undici: When Content-Length is specified, fetch() raises RequestContentLengthMismatchError on redirection

Bug Description

When the Content-Length request header field is manually specified (instead of automatically generated), fetch() raises a RequestContentLengthMismatchError in case of redirection.

Reproducible By

Send a POST request to the resource identified by the /sirene/public/recherche target URI to search for a non-existing company whose name foobarbaz is provided in the request body:

const uri = 'https://www.sirene.fr/sirene/public/recherche';
const method = 'POST';
const body = 'recherche.sirenSiret=&recherche.raisonSociale=foobarbaz&recherche.adresse=&recherche.commune=&recherche.excludeClosed=true&__checkbox_recherche.excludeClosed=true&recherche.captcha=';
const headers = {'Content-Type': 'application/x-www-form-urlencoded', 'Content-Length': Buffer.byteLength(body)};
const response = await fetch(uri, {method, headers, body});

The resource replies with a 302 response with a Location: /sirene/error/autre.action header field. So fetch automatically (by default) redirects the original request to the target URI /sirene/error/autre.action and raises a RequestContentLengthMismatchError:

Uncaught TypeError: fetch failed
    at Object.fetch (node:internal/deps/undici/undici:11413:11)
    at async REPL5:1:47 {
  cause: RequestContentLengthMismatchError: Request body length does not match content-length header
      at write (node:internal/deps/undici/undici:9907:41)
      at _resume (node:internal/deps/undici/undici:9885:33)
      at resume (node:internal/deps/undici/undici:9787:7)
      at connect (node:internal/deps/undici/undici:9776:7) {
    code: 'UND_ERR_REQ_CONTENT_LENGTH_MISMATCH'
  }
}

Expected Behavior

The specified Content-Length request header field is correct in the original request so I don’t expect a RequestContentLengthMismatchError after automatic redirection.

Logs & Screenshots

I assume that during redirection, fetch() resends the original request but with the following modifications:

• the target URI is changed from /sirene/public/recherche to /sirene/error/autre.action;
• the POST request method is changed to GET;
• the request body is removed.

But it doesn’t remove the content-specific header fields (Content-Type and Content-Length here), contrary to what the latest HTTP specification RFC 9110 recommends:

When automatically following a redirected request, the user agent SHOULD resend the original request message with the following modifications:

1. Replace the target URI with the URI referenced by the redirection response’s Location header field value after resolving it relative to the original request’s target URI.
2. […]
3. […]
4. Change the request method according to the redirecting status code’s semantics, if applicable.
5. If the request method has been changed to GET or HEAD, remove content-specific header fields, including (but not limited to) Content-Encoding, Content-Language, Content-Location, Content-Type, Content-Length, Digest, Last-Modified.

The non-compliance of Undici to point 5 likely causes the RequestContentLengthMismatchError. Indeed, sending a GET request without a body but with a Content-Length header field raises the same RequestContentLengthMismatchError:

fetch('http://example.com/', {headers: {'Content-Length': 0}})

Output:

Uncaught TypeError: fetch failed
    at Object.fetch (node:internal/deps/undici/undici:11413:11) {
  cause: RequestContentLengthMismatchError: Request body length does not match content-length header
      at write (node:internal/deps/undici/undici:9907:41)
      at _resume (node:internal/deps/undici/undici:9885:33)
      at resume (node:internal/deps/undici/undici:9787:7)
      at connect (node:internal/deps/undici/undici:9776:7) {
    code: 'UND_ERR_REQ_CONTENT_LENGTH_MISMATCH'
  }
}

Environment

MacOS 11.6.7, Node 19.7.0.