How can I use fixed-length multipart request with FormData?

Discussed in https://github.com/nodejs/undici/discussions/1420

<div type='discussions-op-text'>

^{Originally posted by nwtgck May 6, 2022} Hi, I’d like to request a fixed-length multipart request. The code below requested with transfer-encoding: chunked without content-length. I assume the length of formData is statically decided.

const undici = require("undici");

(async () => {
  const formData = new undici.FormData();
  formData.append("myname", "myvalue");
  const res = await undici.request("http://localhost:8181", {
    body: formData,
  });
})();

$ nc -lp 8181
PUT / HTTP/1.1
host: localhost:8181
connection: keep-alive
content-type: multipart/form-data; boundary=----formdata-undici-0.5140957512643098
transfer-encoding: chunked

64
------formdata-undici-0.5140957512643098
Content-Disposition: form-data; name="myname"

myvalue

2a
------formdata-undici-0.5140957512643098--
0
```</div>

[thatsmydoing]

It seems like the issue is that the spec is unclear https://fetch.spec.whatwg.org/#concept-bodyinit-extract

https://github.com/nodejs/undici/blob/8549a9402c46034d6d28b2f6f65d934736e51a59/lib/fetch/body.js#L114-L115

I checked Firefox and it looks like they do set the length in their implementation https://searchfox.org/mozilla-central/rev/8dd35cd8f5284fbaa506aab02fe42fc87efb249e/dom/html/HTMLFormSubmission.cpp#373

[jimmywarting]

hmm. couldn’t really convert the formdata to a blob as i’m doing in node-fetch and the formdata polyfill for browsers I’m using fetch-blob instead that can accept 3rd party blobs backed up by the fs. And i couldn’t really use the FileLike as it did only accept one blob part instead of an array of parts.

https://github.com/nodejs/undici/blob/860c2617ba55d911286f967dbc915f03e89e0d94/lib/fetch/file.js#L95-L97

So it’s not spec compatible in that regards…

import { fileFromSync } from 'fetch-blob/from.js'

const fsFile = fileFromSync('file.txt')
const fd = new globalThis.FormData()
fd.set('a', fsFile)
const FileLike = fd.get('a').constructor
new FileLike(['a', 'b']) // don't work...
new FileLike([blob, blob]) // don't work either...