fetch RequestDuplex is "full" even set as "half"

[zizifn]

fetch spec say,

duplex
"half" is the only valid value and it is for initiating a half-duplex fetch (i.e., the user agent sends the entire request before processing the response). "full" is reserved for future use, for initiating a full-duplex fetch (i.e., the user agent can process the response before sending the entire request). This member needs to be set when body is a ReadableStream object. See issue #1254 for defining "full".

But seems to fetch from undici always use full, so does this is expected in undici / nodejs ?

Below is my test case in

nodejs v16.18.0
undici 5.12.0

Nodejs http server

import http from "node:http";

const requestListener = async function (req, res) {
  console.log("start");
  if (req.method === "POST") {
    let index = 0;
    res.writeHead(200);
    for await (const chunk of req) {
      console.log(`client send ${chunk.toString()}`);
      res.write(
        `server response ${index++}, after client send ${chunk.toString()}`
      );
    }
    res.end();
  } else {
    res.setHeader("Allow", "POST");
    res.writeHead(405).end("Method Not Allowed");
  }
};

const server = http.createServer(requestListener);
server.listen(8080);

Below is the fetch client

import {
  ReadableStream,
  TextDecoderStream,
  TextEncoderStream,
} from "node:stream/web";
import { fetch } from "undici";

try {
  const readableStream = new ReadableStream({
    index: 0,
    start(controller) {
      const interval = setInterval(() => {
        controller.enqueue(`client send ${this.index++}`);
      }, 500);

      setTimeout(() => {
        clearInterval(interval);
        controller.close();
      }, 10_000);
    },
  }).pipeThrough(new TextEncoderStream());

  const url = "http://localhost:8080";

  const resp = await fetch(url, {
    method: "POST",
    headers: {
      "Content-Type": "text/plain",
    },
    body: readableStream,
    duplex: "half",
  });

  for await (let chunk of resp.body.pipeThrough(new TextDecoderStream())) {
    console.log(chunk);
  }
} catch (err) {
  console.log(err);
}

Screen the test result

[ronag]

@KhafraDev I don't think undici currently even supports half duplex.

[zizifn]

@ronag update screen result about above code.

[KhafraDev]

Yeah, but the spec tells us to return half for the duplex option; I guess it's not meant to support full duplex just yet. Fetch should probably support half duplex and maybe go against the spec and add in a full option that maintains the current behavior?

[zizifn]

I like full, and spec also allows "full" for future use. Just want to confirm does this is expected in the future as nodejs will also have native fetch.
But this is totally deponed on you or nodejs team, if you or team decide to use full, please update doc, or I can help update doc?

[KhafraDev]

A doc change would be great for now, stating the the "duplex" option doesn't affect behavior currently & fetch requests are currently full duplex.

[zizifn]

PR for update doc... #1765