fetch fails with RequestContentLengthMismatchError when following redirect

[angelyan]

Bug Description

fetch fails with a RequestContentLengthMismatchError error when following a redirect under the following conditions:

• The request has a body.
• The redirect preserves the body, e.g. 307 temporary redirects.
• The redirect response doesn't end immediately after sending the headers.

Reproducible By

import { createServer } from 'http';
import { once } from 'events';
import { fetch } from 'undici';

async function main() {
  const server = createServer((req, res) => {
    if (req.url === '/redirect') {
      res.writeHead(307, { location: '/target' });
      res.write('flush headers');
      setTimeout(() => res.end(), 1000);
    } else {
      res.end();
    }
  }).listen(0).unref();

  await once(server, 'listening');

  await fetch(`http://localhost:${server.address().port}/redirect`, {
    method: 'POST',
    body: 'body',
  });
}

main();

fetch fails with the following error:

TypeError: fetch failed
    at fetch (/node_modules/undici/index.js:115:13)
    at async main (file://index.mjs:18:3) {
  cause: RequestContentLengthMismatchError: Request body length does not match content-length header
      at AsyncWriter.end (/node_modules/undici/lib/client.js:2257:15)
      at writeIterable (/node_modules/undici/lib/client.js:2140:12) {
    code: 'UND_ERR_REQ_CONTENT_LENGTH_MISMATCH'
  }
}

Environment

• Node: 20.9.0
• Undici: 6.2.1

Additional context

This happens because the connection is destroyed after a redirect is received:

undici/lib/fetch/index.js

Line 1209 in d56cd7d

fetchParams.controller.connection.destroy()

This causes the fetch to be aborted:

undici/lib/fetch/index.js

Lines 1721 to 1726 in d56cd7d

	destroy (err) {
	if (!this.destroyed) {
	this.destroyed = true
	this.abort?.(err ?? new DOMException('The operation was aborted.', 'AbortError'))
	}
	}

Afterwards, when the redirect is followed, the body is not written because the fetch was cancelled previously:

undici/lib/fetch/index.js

Lines 1833 to 1835 in d56cd7d

	if (isCancelled(fetchParams)) {
	return
	}

This results in an error because the content-length and the written bytes do not match:

undici/lib/client.js

Lines 2255 to 2261 in 871baa7

	if (contentLength !== null && bytesWritten !== contentLength) {
	if (client[kStrictContentLength]) {
	throw new RequestContentLengthMismatchError()
	} else {
	process.emitWarning(new RequestContentLengthMismatchError())
	}
	}

This doesn't happen when the redirect response ends immediately after sending the headers because the request is marked as completed before it's aborted, so the body is written as expected when the redirect is followed.

I was able to fix this by passing a flag to fetchParams.controller.connection.destroy() to destroy the connection without aborting the entire fetch, but I don't know if this is the right approach. If it is, I can send a PR.

[metcoder95]

cc @KhafraDev

[KhafraDev]

PR would be appreciated.

[angelyan]

PR sent @KhafraDev: #2545