fix: handle immutable response object

.changeset/poor-doors-listen.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Handles case where an immutable Response object is returned from an endpoint

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

@@ -50,7 +50,7 @@ export async function renderEndpoint(
 		return new Response(null, { status: 500 });
 	}
 
-	const response = await handler.call(mod, context);
+	let response = await handler.call(mod, context);
 
 	if (!response || response instanceof Response === false) {
 		throw new AstroError(EndpointDidNotReturnAResponse);
@@ -59,10 +59,20 @@ export async function renderEndpoint(
 	// Endpoints explicitly returning 404 or 500 response status should
 	// NOT be subject to rerouting to 404.astro or 500.astro.
 	if (REROUTABLE_STATUS_CODES.includes(response.status)) {
-		// Only `Response.redirect` headers are immutable, therefore a `try..catch` is not necessary.
-		// Note: `Response.redirect` can only be called 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');
+		try {
+			response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+		} catch (err) {
+			// In some cases the response may have immutable headers
+			// This is the case if, for example, the user directly returns a `fetch` response
+			// There's no clean way to check if the headers are immutable, so we just catch the error
+			// Note that response.clone() still has immutable headers!
+			if((err as Error).message?.includes('immutable')) {
+				response = new Response(response.body, response);
+				response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+			} else {
+				throw err;
+			}
+		}
 	}
 
 	return response;

packages/astro/test/fixtures/ssr-api-route/src/pages/fail.js

@@ -0,0 +1,3 @@
+export async function GET({ request }) {
+  return fetch("https://http.im/status/500", request)
+}

packages/astro/test/ssr-api-route.test.js

@@ -125,23 +125,29 @@ describe('API routes in SSR', () => {
 			assert.equal(count, 2, 'Found two separate set-cookie response headers');
 		});
 
+		it('can return an immutable response object', async () => {
+			const response = await fixture.fetch('/fail');
+			const text = await response.text();
+			assert.equal(response.status, 500);
+			assert.equal(text, '');
+		});
+
 		it('Has valid api context', async () => {
 			const response = await fixture.fetch('/context/any');
 			assert.equal(response.status, 200);
 			const data = await response.json();
-			assert.equal(data.cookiesExist, true);
-			assert.equal(data.requestExist, true);
-			assert.equal(data.redirectExist, true);
-			assert.equal(data.propsExist, true);
+			assert.ok(data.cookiesExist);
+			assert.ok(data.requestExist);
+			assert.ok(data.redirectExist);
+			assert.ok(data.propsExist);
 			assert.deepEqual(data.params, { param: 'any' });
 			assert.match(data.generator, /^Astro v/);
-			assert.equal(
+			assert.ok(
 				['http://[::1]:4321/blog/context/any', 'http://127.0.0.1:4321/blog/context/any'].includes(
 					data.url,
 				),
-				true,
 			);
-			assert.equal(['::1', '127.0.0.1'].includes(data.clientAddress), true);
+			assert.ok(['::1', '127.0.0.1'].includes(data.clientAddress));
 			assert.equal(data.site, 'https://mysite.dev/subsite/');
 		});
 	});