Awesome Ruby Synchronization Behaviors

browser.rb

class Browser
   CreationMutex = Mutex.new
   def _create_monitor_synchronized
     if ((not defined?(@monit)) or @monit.nil?)
       CreationMutex.synchronize do
         if ((not defined?(@monit)) or @monit.nil?)
           puts "Thread #{Thread.current.object_id} created @monit"
           @monit = Object.new
           @monit.extend(MonitorMixin)
         end
       end
     end
     @monit
   end

   def synchronize(&block)
     _create_monitor_synchronized if ((not defined?(@monit)) or @monit.nil?)
     @monit.synchronize(&block)
   end

   alias original_command command
   def command(*args)
     res = nil
     synchronize do
       puts "Thread #{Thread.current.object_id} running #{args[0]}"
       res = original_command(*args)
       puts "Thread #{Thread.current.object_id} exiting monitor"
     end
     res
   end
 end

program.out

Thread 18826100 created @monit
Thread 18826100 running current_url
{"name"=>"current_url", "args"=>[]}
Thread 18820220 created @monit
Thread 18820220 running current_url
{"name"=>"current_url", "args"=>[]}
{"response"=>"about:blank"}
Thread 18820220 exiting monitor

*BOOM* blowup on the websocket server from unsynchronized socket reads. 

Exception: negative string size (or size too big)
/.../poltergeist-0.7.0/lib/capybara/poltergeist/web_socket_server.rb:95:in `<<'
/.../poltergeist-0.7.0/lib/capybara/poltergeist/web_socket_server.rb:95:in `accept'
/.../poltergeist-0.7.0/lib/capybara/poltergeist/web_socket_server.rb:143:in `send'
/.../poltergeist-0.7.0/lib/capybara/poltergeist/server.rb:29:in `send'