Add cache-control header on 304 response (vercel#50408)

Next.js currently does not return `cache-control` header on 304 response. Accordingly to RFC, Next.js should include it as well: https://www.rfc-editor.org/rfc/rfc9110#section-15.4.5

> The server generating a 304 response MUST generate any of the following header fields that would have been sent in a [200 (OK)](https://www.rfc-editor.org/rfc/rfc9110#status.200) response to the same request:
> 
> - [Content-Location](https://www.rfc-editor.org/rfc/rfc9110#field.content-location), [Date](https://www.rfc-editor.org/rfc/rfc9110#field.date), [ETag](https://www.rfc-editor.org/rfc/rfc9110#field.etag), and [Vary](https://www.rfc-editor.org/rfc/rfc9110#field.vary)
> - **_Cache-Control_** and Expires (see [[CACHING](https://www.rfc-editor.org/rfc/rfc9110#CACHING)])

packages/next/src/server/send-payload/index.ts

@@ -63,6 +63,10 @@ export async function sendRenderResult({
     res.setHeader('X-Powered-By', 'Next.js')
   }
 
+  if (options != null) {
+    setRevalidateHeaders(res, options)
+  }
+
   const payload = result.isDynamic() ? null : await result.toUnchunkedString()
 
   if (payload !== null) {
@@ -91,10 +95,6 @@ export async function sendRenderResult({
     res.setHeader('Content-Length', Buffer.byteLength(payload))
   }
 
-  if (options != null) {
-    setRevalidateHeaders(res, options)
-  }
-
   if (req.method === 'HEAD') {
     res.end(null)
   } else if (payload !== null) {

test/integration/prerender-revalidate/pages/static.js

@@ -0,0 +1,18 @@
+export async function getStaticProps() {
+  return {
+    props: {
+      world: 'world',
+    },
+    revalidate: 10,
+  }
+}
+
+const Page = ({ world }) => {
+  return (
+    <div>
+      <p>hello {world}</p>
+    </div>
+  )
+}
+
+export default Page

test/integration/prerender-revalidate/test/index.test.js

@@ -9,6 +9,7 @@ import {
   renderViaHTTP,
   waitFor,
   getPageFileFromPagesManifest,
+  fetchViaHTTP,
 } from 'next-test-utils'
 import { join } from 'path'
 
@@ -79,6 +80,18 @@ describe('SSG Prerender Revalidate', () => {
     runTests('/named', '/named')
     runTests('/nested', '/nested')
     runTests('/nested/named', '/nested/named')
+
+    it('should return cache-control header on 304 status', async () => {
+      const url = `http://localhost:${appPort}`
+      const res1 = await fetchViaHTTP(url, '/static')
+      const cacheControl200 = res1.headers.get('Cache-Control')
+      const etag = res1.headers.get('ETag')
+
+      const headers = { 'If-None-Match': etag }
+      const res2 = await fetchViaHTTP(url, '/static', undefined, { headers })
+      const cacheControl304 = res2.headers.get('Cache-Control')
+      expect(cacheControl304).toEqual(cacheControl200)
+    })
   })
 
   // Regression test for https://github.com/vercel/next.js/issues/24806