[engine.io-client] polling transport: the long-polling request stays open after the socket is closed #5628

Describe the bug

When the polling transport is closed, doClose() only sends a close packet (one more POST request). It does not abort the requests that are still pending, like the long-polling GET request (BaseXHR keeps it in this.pollXhr, but does not use it).

With an Engine.IO server, this is not visible: when the server receives the close packet, it answers the pending GET request. But when the GET request does not get an answer (a proxy that holds it, or a request on a connection that is no longer usable, for example after a network change), the request stays open after the close event. It holds a connection (in a browser, one of the few connections per origin), and in Node.js the process does not exit.

To Reproduce

Engine.IO client version: 6.6.8 (also present on main)

The server is a small HTTP server that answers the handshake and the POST requests, but never answers the long-polling GET request. The client closes the socket 100 ms after it is open, while the long-polling request is pending. The repro uses the Fetch transport (see the note below about xmlhttprequest-ssl).

package.json:

{
  "private": true,
  "type": "module",
  "dependencies": {
    "engine.io-client": "6.6.8"
  },
  "devDependencies": {
    "@types/node": "24.19.2",
    "typescript": "7.0.2"
  }
}

repro.ts:

// npm install, then: node repro.ts (Node.js 22.18 or later runs TypeScript directly)
import { createServer } from "node:http";
import type { AddressInfo } from "node:net";
import { Fetch, Socket } from "engine.io-client";

let pending = false;

// A server (or a proxy) that answers the POST requests, but never answers the long-polling GET request.
const httpServer = createServer((req, res) => {
  if (req.method === "POST") {
    req.resume();
    req.on("end", () => res.end("ok"));
    return;
  }
  if (!new URL(req.url ?? "/", "http://x").searchParams.has("sid")) {
    res.end('0{"sid":"abc","upgrades":[],"pingInterval":10000,"pingTimeout":5000,"maxPayload":1000000}');
    return;
  }
  pending = true;
  console.log("server: long-polling request received (never answered)");
  res.on("close", () => {
    pending = false;
    console.log("server: long-polling request closed");
  });
});

httpServer.listen(0, "127.0.0.1", () => {
  const { port } = httpServer.address() as AddressInfo;
  const socket = new Socket(`http://127.0.0.1:${port}`, { transports: [Fetch] });
  // Close the socket while the long-polling request is pending.
  socket.on("open", () => setTimeout(() => socket.close(), 100));
  socket.on("close", (reason) => console.log(`client: close "${reason}"`));

  setTimeout(() => {
    console.log(`after 2 s, the long-polling request is still open: ${pending}`);
    process.exit(pending ? 1 : 0);
  }, 2000);
});

npm install, then node repro.ts (Node.js 22.18 or later runs TypeScript directly). Output:

server: long-polling request received (never answered)
client: close "forced close"
after 2 s, the long-polling request is still open: true

The server never sees the long-polling request closed: it stays open after the close event, until process.exit().

Expected behavior

Once the close packet is sent, the pending requests are aborted, and the server prints server: long-polling request closed.

Platform:

• Device: a GitHub Actions runner
• OS: Linux, Node.js 24.22.2

Additional context

https://github.com/socketio/socket.io/blob/engine.io-client%406.6.8/packages/engine.io-client/lib/transports/polling.ts#L122-L137
https://github.com/socketio/socket.io/blob/engine.io-client%406.6.8/packages/engine.io-client/lib/transports/polling-xhr.ts#L82

The requests must not be aborted before the server receives the close packet: if the server sees the GET request closed first, it closes the session with the reason transport error instead of transport close.

Note: with the default polling transport of Node.js (xmlhttprequest-ssl), a pending request cannot be aborted today: from version 2.1.2 (the version that ~2.1.1 installs), abort() on a pending request throws an uncaught TypeError: Cannot read properties of null (reading 'reusedSocket') (mjwwit/node-XMLHttpRequest#37).

A pull request with a fix and a test follows: the pending requests are aborted once the close packet is sent (or cannot be sent), with the XMLHttpRequest object of a browser and with the Fetch transport.

I found this with AI assistance (Claude) during a review of WebSocket client libraries. The repro above was run with engine.io-client 6.6.8 on Node.js 24.22.2 on Linux (a GitHub Actions runner), and the output above is from that run.