Socket server tries to close external http server, causing `ERR_SERVER_NOT_RUNNING` error

Describe the bug

When you initialize socket server, you can either give it an external instance of HTTPServer, or let socket.io create its own internal instance.

However, in either case, SocketIO tries to close the server when shutting down:

https://github.com/socketio/socket.io/blob/3b7ced7af7e0a2a66392577f94af1ee5ed190ab1/lib/index.ts#L685

In my case, I have my own shutdown procedure elsewhere, and it causes server to throw ERR_SERVER_NOT_RUNNING error, because server.close() was called twice.

This might be appearing now due to new stricter behavior in node.js, I am not sure. This is running on v14.17.3.

Expected behavior

IMO socket.io should only close HTTP server if it's its own instance. If an instance is provided, it's someone else's responsibility to close that server. Socket.IO shouldn't interfere.

Mitigation

I am currently using this hack to prevent the uncaught exception:

ioServer['httpServer'] = null;
ioServer.close();

Start in lib/index.ts around line 685 and inspect the shutdown path for externally supplied HTTPServer instances. Reproduce the double-close scenario described in the issue, then verify that an externally owned server is not closed by Socket.IO while an internally created server still shuts down correctly.