FormData.hasKnownLength always returns true

[cchudant]

FormData.hasKnownLength() returns true whether my form data has a known length or not (ie. inserting a stream)
This break node-fetch, because they use the known length to send Content-Length header.

[alexindigo]

Do you mind to elaborate on how it break node-fetch?
And when there is no known content-length, I'm not sure how it could set Content-Length header.

[cchudant]

Try this:

const fetch = require('node-fetch')
const FormData = require('form-data')
const { Readable } = require('stream')

let i = 0
const stream = new Readable({
  read(size) {
    i++
    this.push(i <= 16 ? Buffer.alloc(size) : null) // Buffer full of 0's
  }
})

const apiUrl = 'https://bayfiles.com/api/upload'

const form = new FormData()
form.on('data', buf => console.log(buf.length, buf))

// form.hasKnownLength = () => false
console.log(`Form has known length: ${form.hasKnownLength()}`)

form.append('file', stream, 'myFile')

fetch(apiUrl, {
  method: 'POST',
  body: form
})
  .then(res => res.text())
  .then(json => {
    console.log(json)
  })

FormData says the stream has a known length even though it hasn't.
The upload doesn't work because a wrong Content-Length header is sent.

Then try uncommenting form.hasKnownLength = () => false; the Content-Length header is not sent and the upload works.

[cchudant]

any update on that?

[yordis]

I believe that form-data package shouldn't expose that function since it doesn't exist on FormData spec.

[cchudant]

This function is needed for node-fetch to work.
node-fetch need to know the content length in order to send the Content-Length header when the length is available.

[yordis]

@SkyBeastMC I don't know the details to be honest, but even when that is the case, it is leaking abstractions.

It is fine if node-fetch ask for all the entries and figure out Content-Length based on the form values or something like that.

But if you have to expose a function for that,

1. Leak abstraction
2. It is not following the spec

So probably things need to be clean up.

Just a feedback.

[dalcde]

I believe the offending line is https://github.com/form-data/form-data/blob/master/lib/form_data.js#L104 . When value is a body returned from node-fetch, the condition evaluates to true, because

body.path == undefined
value.readable == true
value.hasOwnProperty('httpVersion') == false

The body Readable Stream is then considered to have known length. So the form.hasKnownLength would incorrectly return true, and the "known" length is 0. This means node-fetch gets an incorrect Content-Length, and webservers get confused.

[nytamin]

I spent a fair bit of time troubleshooting a similar issue, the PR #382 solved the issue for me.