fix: detect build-emitted assets in prerenderer fetch

.changeset/chilled-humans-roll.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: detect assets emitted by vite plugins in prerenderer `fetch`.

packages/kit/src/core/generate_manifest/index.js

@@ -80,6 +80,15 @@ export function generate_manifest({ build_data, relative_path, routes }) {
 		mime_types[ext] ??= mime.lookup(ext) || '';
 	}
 
+	const emitted_asset_dir = path.resolve(build_data.out_dir, 'server', build_data.asset_prefix);
+	if (fs.existsSync(emitted_asset_dir)) {
+		for (const file of fs.readdirSync(emitted_asset_dir)) {
+			assets.push(path.join(build_data.asset_prefix, file).replaceAll(path.sep, '/'));
+			const ext = path.extname(file);
+			mime_types[ext] ??= mime.lookup(ext) || '';
+		}
+	}
+
 	// prettier-ignore
 	// String representation of
 	/** @template {import('@sveltejs/kit').SSRManifest} T */

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

@@ -211,6 +211,11 @@ async function prerender({ out, manifest_path, metadata, verbose, env }) {
 				const filepath = saved.get(file);
 				if (filepath) return readFileSync(filepath);
 
+				// Static assets emitted during build
+				if (file.startsWith(config.appDir)) {
+					return readFileSync(join(config.outDir, 'output', 'server', file));
+				}
+
 				// stuff in `static`
 				return readFileSync(join(config.files.assets, file));
 			},

packages/kit/src/exports/vite/index.js

@@ -754,6 +754,7 @@ async function kit({ svelte_config }) {
 					app_path: `${kit.paths.base.slice(1)}${kit.paths.base ? '/' : ''}${kit.appDir}`,
 					manifest_data,
 					out_dir: out,
+					asset_prefix: `${kit.appDir}/immutable/assets`,
 					service_worker: service_worker_entry_file ? 'service-worker.js' : null, // TODO make file configurable?
 					client: null,
 					server_manifest

packages/kit/src/types/internal.d.ts

@@ -63,6 +63,7 @@ export interface BuildData {
 	app_path: string;
 	manifest_data: ManifestData;
 	out_dir: string;
+	asset_prefix: string;
 	service_worker: string | null;
 	client: {
 		start: string;

packages/kit/test/prerendering/paths-base/custom_asset_plugin.js

@@ -0,0 +1,41 @@
+import fs from 'node:fs';
+import path from 'node:path';
+
+export default function () {
+	let config;
+	const ext = '.csv';
+	return {
+		name: 'vite-plugin-sveltekit-custom-asset',
+		resolveId(id) {
+			if (id.endsWith(ext)) {
+				return id;
+			}
+		},
+		configResolved(resolvedConfig) {
+			config = resolvedConfig;
+		},
+		load(id) {
+			if (!id.endsWith(ext)) return;
+			if (config.command === 'serve')
+				return `export default "${path.relative(process.cwd(), id).replaceAll(path.sep, '/')}";`;
+
+			const outName = config.build.rollupOptions.output.assetFileNames
+				.replace('[name]', path.basename(id, ext))
+				.replace('[hash]', '000000')
+				.replace('[extname]', ext);
+
+			if (config.build?.ssr) {
+				const ref = this.emitFile({
+					type: 'asset',
+					fileName: outName
+				});
+				fs.readFile(id, (err, data) => {
+					if (err) this.error(err);
+					this.setAssetSource(ref, data);
+				});
+			}
+
+			return `export default "${config.base}${outName}";`;
+		}
+	};
+}

packages/kit/test/prerendering/paths-base/src/routes/assets/emitted/+page.js

@@ -0,0 +1,8 @@
+// @ts-ignore
+import url from './message.csv';
+
+export async function load({ fetch }) {
+	const response = await fetch(url);
+	const asset = await response.text();
+	return { asset };
+}

packages/kit/test/prerendering/paths-base/src/routes/assets/emitted/+page.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let data;
+</script>
+
+<p>{data.asset}</p>

packages/kit/test/prerendering/paths-base/src/routes/assets/emitted/message.csv

@@ -0,0 +1 @@
+A custom asset emitted by a vite plugin during build.

packages/kit/test/prerendering/paths-base/test/tests.spec.js

@@ -29,3 +29,8 @@ test('prerenders /path-base/assets', () => {
 	const content = read('assets.html');
 	assert.match(content, /<img[^>]+src="\/path-base\//u);
 });
+
+test('prerenders /path-base/assets/emitted', () => {
+	const content = read('assets/emitted.html');
+	assert.ok(content.includes('<p>A custom asset emitted by a vite plugin during build.</p>'));
+});

packages/kit/test/prerendering/paths-base/vite.config.js

@@ -1,5 +1,6 @@
 import * as path from 'node:path';
 import { sveltekit } from '@sveltejs/kit/vite';
+import csvasset from './custom_asset_plugin.js';
 
 /** @type {import('vite').UserConfig} */
 const config = {
@@ -13,7 +14,7 @@ const config = {
 
 	logLevel: 'silent',
 
-	plugins: [sveltekit()],
+	plugins: [csvasset(), sveltekit()],
 
 	server: {
 		fs: {

packages/kit/types/index.d.ts

@@ -1587,6 +1587,7 @@ declare module '@sveltejs/kit' {
 		app_path: string;
 		manifest_data: ManifestData;
 		out_dir: string;
+		asset_prefix: string;
 		service_worker: string | null;
 		client: {
 			start: string;