fetch: using for await with .body does not throw error after being consumed

[KhafraDev]

Bug Description

Using for await (const whatever of response.body) {...} you can kinda-kinda not consume a body multiple times.

Reproducible By

import { fetch } from 'undici'

const response = await fetch('https://www.google.com')

for await (const chunk of response.body) {}
for await (const chunk of response.body) {}

Expected Behavior

I would consider any output other than throwing an error a bug. For all body mixin methods, an error is thrown when consuming the body more than once. Currently the second iterator won't return anything which is also very confusing behavior.

I guess something along the lines of:

const readable = new ReadableStream({
  start () {},
  pull () {}
})

readable[Symbol.asyncIterator] = undefined

This will match browser behavior by causing an error to be thrown when using it in a for await loop like browsers.

Or it could be reimplemented to include a body used check and throw an error if its been consumed?

Logs & Screenshots

Environment

Additional context

Node implements an async iterator onto the web ReadableStream, unlike browsers.

Run in repl/browser console:

ReadableStream.prototype[Symbol.asyncIterator]

This is where a ReadableStream is constructed:

undici/lib/core/util.js

Line 351 in 674875a

function ReadableStreamFrom (iterable) {

I didn't really know what to do with this one. Browsers (Firefox & chrome) throw an error but deno has the same "buggy"/confusing behavior as node.

[KhafraDev]

So the stream spec was updated to include async iterators in whatwg/streams#980, which means that we can't override the async iterator implementation, or get rid of it completely.

I opened an issue on the spec repo for this at whatwg/fetch#1488 which seems like the more applicable place to post this.