fix: append server-timing header value if key exist when using setHeaders to avoid exception

.changeset/clear-spies-wear.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+allow multiple server-timing headers

packages/kit/src/runtime/server/respond.js

@@ -183,7 +183,11 @@ export async function internal_respond(request, options, manifest, state) {
 						'Use `event.cookies.set(name, value, options)` instead of `event.setHeaders` to set cookies'
 					);
 				} else if (lower in headers) {
-					throw new Error(`"${key}" header is already set`);
+					if (lower === 'server-timing') {
+						headers[lower] += ', ' + value;
+					} else {
+						throw new Error(`"${key}" header is already set`);
+					}
 				} else {
 					headers[lower] = value;
 

packages/kit/test/apps/basics/src/routes/headers/server-timing/+layout.server.js

@@ -0,0 +1,6 @@
+/** @type {import('./$types').LayoutServerLoad} */
+export function load({ setHeaders }) {
+	setHeaders({
+		'server-timing': 'db;dur=53'
+	});
+}

packages/kit/test/apps/basics/src/routes/headers/server-timing/+layout.svelte

@@ -0,0 +1 @@
+<slot />

packages/kit/test/apps/basics/src/routes/headers/server-timing/sub/+page.server.js

@@ -0,0 +1,6 @@
+/** @type {import('./$types').PageServerLoad} */
+export function load({ setHeaders }) {
+	setHeaders({
+		'server-timing': 'external-api;dur=250'
+	});
+}

packages/kit/test/apps/basics/test/server.test.js

@@ -785,6 +785,12 @@ test.describe('setHeaders', () => {
 		expect(cookies).toMatch('cookie1=value1');
 		expect(cookies).toMatch('cookie2=value2');
 	});
+	test('allows multiple server-timing headers with different values', async ({ page }) => {
+		const response = await page.goto('/headers/server-timing/sub');
+		const serverTiming = response ? (await response.allHeaders())['server-timing'] : '';
+
+		expect(serverTiming).toMatch('db;dur=53, external-api;dur=250');
+	});
 });
 
 test.describe('cookies', () => {