Stash extra headers on the metadata of the renderresult instead

This ensures that we only track the static headers we want to track.

packages/next/src/export/routes/app-page.ts

@@ -184,7 +184,7 @@ export async function exportAppPage(
       )
     }
 
-    const headers = toNodeOutgoingHttpHeaders(res.headers)
+    const headers = { ...metadata.extraHeaders }
 
     if (fetchTags) {
       headers[NEXT_CACHE_TAGS_HEADER] = fetchTags

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

@@ -741,6 +741,10 @@ async function renderToHTMLOrFlightImpl(
         // Copy headers created by React into the response object.
         headers.forEach((value: string, key: string) => {
           res.appendHeader(key, value)
+          if (!extraRenderResultMeta.extraHeaders) {
+            extraRenderResultMeta.extraHeaders = {}
+          }
+          extraRenderResultMeta.extraHeaders[key] = value
         })
       }
 

packages/next/src/server/base-server.ts

@@ -2374,8 +2374,7 @@ export default abstract class Server<ServerOptions extends Options = Options> {
       // Add any fetch tags that were on the page to the response headers.
       const cacheTags = metadata.fetchTags
 
-      // Copy the headers from the response.
-      headers = { ...res.getHeaders() }
+      headers = { ...metadata.extraHeaders }
 
       if (cacheTags) {
         headers[NEXT_CACHE_TAGS_HEADER] = cacheTags

packages/next/src/server/render-result.ts

@@ -1,4 +1,4 @@
-import type { ServerResponse } from 'http'
+import type { OutgoingHttpHeaders, ServerResponse } from 'http'
 import type { StaticGenerationStore } from '../client/components/static-generation-async-storage.external'
 import type { Revalidate } from './lib/revalidate'
 
@@ -23,6 +23,7 @@ export type RenderResultMetadata = {
   isRedirect?: boolean
   fetchMetrics?: StaticGenerationStore['fetchMetrics']
   fetchTags?: string
+  extraHeaders?: OutgoingHttpHeaders
   waitUntil?: Promise<any>
   postponed?: string
 }