fix: RTCDataChannel readyState race condition and improve typings

[mertushka]

As identified here: #326 (comment) and mertushka/haxball.js#64 (comment)

This fixes a race condition where <RTCDataChannel>.readyState could remain stuck in 'open' even after the underlying native DataChannel was closed. It also improves typings a bit.

[mertushka]

Test Case

import { RTCPeerConnection } from "node-datachannel/polyfill";

const peer1 = new RTCPeerConnection();
const peer2 = new RTCPeerConnection();

peer2.addEventListener("datachannel", (e) => {
  const channel = e.channel;
  channel.binaryType = "arraybuffer";
  channel.addEventListener("message", (evt) => {
    // echo back
    if (channel.readyState === "open") channel.send(evt.data);
  });
});

const channel = peer1.createDataChannel("race");
await connectPeers(peer1, peer2);
await waitForOpen(channel);

let tick = 0;
let sent = 0;
let interval;

channel.binaryType = "arraybuffer";

// Start sending every tick
interval = setInterval(() => {
  console.log("readyState: ", channel.readyState);
  if (channel.readyState === "open") {
    try {
      channel.send(new Uint8Array(128));
      sent++;
    } catch (err) {
      console.error(`Tick ${tick}: Send failed despite readyState === "open"`);
      console.error("Error:", err.message);
      clearInterval(interval);
      cleanup();
    }
  } else {
    console.warn(`Tick ${tick}: Not open, readyState is`, channel.readyState);
    clearInterval(interval);
    cleanup();
  }

  tick++;

  // Simulate sudden closure of peer2 after a few ticks
  if (tick === 200) {
    console.warn(">> Closing peer2 abruptly");
    peer2.close();
  }
}, 0);

function cleanup() {
  console.log(`Sent ${sent} messages before failure`);
  peer1.close();
}

async function connectPeers(peer1, peer2) {
  const offer = await peer1.createOffer();
  await peer2.setRemoteDescription(offer);

  const answer = await peer2.createAnswer();
  await peer1.setRemoteDescription(answer);

  peer1.addEventListener("icecandidate", (e) => {
    peer2.addIceCandidate(e.candidate);
  });

  peer2.addEventListener("icecandidate", (e) => {
    peer1.addIceCandidate(e.candidate);
  });

  await Promise.all([waitForConnection(peer1), waitForConnection(peer2)]);
}

function waitForConnection(peer) {
  return new Promise((resolve, reject) => {
    peer.addEventListener("connectionstatechange", () => {
      if (peer.connectionState === "connected") resolve();
      if (peer.connectionState === "failed") reject(new Error("ICE failed"));
    });
  });
}

function waitForOpen(dc) {
  return new Promise((resolve, reject) => {
    if (dc.readyState === "open") return resolve();
    dc.addEventListener("open", () => resolve());
    dc.addEventListener("error", () => reject(new Error("DC failed")));
  });
}

Before Fix

readyState: open
>> Closing peer2 abruptly
readyState: open
❌ Error: libdatachannel error while sending data channel message: DataChannel is closed

After Fix

readyState: open
>> Closing peer2 abruptly
readyState: closed
✅ Tick 200: Not open, readyState is closed

[mertushka]

@murat-dogan This is ready to merge after you review.