Change Flight response content type to application/octet-stream (#40665)

Ensures Flight responses are not loaded as HTML.


## Bug

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

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/server/app-render.tsx

@@ -51,6 +51,15 @@ export type RenderOptsPartial = {
 
 export type RenderOpts = LoadComponentsReturnType & RenderOptsPartial
 
+/**
+ * Flight Response is always set to application/octet-stream to ensure it does not
+ */
+class FlightRenderResult extends RenderResult {
+  constructor(response: string | ReadableStream<Uint8Array>) {
+    super(response, { contentType: 'application/octet-stream' })
+  }
+}
+
 /**
  * Interop between "export default" and "module.exports".
  */
@@ -500,7 +509,7 @@ export async function renderToHTMLOrFlight(
 
     // Empty so that the client-side router will do a full page navigation.
     const flightData: FlightData = pathname + (search ? `?${search}` : '')
-    return new RenderResult(
+    return new FlightRenderResult(
       renderToReadableStream(flightData, serverComponentManifest).pipeThrough(
         createBufferedTransformStream()
       )
@@ -1054,7 +1063,7 @@ export async function renderToHTMLOrFlight(
       ).slice(1),
     ]
 
-    return new RenderResult(
+    return new FlightRenderResult(
       renderToReadableStream(flightData, serverComponentManifest, {
         context: serverContexts,
       }).pipeThrough(createBufferedTransformStream())

packages/next/server/render-result.ts

@@ -1,10 +1,21 @@
 import type { ServerResponse } from 'http'
 
+type ContentTypeOption = string | undefined
+
 export default class RenderResult {
-  _result: string | ReadableStream<Uint8Array>
+  private _result: string | ReadableStream<Uint8Array>
+  private _contentType: ContentTypeOption
 
-  constructor(response: string | ReadableStream<Uint8Array>) {
+  constructor(
+    response: string | ReadableStream<Uint8Array>,
+    { contentType }: { contentType?: ContentTypeOption } = {}
+  ) {
     this._result = response
+    this._contentType = contentType
+  }
+
+  contentType(): ContentTypeOption {
+    return this._contentType
   }
 
   toUnchunkedString(): string {

packages/next/server/send-payload/index.ts

@@ -71,10 +71,16 @@ export async function sendRenderResult({
     }
   }
 
+  const resultContentType = result.contentType()
+
   if (!res.getHeader('Content-Type')) {
     res.setHeader(
       'Content-Type',
-      type === 'json' ? 'application/json' : 'text/html; charset=utf-8'
+      resultContentType
+        ? resultContentType
+        : type === 'json'
+        ? 'application/json'
+        : 'text/html; charset=utf-8'
     )
   }
 

packages/next/server/web-server.ts

@@ -370,10 +370,14 @@ export default class NextWebServer extends BaseServer<WebServerOptions> {
     if (options.poweredByHeader && options.type === 'html') {
       res.setHeader('X-Powered-By', 'Next.js')
     }
+    const resultContentType = options.result.contentType()
+
     if (!res.getHeader('Content-Type')) {
       res.setHeader(
         'Content-Type',
-        options.type === 'json'
+        resultContentType
+          ? resultContentType
+          : options.type === 'json'
           ? 'application/json'
           : 'text/html; charset=utf-8'
       )

test/e2e/app-dir/index.test.ts

@@ -44,6 +44,19 @@ describe('app dir', () => {
     })
     afterAll(() => next.destroy())
 
+    it('should use application/octet-stream for flight', async () => {
+      const res = await fetchViaHTTP(
+        next.url,
+        '/dashboard/deployments/123?__flight__'
+      )
+      expect(res.headers.get('Content-Type')).toBe('application/octet-stream')
+    })
+
+    it('should use application/octet-stream for flight with edge runtime', async () => {
+      const res = await fetchViaHTTP(next.url, '/dashboard?__flight__')
+      expect(res.headers.get('Content-Type')).toBe('application/octet-stream')
+    })
+
     it('should pass props from getServerSideProps in root layout', async () => {
       const html = await renderViaHTTP(next.url, '/dashboard')
       const $ = cheerio.load(html)