[ading2210]

This PR adds new features to the POSIX socket emulation over WebSockets. This is a very small change that allows for a great deal of flexibility in Emscripten's networking.

It adds the ability to change the WebSocket constructor to any custom class/function, by changing Module['websocket']['constructor'];. For example:

Module['websocket']['constructor'] = CustomWebSocket;

Or if we want to control the arguments passed into the constructor:

const custom_net_options = {...};
Module['websocket']['constructor'] = function() {
  return new CustomWebSocket(custom_net_options);
}

This lets us change the underlying network transport for the socket to whatever we want, so long as a WebSocket-compatible class with the same API surface is used. For instance, you could create a wrapper for WebTransport data channels, WebRTC data channels, or direct sockets.

This PR also adds the ability to make Module['websocket']['url'] a function rather than a string, which maps the hostname and port to a URL. For instance, you could use:

Module['websocket']['url'] = (host, port) => `wss://example.com/proxy/${host}:${port}`;

In this example, it would cause every TCP socket opened to create a new WebSocket with the hostname and port in the URL path.

I'm making this change because I've encountered a lot of uses cases where I'd want to write a custom network transport for Emscripten programs. I'd usually have to intercept the global window.WebSocket object or patch Emscripten's emitted JS code. I could avoid all of that if the features described in this PR are merged.

[gemini-code-assist]

Code Review

This pull request adds support for dynamically resolving the WebSocket URL via a function and customizing the WebSocket constructor at runtime, updating both the implementation in libsockfs.js and the documentation. The review comments point out a critical issue where runtime changes to Module['websocket'] may not be detected if it was not defined at startup due to decoupling from SOCKFS.websocketArgs. It is recommended to use Module['websocket'] || SOCKFS.websocketArgs to correctly resolve these properties at runtime.

[ading2210]

All the Gemini comments have been resolved.