Fix handling for 204 status code with a body (#28479)

This ensures we handle 204 and 304 status codes correctly in API routes.  

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`

Fixes: #28464

errors/invalid-api-status-body.md

@@ -0,0 +1,32 @@
+Invalid API Route Status/Body Response
+
+#### Why This Error Occurred
+
+In one of your API routes a 204 or 304 status code was used as well as sending a response body.
+
+This is invalid as a 204 or 304 status code dictates no response body should be present.
+
+#### Possible Ways to Fix It
+
+Send an empty body when using a 204 or 304 status code or use a different status code while sending a response body.
+
+Before
+
+```js
+export default function handler(req, res) {
+  res.status(204).send('invalid body')
+}
+```
+
+After
+
+```js
+export default function handler(req, res) {
+  res.status(204).send()
+}
+```
+
+### Useful Links
+
+- [204 status code documentation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/204)
+- [304 status code documentation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304)

errors/manifest.json

@@ -454,6 +454,10 @@
         {
           "title": "next-config-error",
           "path": "/errors/next-config-error.md"
+        },
+        {
+          "title": "invalid-api-status-body",
+          "path": "/errors/invalid-api-status-body.md"
         }
       ]
     }

packages/next/server/api-utils.ts

@@ -258,6 +258,22 @@ export function sendData(
     return
   }
 
+  // strip irrelevant headers/body
+  if (res.statusCode === 204 || res.statusCode === 304) {
+    res.removeHeader('Content-Type')
+    res.removeHeader('Content-Length')
+    res.removeHeader('Transfer-Encoding')
+
+    if (process.env.NODE_ENV === 'development' && body) {
+      console.warn(
+        `A body was attempted to be set with a 204 statusCode for ${req.url}, this is invalid and the body was ignored.\n` +
+          `See more info here https://nextjs.org/docs/messages/invalid-api-status-body`
+      )
+    }
+    res.end()
+    return
+  }
+
   const contentType = res.getHeader('Content-Type')
 
   if (body instanceof Stream) {

test/integration/api-support/pages/api/status-204.js

@@ -0,0 +1,7 @@
+export default function handler(req, res) {
+  if (req.query.invalid) {
+    // test the warning when content is added for a 204 response
+    return res.status(204).json({ hello: 'world' })
+  }
+  return res.status(204).send()
+}

test/integration/api-support/test/index.test.js

@@ -26,6 +26,36 @@ let mode
 let app
 
 function runTests(dev = false) {
+  it('should handle 204 status correctly', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/status-204', undefined, {
+      redirect: 'manual',
+    })
+    expect(res.status).toBe(204)
+    expect(res.headers.get('content-type')).toBe(null)
+    expect(res.headers.get('content-length')).toBe(null)
+    expect(res.headers.get('transfer-encoding')).toBe(null)
+
+    const stderrIdx = stderr.length
+    const res2 = await fetchViaHTTP(
+      appPort,
+      '/api/status-204',
+      { invalid: '1' },
+      {
+        redirect: 'manual',
+      }
+    )
+    expect(res2.status).toBe(204)
+    expect(res2.headers.get('content-type')).toBe(null)
+    expect(res2.headers.get('content-length')).toBe(null)
+    expect(res2.headers.get('transfer-encoding')).toBe(null)
+
+    if (dev) {
+      expect(stderr.substr(stderrIdx)).toContain(
+        'A body was attempted to be set with a 204 statusCode'
+      )
+    }
+  })
+
   it('should render page', async () => {
     const html = await renderViaHTTP(appPort, '/')
     expect(html).toMatch(/API - support/)