Fix middleware header propagation (#30288)

packages/next/server/next-server.ts

@@ -624,7 +624,7 @@ export default class Server {
 
     const subreq = params.request.headers[`x-middleware-subrequest`]
     const subrequests = typeof subreq === 'string' ? subreq.split(':') : []
-
+    const allHeaders = new Headers()
     let result: FetchEventResult | null = null
 
     for (const middleware of this.middleware || []) {
@@ -667,6 +667,10 @@ export default class Server {
           },
         })
 
+        for (let [key, value] of result.response.headers) {
+          allHeaders.append(key, value)
+        }
+
         if (!this.renderOpts.dev) {
           result.waitUntil.catch((error) => {
             console.error(`Uncaught: middleware waitUntil errored`, error)
@@ -681,6 +685,10 @@ export default class Server {
 
     if (!result) {
       this.render404(params.request, params.response, params.parsed)
+    } else {
+      for (let [key, value] of allHeaders) {
+        result.response.headers.set(key, value)
+      }
     }
 
     return result
@@ -1156,9 +1164,13 @@ export default class Server {
 
           for (const [key, value] of result.response.headers.entries()) {
             if (key !== 'content-encoding') {
+              if (key.toLowerCase() === 'set-cookie') {
+                res.setHeader(key, value.split(', '))
+              } else {
                 res.setHeader(key, value)
               }
             }
+          }
 
           const preflight =
             req.method === 'HEAD' && req.headers['x-middleware-preflight']

packages/next/server/web/utils.ts

@@ -20,25 +20,28 @@ export function notImplemented(name: string, method: string): any {
   )
 }
 
-export function fromNodeHeaders(object: NodeHeaders) {
-  const headers: { [k: string]: string } = {}
-  for (let headerKey in object) {
-    const headerValue = object[headerKey]
-    if (Array.isArray(headerValue)) {
-      headers[headerKey] = headerValue.join('; ')
-    } else if (headerValue) {
-      headers[headerKey] = String(headerValue)
+export function fromNodeHeaders(object: NodeHeaders): Headers {
+  const headers = new Headers()
+  for (let [key, value] of Object.entries(object)) {
+    const values = Array.isArray(value) ? value : [value]
+    for (let v of values) {
+      if (v !== undefined) {
+        headers.append(key, v)
+      }
     }
   }
   return headers
 }
 
 export function toNodeHeaders(headers?: Headers): NodeHeaders {
-  const object: NodeHeaders = {}
+  const result: NodeHeaders = {}
   if (headers) {
     for (const [key, value] of headers.entries()) {
-      object[key] = value.includes(';') ? value.split(';') : value
+      result[key] = value
+      if (key.toLowerCase() === 'set-cookie') {
+        result[key] = value.split(', ')
       }
     }
-  return object
+  }
+  return result
 }

test/integration/middleware-core/pages/responses/_middleware.js

@@ -15,6 +15,16 @@ export async function middleware(request, ev) {
     next.headers.set('x-nested-header', 'valid')
   }
 
+  // Ensure deep can append to this value
+  if (url.searchParams.get('append-me') === 'true') {
+    next.headers.append('x-append-me', 'top')
+  }
+
+  // Ensure deep can append to this value
+  if (url.searchParams.get('cookie-me') === 'true') {
+    next.headers.append('set-cookie', 'chocochip')
+  }
+
   // Sends a header
   if (url.pathname === '/responses/header') {
     next.headers.set('x-first-header', 'valid')

test/integration/middleware-core/pages/responses/deep/_middleware.js

@@ -0,0 +1,9 @@
+import { NextResponse } from 'next/server'
+
+export async function middleware(request, _event) {
+  const next = NextResponse.next()
+  next.headers.set('x-deep-header', 'valid')
+  next.headers.append('x-append-me', 'deep')
+  next.headers.append('set-cookie', 'oatmeal')
+  return next
+}

test/integration/middleware-core/pages/responses/deep/index.js

@@ -0,0 +1,10 @@
+function Deep() {
+  return (
+    <div>
+      <h1>Deep</h1>
+      <p>This is a deep page with deep middleware, check the headers</p>
+    </div>
+  )
+}
+
+export default Deep

test/integration/middleware-core/test/index.test.js

@@ -325,6 +325,17 @@ function responseTests(locale = '') {
     )
     expect(res.headers.get('x-first-header')).toBe('valid')
   })
+
+  it(`${locale} should respond with top level headers and append deep headers`, async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/responses/deep?nested-header=true&append-me=true&cookie-me=true`
+    )
+    expect(res.headers.get('x-nested-header')).toBe('valid')
+    expect(res.headers.get('x-deep-header')).toBe('valid')
+    expect(res.headers.get('x-append-me')).toBe('top, deep')
+    expect(res.headers.raw()['set-cookie']).toEqual(['chocochip', 'oatmeal'])
+  })
 }
 
 function interfaceTests(locale = '') {