From b3f074432076b2ebaa597fb5a2c7a360acdbd780 Mon Sep 17 00:00:00 2001
From: Abhishek Mehandiratta <36722596+abhi12299@users.noreply.github.com>
Date: Tue, 24 Sep 2024 00:21:12 +0530
Subject: [PATCH] Fix duplicate headers in response to static assets if
 overrides specified by user (#70127)

Fixes #70086 and #64864 (partially)

### What?
When user wants to override for certain headers such as `content-type`
or `cache-control` via `next.config` file or through middleware, nextjs
sends multiple header values by combining the user's headers with its
own default values for such headers. An example is demonstrated here:


https://github.com/user-attachments/assets/7b38331b-9137-485d-9285-d0b0d0e1e5ac


### Why?
Duplicate header values are a problem.

### How?
Maintaining a list of headers which cannot have duplicate values, and
then checking if the user has overridden any of those headers. If so,
user overrides are respected and sent over.

Demonstration of the correct behaviour after this fix:

Using `next.config` file:


https://github.com/user-attachments/assets/65e2aafb-dffc-47f4-bfcf-cf26a66865db



Using `middleware`:


https://github.com/user-attachments/assets/67636145-10eb-4504-ad78-800c1307c550

---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>
---
 packages/next/src/server/send-response.ts     |  17 ++++++++++++-
 .../app/favicon.ico                           | Bin 0 -> 25931 bytes
 .../app/layout.tsx                            |   8 ++++++
 .../app/page.tsx                              |   3 +++
 .../middleware.ts                             |  13 ++++++++++
 .../next.config.js                            |   6 +++++
 .../no-duplicate-headers-middleware.test.ts   |  14 ++++++++++
 .../app/favicon.ico                           | Bin 0 -> 25931 bytes
 .../app/layout.tsx                            |   8 ++++++
 .../app/page.tsx                              |   3 +++
 .../next.config.js                            |  24 ++++++++++++++++++
 .../no-duplicate-headers-next-config.test.ts  |  14 ++++++++++
 12 files changed, 109 insertions(+), 1 deletion(-)
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-middleware/app/favicon.ico
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-middleware/app/layout.tsx
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-middleware/app/page.tsx
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-middleware/middleware.ts
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-middleware/next.config.js
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-middleware/no-duplicate-headers-middleware.test.ts
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-next-config/app/favicon.ico
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-next-config/app/layout.tsx
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-next-config/app/page.tsx
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-next-config/next.config.js
 create mode 100644 test/e2e/app-dir/no-duplicate-headers-next-config/no-duplicate-headers-next-config.test.ts

diff --git a/packages/next/src/server/send-response.ts b/packages/next/src/server/send-response.ts
index 070b400e7d053..c62c228d306ad 100644
--- a/packages/next/src/server/send-response.ts
+++ b/packages/next/src/server/send-response.ts
@@ -27,6 +27,13 @@ export async function sendResponse(
     res.statusCode = response.status
     res.statusMessage = response.statusText
 
+    // can add more headers to this list if needed
+    const headersWithMultipleValuesAllowed = [
+      'set-cookie',
+      'www-authenticate',
+      'proxy-authenticate',
+    ]
+
     // Copy over the response headers.
     response.headers?.forEach((value, name) => {
       // The append handling is special cased for `set-cookie`.
@@ -36,7 +43,15 @@ export async function sendResponse(
           res.appendHeader(name, cookie)
         }
       } else {
-        res.appendHeader(name, value)
+        // only append the header if it is either not present in the outbound response
+        // or if the header supports multiple values
+        const isHeaderPresent = typeof res.getHeader(name) !== 'undefined'
+        if (
+          headersWithMultipleValuesAllowed.includes(name.toLowerCase()) ||
+          !isHeaderPresent
+        ) {
+          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
new file mode 100644
index 0000000000000000000000000000000000000000..718d6fea4835ec2d246af9800eddb7ffb276240c
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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
new file mode 100644
index 0000000000000..888614deda3ba
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..ff7159d9149fe
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..6de9184fcf290
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..807126e4cf0bf
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..34c99d876200e
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000000000000000000000000000000..718d6fea4835ec2d246af9800eddb7ffb276240c
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HcmV?d00001

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
new file mode 100644
index 0000000000000..888614deda3ba
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..ff7159d9149fe
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..75be548b9f41c
--- /dev/null
+++ 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
new file mode 100644
index 0000000000000..b5e0c6d4e295e
--- /dev/null
+++ 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')
+  })
+})