ISR page deployed to Vercel does not respect `notFound` if its been previously built

Bug report

Describe the bug

A ISR page, deployed to Vercel, that’s been rendered and cached once will never show a 404 again even if notFound: true is returned from getStaticProps.

To Reproduce

1. Create a new app using npx create-next-app
2. Create the page pages/[slug].js looking something like this:

export default function PostTemplate({ post: { title }}) {
  return (
    <h1>{title}</h1>
  )
}

export async function getStaticPaths() {
  return {
    paths: [],
    fallback: "blocking",
  }
}

export async function getStaticProps({ params: { slug } }) {
  const res = await fetch(`https://yourapi.com/posts/${slug}`)
  const post = await res.json()

  return {
    props: {
      post
    },
    notFound: !post, // For this example, let's pretend my API returns a nullish value when a post is missing
    revalidate: 1
  }
}

1. Deploy to Vercel
2. Publish a post with the slug my-post
3. Visit yourdeployment.vercel.app/my-post and the post is successfully rendered
4. Unpublish the post
5. Reload the page and get a stale version (expected)
6. Reload again and you will still get the post even though getStaticProps returned notFound: true during the previous revalidation.

Expected behavior

Im expecting the 404 page to be rendered as the previous revalidation returned notFound: true

Screenshots

N/A

System information

• OS: macOS
• Chrome 87
• Version of Next.js: 10.0.3
• Version of Node.js: 12.x
• Deployment: Vercel

Additional context

If i run the same app locally (npm start) and try this, the 404 will get displayed as expected which makes me suspect it might be related to the Vercel platform.

Let me know if you need me to provide an example app or if anything is unclear.

[jensonb]

I believe the previously generated page.html is left in .next/server/pages/page.html and not cleared when the page is rebuilt with notFound: true. I’m experiencing the following behavior locally:

1. next build && next export of a page with notFound: true fails due to .next/server/pages/page.html not existing
2. remove notFound: true, now we are able to next build && next export
3. add notFound: true, now we are still able to next build && next export
4. remove .next/, now we get a failure again when attempting to next build && next export

The issue I’m seeing is that next export is attempting to look for the built html file for a notFound: true page, and then either failing due to it not existing or incorrectly using the old version if it exists.

[ijjk]

Hi, this should now be fixed! I’m going to close this issue, if you are still experiencing problems with this please reply with additional information and we can investigate further.