streams: stream._readableState is undefined on every Readable (undici's BodyReadable writes _readableState.dataEmitted) #11197

stream._readableState is undefined on every Readable in Perry: a plain new Readable(), a bare class X extends Readable {}, and a subclass reading it in its own constructor after super(). Node gives every Readable a ReadableState object, and a lot of real stream code reads and writes it directly.

This is the next undici blocker (#11046). With #11176, #11194 and the #11193 fix, the real undici 8.9.0 fixture's first request() now gets a response and builds its body. undici's BodyReadable (lib/api/readable.js) then runs this._readableState.dataEmitted = false in its constructor, and request() rejects with:

TypeError: Cannot set properties of null or undefined (setting 'dataEmitted')

undici also reads _readableState.endEmitted, .closeEmitted, .ended, .destroyed, .encoding and .length (lib/api/readable.js, lib/api/api-pipeline.js, lib/core/util.js, lib/core/request.js). So a static placeholder would move the failure, not fix it: these need to reflect live stream state.

Repro (no packages)

import { Readable } from "node:stream";
class Body extends Readable {
  constructor(opts: any) {
    super({ autoDestroy: true, read: opts.resume, highWaterMark: 64 * 1024 });
    const rs = (this as any)._readableState;
    console.log("in ctor:", typeof rs);
  }
}
const b = new Body({ resume() {} });
console.log("after ctor:", typeof (b as any)._readableState);
const plain = new Readable({ read() {} });
console.log("plain Readable:", typeof (plain as any)._readableState, (plain as any)._readableState && (plain as any)._readableState.highWaterMark);
class Bare extends Readable {}
console.log("bare subclass:", typeof (new Bare() as any)._readableState);

Node 26.5.1:

in ctor: object
after ctor: object
plain Readable: object 65536
bare subclass: object

Perry origin/main 2762714 (+ the #11193 fix; perry-dev, Linux x64, both PERRY_NO_AUTO_OPTIMIZE=1 and auto-optimize, .ts and .cts):

in ctor: undefined
after ctor: undefined
plain Readable: undefined undefined
bare subclass: undefined

Notes

Perry's native streams keep their state internally (node_stream*.rs). There is already a serialized _readableState in node_stream_json.rs, but no live object on the instance. A fix probably needs _readableState (and _writableState, which undici's api-stream.js reads via ?.) as an accessor-backed view over the internal state, with at least the fields above live. The dataEmitted write must be accepted too.