fix: use the UTF-8 byte length when reading inline assets

.changeset/shaggy-mails-eat.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: use the UTF-8 byte length when reading inline assets

packages/kit/src/runtime/app/server/index.js

@@ -1,6 +1,6 @@
 import { read_implementation, manifest } from '../../server/internal.js';
 import { assets } from '#app/paths';
-import { base64_decode } from '../../utils.js';
+import { base64_decode, text_encoder } from '../../utils.js';
 import * as e from '../../../messages/server-errors.js';
 
 /**
@@ -42,11 +42,11 @@ export function read(asset) {
 			});
 		}
 
-		const decoded = decodeURIComponent(data);
+		const decoded = text_encoder.encode(decodeURIComponent(data));
 
 		return new Response(decoded, {
 			headers: {
-				'Content-Length': decoded.length.toString(),
+				'Content-Length': decoded.byteLength.toString(),
 				'Content-Type': type
 			}
 		});

packages/kit/src/runtime/app/server/index.spec.js

@@ -0,0 +1,33 @@
+import { afterAll, expect, test, vi } from 'vitest';
+
+vi.stubGlobal('__SVELTEKIT_DEV__', false);
+vi.stubGlobal('__SVELTEKIT_APP_VERSION__', 'test');
+vi.stubGlobal('__SVELTEKIT_TRACK__', vi.fn());
+afterAll(() => vi.unstubAllGlobals());
+
+const { read } = await import('./index.js');
+const { set_read_implementation } = await import('../../server/internal.js');
+set_read_implementation(() => new ReadableStream());
+
+test.each(['', 'hello', 'café', '中文', '\u{1f680}'])(
+	'read uses the byte length of an inline asset: %j',
+	async (title) => {
+		const text = `<svg xmlns="http://www.w3.org/2000/svg"><title>${title}</title></svg>`;
+		const response = read(`data:image/svg+xml,${encodeURIComponent(text)}`);
+		const body = await response.arrayBuffer();
+
+		expect(new TextDecoder().decode(body)).toBe(text);
+		expect(response.headers.get('content-type')).toBe('image/svg+xml');
+		expect(response.headers.get('content-length')).toBe(body.byteLength.toString());
+	}
+);
+
+test.each(['data:text/plain,café', 'data:text/plain;base64,Y2Fmw6k='])(
+	'read preserves unescaped and base64 inline assets: %s',
+	async (asset) => {
+		const response = read(asset);
+		expect(await response.text()).toBe('café');
+		expect(response.headers.get('content-type')).toBe('text/plain');
+		expect(response.headers.get('content-length')).toBe('5');
+	}
+);