vercel · ijjk · Sep 23, 2024 · Sep 15, 2024 · Sep 15, 2024 · Sep 17, 2024
diff --git a/packages/next/src/server/send-response.ts b/packages/next/src/server/send-response.ts
@@ -27,6 +27,15 @@ export async function sendResponse(
     res.statusCode = response.status
     res.statusMessage = response.statusText
 
+    const headersWithNoMultipleValuesAllowed = [
+      'content-type',
+      'content-length',
+      'content-encoding',
+      'cache-control',
+      'expires',
+      'etag',
+    ]
+
     // Copy over the response headers.
     response.headers?.forEach((value, name) => {
       // The append handling is special cased for `set-cookie`.
@@ -36,6 +45,15 @@ export async function sendResponse(
           res.appendHeader(name, cookie)
         }
       } else {
+        // if there is a common header from the list of headers which should not
+        // have multiple values do not overwrite it
+        const isHeaderPresentInRes = typeof res.getHeader(name) !== 'undefined'
+        if (
+          headersWithNoMultipleValuesAllowed.includes(name.toLowerCase()) &&
+          isHeaderPresentInRes
+        ) {
+          return
+        }
         res.appendHeader(name, value)
       }
     })

diff --git a/test/e2e/app-dir/no-duplicate-headers-middleware/app/favicon.ico b/test/e2e/app-dir/no-duplicate-headers-middleware/app/favicon.ico
diff --git a/test/e2e/app-dir/no-duplicate-headers-middleware/app/layout.tsx b/test/e2e/app-dir/no-duplicate-headers-middleware/app/layout.tsx
@@ -0,0 +1,8 @@
+import { ReactNode } from 'react'
+export default function Root({ children }: { children: ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}
diff --git a/test/e2e/app-dir/no-duplicate-headers-middleware/app/page.tsx b/test/e2e/app-dir/no-duplicate-headers-middleware/app/page.tsx
@@ -0,0 +1,3 @@
+export default function Page() {
+  return <p>hello world</p>
+}
diff --git a/test/e2e/app-dir/no-duplicate-headers-middleware/middleware.ts b/test/e2e/app-dir/no-duplicate-headers-middleware/middleware.ts
@@ -0,0 +1,13 @@
+import { NextResponse } from 'next/server'
+import type { NextRequest } from 'next/server'
+
+export function middleware(request: NextRequest) {
+  if (request.nextUrl.pathname === '/favicon.ico') {
+    return NextResponse.next({
+      headers: {
+        'Cache-Control': 'max-age=1234',
+        'Content-Type': 'image/vnd.microsoft.icon',
+      },
+    })
+  }
+}
diff --git a/test/e2e/app-dir/no-duplicate-headers-middleware/next.config.js b/test/e2e/app-dir/no-duplicate-headers-middleware/next.config.js
@@ -0,0 +1,6 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {}
+
+module.exports = nextConfig
diff --git a/test/e2e/app-dir/no-duplicate-headers-middleware/no-duplicate-headers-middleware.test.ts b/test/e2e/app-dir/no-duplicate-headers-middleware/no-duplicate-headers-middleware.test.ts
@@ -0,0 +1,14 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('no-duplicate-headers-next-config', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should prioritise headers in middleware for static assets', async () => {
+    const res = await next.fetch('favicon.ico')
+    expect(res.status).toBe(200)
+    expect(res.headers.get('cache-control')).toBe('max-age=1234')
+    expect(res.headers.get('content-type')).toBe('image/vnd.microsoft.icon')
+  })
+})
diff --git a/test/e2e/app-dir/no-duplicate-headers-next-config/app/favicon.ico b/test/e2e/app-dir/no-duplicate-headers-next-config/app/favicon.ico
diff --git a/test/e2e/app-dir/no-duplicate-headers-next-config/app/layout.tsx b/test/e2e/app-dir/no-duplicate-headers-next-config/app/layout.tsx
@@ -0,0 +1,8 @@
+import { ReactNode } from 'react'
+export default function Root({ children }: { children: ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}
diff --git a/test/e2e/app-dir/no-duplicate-headers-next-config/app/page.tsx b/test/e2e/app-dir/no-duplicate-headers-next-config/app/page.tsx
@@ -0,0 +1,3 @@
+export default function Page() {
+  return <p>hello world</p>
+}
diff --git a/test/e2e/app-dir/no-duplicate-headers-next-config/next.config.js b/test/e2e/app-dir/no-duplicate-headers-next-config/next.config.js
@@ -0,0 +1,24 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {
+  async headers() {
+    return [
+      {
+        source: '/favicon.ico',
+        headers: [
+          {
+            key: 'cache-control',
+            value: 'max-age=1234',
+          },
+          {
+            key: 'content-type',
+            value: 'image/vnd.microsoft.icon',
+          },
+        ],
+      },
+    ]
+  },
+}
+
+module.exports = nextConfig
diff --git a/test/e2e/app-dir/no-duplicate-headers-next-config/no-duplicate-headers-next-config.test.ts b/test/e2e/app-dir/no-duplicate-headers-next-config/no-duplicate-headers-next-config.test.ts
@@ -0,0 +1,14 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('no-duplicate-headers-next-config', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should prioritise headers in next config for static assets', async () => {
+    const res = await next.fetch('favicon.ico')
+    expect(res.status).toBe(200)
+    expect(res.headers.get('cache-control')).toBe('max-age=1234')
+    expect(res.headers.get('content-type')).toBe('image/vnd.microsoft.icon')
+  })
+})