websocket: send callbacks are not all called if multiple messages are sent rapidly

[jonathanperret]

Describe the bug

When using the websocket transport, if socket.send() is called multiple times in the same tick with a callback provided, some of the callbacks may not be called even though the messages were sent.

To Reproduce

Engine.IO server version: 6.5.4

Server

const engine = require("engine.io");
const server = engine.listen(3000, { transports: ["websocket"] });

server.on("connection", (socket) => {
  console.log("connection");

  socket.send("1", ()=>console.log("callback 1 called"));
  socket.send("2", ()=>console.log("callback 2 called"));
  socket.send("3", ()=>console.log("callback 3 called"));
  socket.send("4", ()=>console.log("callback 4 called"));

  socket.on("close", () => {
    console.log("close");
  });
});

Engine.IO client version: 6.5.0

Client

const socket = new (require("engine.io-client").Socket)("ws://localhost:3000", { transports:["websocket"] });

socket.on("open", () => {
  console.log("open");

  socket.on("message", (data) => {
    console.log("data", data);
  });

  socket.on("close", () => {
    console.log("close");
  });
});

Expected behavior

The server above should log:

connection
callback 1 called
callback 2 called
callback 3 called
callback 4 called

Instead it logs:

connection
callback 1 called
callback 2 called

It does log callback 3 called after about 25 seconds, and callback 4 called 25 seconds later — enabling debug logs shows that ping packets are triggering those late callbacks.

The client logs that the messages are all received immediately.

Platform:

• Device: M1 MacBook Pro
• OS: macOS Sonoma 14.2

Additional context

There is an existing test that is supposed to check this scenario. However it does not expose this bug, because it does not specify a transport and defaults to polling, which also makes it redundant with the next test. That first test also only sends two messages, when the test program above shows that only the third and further callbacks are delayed.