fix: Merge link header from middleware with the ones from React (#73431)

Fixes #69000

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---------

Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>
Co-authored-by: Sam Ko <sam@vercel.com>

Author: 3 people

packages/next/src/server/app-render/app-render.tsx

@@ -1735,6 +1735,7 @@ async function renderToStream(
   let reactServerResult: null | ReactServerResult = null
 
   const setHeader = res.setHeader.bind(res)
+  const appendHeader = res.appendHeader.bind(res)
 
   try {
     if (
@@ -1904,7 +1905,7 @@ async function renderToStream(
         nonce: ctx.nonce,
         onHeaders: (headers: Headers) => {
           headers.forEach((value, key) => {
-            setHeader(key, value)
+            appendHeader(key, value)
           })
         },
         maxHeadersLength: renderOpts.reactMaxHeadersLength,
@@ -2567,14 +2568,25 @@ async function prerenderToStream(
     | null
     | ReactServerPrerenderResult
     | ServerPrerenderStreamResult = null
-  const setHeader = (name: string, value: string | string[]) => {
-    res.setHeader(name, value)
-
+  const setMetadataHeader = (name: string) => {
     metadata.headers ??= {}
     metadata.headers[name] = res.getHeader(name)
-
+  }
+  const setHeader = (name: string, value: string | string[]) => {
+    res.setHeader(name, value)
+    setMetadataHeader(name)
     return res
   }
+  const appendHeader = (name: string, value: string | string[]) => {
+    if (Array.isArray(value)) {
+      value.forEach((item) => {
+        res.appendHeader(name, item)
+      })
+    } else {
+      res.appendHeader(name, value)
+    }
+    setMetadataHeader(name)
+  }
 
   let prerenderStore: PrerenderStore | null = null
 
@@ -2923,7 +2935,7 @@ async function prerenderToStream(
                 },
                 onHeaders: (headers: Headers) => {
                   headers.forEach((value, key) => {
-                    setHeader(key, value)
+                    appendHeader(key, value)
                   })
                 },
                 maxHeadersLength: renderOpts.reactMaxHeadersLength,
@@ -3543,7 +3555,7 @@ async function prerenderToStream(
           onError: htmlRendererErrorHandler,
           onHeaders: (headers: Headers) => {
             headers.forEach((value, key) => {
-              setHeader(key, value)
+              appendHeader(key, value)
             })
           },
           maxHeadersLength: renderOpts.reactMaxHeadersLength,

test/e2e/app-dir/app-middleware/app-middleware.test.ts

@@ -123,6 +123,13 @@ describe('app-dir with middleware', () => {
     })
   })
 
+  it('retains a link response header from the middleware', async () => {
+    const res = await next.fetch('/preloads')
+    expect(res.headers.get('link')).toContain(
+      '<https://example.com/page>; rel="alternate"; hreflang="en"'
+    )
+  })
+
   it('should be possible to modify cookies & read them in an RSC in a single request', async () => {
     const browser = await next.browser('/rsc-cookies')
 

test/e2e/app-dir/app-middleware/app/preloads/page.js

@@ -0,0 +1,13 @@
+import Image from 'next/image'
+import { Inter } from 'next/font/google'
+
+const inter = Inter({ subsets: ['latin'] })
+
+export default function Page() {
+  return (
+    <>
+      <Image priority src="/favicon.ico" alt="favicon" width={16} height={16} />
+      <h1 className={inter.className}>Hello World</h1>
+    </>
+  )
+}

test/e2e/app-dir/app-middleware/middleware.js

@@ -69,6 +69,15 @@ export async function middleware(request) {
     return res
   }
 
+  if (request.nextUrl.pathname === '/preloads') {
+    const res = NextResponse.next({
+      headers: {
+        link: '<https://example.com/page>; rel="alternate"; hreflang="en"',
+      },
+    })
+    return res
+  }
+
   return NextResponse.next({
     request: {
       headers: headersFromRequest,