[whatwg/fetch] [Question] What is the expected behavior for reusing the Request body in a HTTP redirect fetch (status code 307 / 308)? (Issue #1662)

I am performing maintenance work for some some internal implementation of the `fetch` API for the company I am working in. Recently I discovered some bugs of that implementation related to HTTP redirect fetches with a `POST` request and a 3xx return code, and I am working on fixing them.

The typical use case is as follows:

1. The user makes a `POST` request to **xxx.com** with a body `12345`
2. **xxx.com** returns a **3XX** return code and the new URL **yyy.com** to redirect to
3. The user then requests **yyy.com** with the `POST` request and the same body `12345`

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My major confusion arises when I discovered bugs related to the **307** and **308** return codes.

Basically, MDN says this for [307](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/307):
> The only difference between 307 and [302](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/302) is that 307 guarantees that the method and the body will not be changed when the redirected request is made.

And for [308](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/308):
> The request method and the body will not be altered, whereas [301](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/301) may incorrectly sometimes be changed to a [GET](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/GET) method.

But the [Fetch Spec](https://fetch.spec.whatwg.org/#http-redirect-fetch) suggests otherwise:
> 11. If internalResponse’s [status](https://fetch.spec.whatwg.org/#concept-response-status) is not 303, request’s [body](https://fetch.spec.whatwg.org/#concept-request-body) is non-null, and request’s [body](https://fetch.spec.whatwg.org/#concept-request-body)’s [source](https://fetch.spec.whatwg.org/#concept-body-source) is null, then return a [network error](https://fetch.spec.whatwg.org/#concept-network-error).

This is my interpretation:

* For 307 and 308 status codes, MDN suggests that the same _method_ and same _body_ shall be used for the subsequent access to the redirected URL.
* But in fetch spec, if I follow the "HTTP-redirect fetch" item 11 (as given above), then it would end up in _network error_.

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So my question arises here: **What is the expected behavior for reusing the Request body in a HTTP redirect fetch (307 / 308 return codes)?**


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