DispatchInterceptor with POST request doesn't work

[battall]

Bug Description

When a POST request is made using a dispatch interceptor and undici.DecoratorHandler, an error is thrown if the request includes a body. However, if the request does not include a body, no error is thrown.

/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/index.js:109
      Error.captureStackTrace(err, this)
            ^
TypeError: fetch failed
    at fetch (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/index.js:109:13) {
  cause: TypeError: this.handler.onBodySent is not a function
      at undiciInterceptorTesto.onBodySent (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/handler/DecoratorHandler.js:33:25)
      at Request.onBodySent (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/core/request.js:205:24)
      at AsyncWriter.write (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/client.js:1714:13)
      at writeIterable (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/client.js:1637:19)
}

Reproducible By

import * as undici from "undici";
import { fetch } from "undici";

const undiciInterceptorHeaders = (dispatch: any) => {
  class undiciInterceptorTesto extends undici.DecoratorHandler {
    onHeaders(statusCode: any, headers: any, resume: any) {
      // @ts-ignore
      return super.onHeaders(statusCode, headers, resume);
    }
  }
  return function InterceptedDispatch(opts: any, handler: any) {
    return dispatch(opts, new undiciInterceptorTesto(handler));
  };
};

let dispatcher = new undici.Agent({
  interceptors: {
    Client: [undiciInterceptorHeaders as unknown as undici.Dispatcher],
  },
});

fetch("https://postman-echo.com/post", {
  method: "POST",
  body: new URLSearchParams("amk=5&asdm=6"),
  // body: new URLSearchParams(""), // works fine
  dispatcher,
})
  .then((res) => res.json())
  .then((body) => {
    console.log(body);
  });

Expected Behavior

it should not throw error

Environment

Macos Sonoma Beta 4 Node v20.5.0

[battall]

after some fiddling, creating a empty onBodySent method on undici.DecoratorHandler makes things work, not sure why.

const undiciInterceptorHeaders = (dispatch: any) => {
  class undiciInterceptorTesto extends undici.DecoratorHandler {
    onHeaders(statusCode: any, headers: any, resume: any) {
      // @ts-ignore
      return super.onHeaders(statusCode, headers, resume);
    }
    onBodySent() {} // <---- here
  }
  return function InterceptedDispatch(opts: any, handler: any) {
    return dispatch(opts, new undiciInterceptorTesto(handler));
  };
};

looks like on request.js:201, even tho "this[kHandler]" doesnt have a onBodySent function, code sees it like there is one, black magic to me how this happens

onBodySent (chunk) {
    if (this[kHandler].onBodySent) {
      // i edited this line
      console.log(this[kHandler], this[kHandler].onBodySent)
      try {
        this[kHandler].onBodySent(chunk)
      } catch (err) {
        this.onError(err)
      }
    }
  }

undiciInterceptorTesto {
  handler: {
    body: null,
    abort: [Function (anonymous)],
    onConnect: [Function: onConnect],
    onHeaders: [Function: onHeaders],
    onData: [Function: onData],
    onComplete: [Function: onComplete],
    onError: [Function: onError],
    onUpgrade: [Function: onUpgrade]
  }
} [Function: onBodySent]

[KhafraDev]

https://github.com/nodejs/undici/blob/c83b084879fa0bb8e0469d31ec61428ac68160d5/docs/api/Dispatcher.md?plain=1#L214

it would appear as though either the docs are wrong or the code should work without an onBodySent, not entirely sure which is more correct.