unexpected behaviours between MockAgent and node's bundled fetch

Bug Description

Intercepting undici’s own fetch body works but node’s doesn’t. This is building on top of the global global dispatcher shipped with 5.2.0 which is then used by Node 18.2.0 bundled fetch.

Reproducible By

import * as undici from 'undici' // 5.2.0

const mockAgent = new undici.MockAgent()
mockAgent.disableNetConnect()
undici.setGlobalDispatcher(mockAgent)
const pool = mockAgent.get('https://example.com')

pool.intercept({
  path: '/path',
  method: 'POST',
  body(body) {
    console.log(body)
    return true
  }
}).reply(200, { ok: 1 }).times(2)

const url = new URL('https://example.com/path').href

// undici fetch v5.2.0 from node_modules
await undici.fetch(url, {
  method: 'POST',
  body: JSON.stringify({ foo: 'bar' })
})

// undici fetch v5.2.0 from node 18.2.0 global
await globalThis.fetch(url, {
  method: 'POST',
  body: JSON.stringify({ foo: 'bar' })
})

The above logs

{"foo":"bar"} // undici fetch v5.2.0 from node_modules
Object [AsyncGenerator] {} // undici fetch v5.2.0 from node 18.2.0 global

Expected Behavior

It is expected that the string body is passed into the matching function both times.

Environment

Node.js 18.2.0 Undici 5.2.0

[KhafraDev]

Yep, using global symbols (idk the correct term; using Symbol.for instead of Symbol) fixes the issues, although it would be great to remove the symbols altogether. I had two ideas regarding removing the symbol:

• Async mocking would allow us to resolve the async iterable body instead of using request.body.source.
• Check if the current agent is a MockAgent instead. Wouldn’t work; mocks could be active/inactive, still need to check the symbol.

I haven’t gotten around to working on either, though. I have a PR that changes the is mock active symbol to using Symbol.for but I haven’t written tests (no idea how to) and I haven’t checked if mock symbols are used elsewhere in fetch where it’d also cause issues.

edit: I found a solution without modifying the symbols - just need to make an active getter on MockAgent that returns this[kIsMockActive] and everything seems to work.

[mcollina]

go for the updated solution, I’ll ship a release early next week.