Ruby chat server

chat.rb

#!/usr/bin/env ruby

# Run with
# $ chmod +x chat.rb && ./chat.rb
# or
# $ ruby chat.rb

require 'socket'
require 'thread'

class User
    attr_accessor :name, :socket, :thread
    def initialize(name, socket, thread)
        @name = name
        @socket = socket
        @thread = thread
    end
end

$users = []
$users_mutex = Mutex.new
PORT = 4625

server = TCPServer.new PORT
puts "[#{Time.now}]: Server started...\nListening on port #{PORT}\n"
loop do
    Thread.start(server.accept) do |client|
        client.puts "Welcome to AkChat!"
        client.puts "Please login!\n"
        client.write "Login: "
        username = client.gets.chomp
        u = User.new(username, client, Thread.current)
        $users_mutex.synchronize do
            $users.each do |us|
                if us.name == username
                    client.puts "This username is already taken."
                    client.close
                    Thread.stop
                else
                    us.socket.puts "\n### #{username} joined to the conversation ###"
                    us.socket.write "[#{us.name}]: "
                end
                
            end
            $users << u
            client.puts "Welcome aboard #{username}!"
            puts "[#{Time.now}]: User #{username} signed in..."
            if ($users.count > 1)
                client.puts "We have #{$users.count} users logged in:"
                $users.each do |us|
                    client.puts "\t#{us.name}"
                end
            end
        end
        loop do
            client.write "[#{username}]: "
            msg = client.readline.chomp
            if (msg == "quit" || msg == "q")
                $users_mutex.synchronize do
                    $users -= [u]
                end
                client.close
                puts "[#{Time.now}]: #{username} quited from chat"
                $users_mutex.synchronize do
                    $users.each do |us|
                        us.socket.puts "\n### #{username} quited from the conversation ###"
                        us.socket.write "[#{us.name}]: "
                    end
                end
                Thread.kill(Thread.current)
            end
             $users_mutex.synchronize do
                $users.each do |us|
                    if (us.name != username)
                        puts "[#{Time.now}]: Broadcasting message '#{msg}' from #{username} to #{us.name}."
                        us.socket.puts "\n[#{username}]: #{msg}"
                        us.socket.write "[#{us.name}]: "
                    end
                end
            end
        end
    end
end

njnj