next.js: Fetch being called twice when invalidating

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 22.2.0: Fri Nov 11 02:03:51 PST 2022; root:xnu-8792.61.2~4/RELEASE_ARM64_T6000
Binaries:
  Node: 16.15.0
  npm: 8.5.5
  Yarn: 1.22.19
  pnpm: 7.5.0
Relevant packages:
  next: 13.1.2-canary.1
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0

Which area(s) of Next.js are affected? (leave empty if unsure)

App directory (appDir: true)

Link to the code that reproduces this issue

https://github.com/patrick91/next-cache-exploration

To Reproduce

Using the repo:

To run, start the backend first, you can do so by using docker:

cd backend
docker build -t next-backend-for-cache .
docker run -p 8000:8000 next-backend-for-cache

Then in another terminal:

cd frontend
pnpm i
pnpm run build
pnpm start

Now you can go to http://localhost:3000 and see the requests done to the backend service in its terminal

Describe the Bug

I’ve tried the following code to do a fetch request in a page in the new app dir:

export default async function Home() {
  const query = `
    query {
      country(code: "CH") {
        name
      }
    }`;

  const qs = new URLSearchParams({ query });
  // const url = `http://localhost:8000?${qs.toString()}`;
  const url = `http://localhost:8000`;

  const data = await fetch(url, {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ query }),
    next: {
      revalidate: 5,
    },
  }).then((res) => res.json());

  return (
    <pre>
      <code>{JSON.stringify(data, null, 2)}</code>
    </pre>
  );
}

It seems to work well and cache things as explained in the doc, but when it tries to invalidate it calls the endpoint twice as shown in the video below:

https://user-images.githubusercontent.com/667029/211041628-8bdd4ca2-342c-43f5-8fcb-91dfec0c0b6f.mp4

I’m uncertain if this is expected. I think it should only do a call and I haven’t found anything related to this behaviour.

On top of that, the request is a POST request, not sure if we intend to cache them as well 🤔

Expected Behavior

The backend service should be only called once per invalidation.

Which browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

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