 			if (fallback) {
-				builder.generateFallback(path.join(pages, fallback));
+				await builder.generateFallback(path.join(pages, fallback));
 			}
 
 			if (precompress) {

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

@@ -1,14 +1,14 @@
-import { fork } from 'node:child_process';
 import { existsSync, statSync, createReadStream, createWriteStream } from 'node:fs';
 import { pipeline } from 'node:stream';
-import { fileURLToPath } from 'node:url';
 import { promisify } from 'node:util';
 import zlib from 'node:zlib';
 import glob from 'tiny-glob';
 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';
+import generate_fallback from '../postbuild/fallback.js';
+import { write } from '../sync/utils.js';
 
 const pipe = promisify(pipeline);
 
@@ -142,31 +142,16 @@ export function create_builder({
 			}
 		},
 
-		generateFallback(dest) {
-			// do prerendering in a subprocess so any dangling stuff gets killed upon completion
-			const script = fileURLToPath(new URL('../postbuild/fallback.js', import.meta.url));
-
+		async generateFallback(dest) {
 			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);
-					}
-				});
+			const fallback = await generate_fallback({
+				manifest_path,
+				env: { ...env.private, ...env.public }
 			});
+
+			write(dest, fallback);
 		},
 
 		generateManifest: ({ relativePath, routes: subset }) => {

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

@@ -1,43 +1,54 @@
-import { readFileSync, writeFileSync } from 'node:fs';
-import { dirname, join } from 'node:path';
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
 import { pathToFileURL } from 'node:url';
-import { mkdirp } from '../../utils/filesystem.js';
 import { installPolyfills } from '../../exports/node/polyfills.js';
 import { load_config } from '../config/index.js';
+import { forked } from '../../utils/fork.js';
 
-const [, , dest, manifest_path, env] = process.argv;
+export default forked(import.meta.url, generate_fallback);
 
-/** @type {import('types').ValidatedKitConfig} */
-const config = (await load_config()).kit;
+/**
+ * @param {{
+ *   manifest_path: string;
+ *   env: Record<string, string>
+ * }} opts
+ */
+async function generate_fallback({ manifest_path, env }) {
+	/** @type {import('types').ValidatedKitConfig} */
+	const config = (await load_config()).kit;
 
-installPolyfills();
+	installPolyfills();
 
-const server_root = join(config.outDir, 'output');
+	const server_root = join(config.outDir, 'output');
 
-/** @type {import('types').ServerInternalModule} */
-const { set_building } = await import(pathToFileURL(`${server_root}/server/internal.js`).href);
+	/** @type {import('types').ServerInternalModule} */
+	const { set_building } = await import(pathToFileURL(`${server_root}/server/internal.js`).href);
 
-/** @type {import('types').ServerModule} */
-const { Server } = await import(pathToFileURL(`${server_root}/server/index.js`).href);
+	/** @type {import('types').ServerModule} */
+	const { Server } = await import(pathToFileURL(`${server_root}/server/index.js`).href);
 
-/** @type {import('types').SSRManifest} */
-const manifest = (await import(pathToFileURL(manifest_path).href)).manifest;
+	/** @type {import('types').SSRManifest} */
+	const manifest = (await import(pathToFileURL(manifest_path).href)).manifest;
 
-set_building(true);
+	set_building(true);
 
-const server = new Server(manifest);
-await server.init({ env: JSON.parse(env) });
+	const server = new Server(manifest);
+	await server.init({ 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()
-	},
-	read: (file) => readFileSync(join(config.files.assets, file))
-});
+	const response = await server.respond(new Request(config.prerender.origin + '/[fallback]'), {
+		getClientAddress: () => {
+			throw new Error('Cannot read clientAddress during prerendering');
+		},
+		prerendering: {
+			fallback: true,
+			dependencies: new Map()
+		},
+		read: (file) => readFileSync(join(config.files.assets, file))
+	});
 
-mkdirp(dirname(dest));
-writeFileSync(dest, await rendered.text());
+	if (response.ok) {
+		return await response.text();
+	}
+
+	throw new Error(`Could not create a fallback page — failed with status ${response.status}`);
+}