From c1d34584f2404b9c80a547c8138264cac2a813b6 Mon Sep 17 00:00:00 2001
From: Conduitry <git@chor.date>
Date: Thu, 25 May 2023 17:26:58 -0400
Subject: [PATCH] fix: clone Headers before mutating them during prerendering
 (#10030)

* clone request and headers before setting x-sveltekit-prerender

* add changeset

* add test
---
 .changeset/curly-onions-compare.md                       | 5 +++++
 packages/kit/src/runtime/server/endpoint.js              | 9 ++++++++-
 .../basics/src/routes/immutable-headers/+server.js       | 8 ++++++++
 packages/kit/test/prerendering/basics/test/tests.spec.js | 5 +++++
 4 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/curly-onions-compare.md
 create mode 100644 packages/kit/test/prerendering/basics/src/routes/immutable-headers/+server.js

diff --git a/.changeset/curly-onions-compare.md b/.changeset/curly-onions-compare.md
new file mode 100644
index 000000000000..5c2516c6dbf7
--- /dev/null
+++ b/.changeset/curly-onions-compare.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: gracefully handle server endpoints that return `Response`s with immutable `Headers` when prerendering
diff --git a/packages/kit/src/runtime/server/endpoint.js b/packages/kit/src/runtime/server/endpoint.js
index 28e57eb047d5..3bb5f15a2863 100644
--- a/packages/kit/src/runtime/server/endpoint.js
+++ b/packages/kit/src/runtime/server/endpoint.js
@@ -39,7 +39,7 @@ export async function render_endpoint(event, mod, state) {
 	}
 
 	try {
-		const response = await handler(
+		let response = await handler(
 			/** @type {import('@sveltejs/kit').RequestEvent<Record<string, any>>} */ (event)
 		);
 
@@ -50,6 +50,13 @@ export async function render_endpoint(event, mod, state) {
 		}
 
 		if (state.prerendering) {
+			// the returned Response might have immutable Headers
+			// so we should clone them before trying to mutate them
+			response = new Response(response.body, {
+				status: response.status,
+				statusText: response.statusText,
+				headers: new Headers(response.headers)
+			});
 			response.headers.set('x-sveltekit-prerender', String(prerender));
 		}
 
diff --git a/packages/kit/test/prerendering/basics/src/routes/immutable-headers/+server.js b/packages/kit/test/prerendering/basics/src/routes/immutable-headers/+server.js
new file mode 100644
index 000000000000..64688cc5b77e
--- /dev/null
+++ b/packages/kit/test/prerendering/basics/src/routes/immutable-headers/+server.js
@@ -0,0 +1,8 @@
+export const prerender = true;
+
+export const GET = () => {
+	const response = new Response('foo');
+	// this simulates immutable Response Headers, like those returned by undici
+	Object.defineProperty(response.headers, 'set', { value: null });
+	return response;
+};
diff --git a/packages/kit/test/prerendering/basics/test/tests.spec.js b/packages/kit/test/prerendering/basics/test/tests.spec.js
index ed29320fcd52..6fee0f46b141 100644
--- a/packages/kit/test/prerendering/basics/test/tests.spec.js
+++ b/packages/kit/test/prerendering/basics/test/tests.spec.js
@@ -223,3 +223,8 @@ test('prerendered.paths omits trailing slashes for endpoints', () => {
 		expect(content, `Missing ${path}`).toMatch(`"${path}"`);
 	}
 });
+
+test('prerenders responses with immutable Headers', () => {
+	const content = read('immutable-headers');
+	expect(content).toMatch('foo');
+});