Response does not resolve until body fetched

[chetbox]

We came across a bug in our code where we expected await fetch('https://example.com/a-large-file.zip', { method: 'HEAD' }) to just get the HTTP status code and not fetch the HTTP body data, however we hit a memory issue because all the data was downloaded and stored in RAM.

I believe I have observed two deviations from the way native fetch works in whatwg-fetch v3.6.20:

1. await fetch(...) does not resolve until the entire body has been downloaded.
   It should be possible to check the status code before the download of the HTTP request body is complete. e.g

const response = await fetch('https://example.com/a-large-file.zip')
if (response.status === 200) {
  // I should be able to do something here before the file download has completed
  console.log('File exists! Downloading...')
  const data = await response.body()
}

2. await fetch(..., { method: 'HEAD' }) exhibits the same behaviour as above but it should not fetch the HTTP body at all. e.g.

const response = await fetch('https://example.com/a-large-file.zip', { method: 'HEAD' })
if (response.status === 200) {
  // I should be able to do something here immediately
  // In practice we only reach here after the file has finished downloading
  console.log('File exists!')
}

We've now upgraded to Node 18 to switch to the native fetch method which does not exhibit the above behaviour.

[Xmader]

It's because of what the polyfill does around XMLHttpRequest: Response object resolves on the XMLHttpRequest load event, i.e. the XMLHttpRequest must already complete successfully.

@JakeChampion Maybe changing it to first resolve when XMLHttpRequest.readyState is HEADERS_RECEIVED, which the headers and status code are available?
(This change would require a lot of refactoring around the Response class)