App dir: notFound() in metadata is ignored with page.tsx

[steebchen]

Link to the code that reproduces this issue

https://github.com/steebchen/nextjs-repro-metadata-404

To Reproduce

1. clone https://github.com/steebchen/nextjs-repro-metadata-404
2. pnpm install
3. pnpm build && pnpm start
4. curl http://localhost:3000/poll/asdf -> returns 404 correctly -> ✅
5. curl http://localhost:3000/user/1 -> returns 200 even with notFound() -> ❌

Current vs. Expected behavior

I expected the /user/[id] route (aka the user/[id]/page.tsx file) to also return a 404, even with streaming enabled, because notFound is returned in generateMetadata().

The generateMetadata function looks like this for both routes:

export async function generateMetadata() {
  return notFound()
}

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.2.0: Wed Nov 15 21:53:18 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T6000
Binaries:
  Node: 20.10.0
  npm: 10.2.3
  Yarn: 1.22.19
  pnpm: 8.10.0-0
Relevant Packages:
  next: 14.0.5-canary.42
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

App Router

Which stage(s) are affected? (Select all that apply)

next start (local)

Additional context

No response

[JCharante]

Is it still a bug if it's documented behavior? https://nextjs.org/docs/app/api-reference/file-conventions/not-found

The not-found file is used to render UI when the notFound function is thrown within a route segment. Along with serving a custom UI, Next.js will return a 200 HTTP status code for streamed responses, and 404 for non-streamed responses.

[steebchen]

It is because the behavior is inconsistent; as you can see above it does indeed return a 404 in some routes even though it's streamed. I believe generatedMetadata is blocking before any response is returend, so it should also return a 404 when you do return notFound() in any case.