Fix endpoint response with immutable header

.changeset/silver-peaches-pump.md

@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Updated `renderEndpoint` implementation to only append the reroute directive if the response HTTP status code is 404 or 500.

packages/astro/src/runtime/server/endpoint.ts

@@ -42,6 +42,12 @@ export async function renderEndpoint(
 	const response = await handler.call(mod, context);
 	// Endpoints explicitly returning 404 or 500 response status should
 	// NOT be subject to rerouting to 404.astro or 500.astro.
-	response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+	if (response.status === 404 || response.status === 500) {
+		// Only `Response.redirect` headers are immutable, therefore a `try..catch` is not necessary.
+		// Note: `Response.redirect` can only constructed with HTTP status codes: 301, 302, 303, 307, 308.
+		// Source: https://developer.mozilla.org/en-US/docs/Web/API/Response/redirect_static#parameters
+		response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+	}
+
 	return response;
 }

packages/astro/test/units/routing/endpoints.test.js

@@ -0,0 +1,61 @@
+import {
+	createBasicSettings,
+	createFs,
+	createRequestAndResponse,
+	defaultLogger,
+} from '../test-utils.js';
+import { fileURLToPath } from 'node:url';
+import { expect } from 'chai';
+import { createContainer } from '../../../dist/core/dev/container.js';
+import testAdapter from '../../test-adapter.js';
+
+const root = new URL('../../fixtures/api-routes/', import.meta.url);
+const fileSystem = {
+	'/src/pages/response-redirect.ts': `export const GET = ({ url }) => Response.redirect("https://example.com/destination", 307)`,
+	'/src/pages/response.ts': `export const GET = ({ url }) => new Response(null, { headers: { Location: "https://example.com/destination" }, status: 307 })`,
+};
+
+describe('endpoints', () => {
+	let container;
+	let settings;
+
+	before(async () => {
+		const fs = createFs(fileSystem, root);
+		settings = await createBasicSettings({
+			root: fileURLToPath(root),
+			output: 'server',
+			adapter: testAdapter(),
+		});
+		container = await createContainer({
+			fs,
+			settings,
+			logger: defaultLogger,
+		});
+	});
+
+	after(async () => {
+		await container.close();
+	});
+
+	it('should return a redirect response with location header', async () => {
+		const { req, res, done } = createRequestAndResponse({
+			method: 'GET',
+			url: '/response-redirect',
+		});
+		container.handle(req, res);
+		await done;
+		expect(res.getHeaders()).to.deep.include({ location: 'https://example.com/destination' });
+		expect(res.statusCode).to.equal(307);
+	});
+
+	it('should return a response with location header', async () => {
+		const { req, res, done } = createRequestAndResponse({
+			method: 'GET',
+			url: '/response',
+		});
+		container.handle(req, res);
+		await done;
+		expect(res.getHeaders()).to.deep.include({ location: 'https://example.com/destination' });
+		expect(res.statusCode).to.equal(307);
+	});
+});