Support for listening on random port and opening browser without delays

[spanezz]

Hello,

I wanted to support opening a browser on a random local port, and got it working with a bit of monkeypatching:

        server = make_server(…)
                
        import tornado.web
        import tornado.httpserver
        import tornado.netutil                   
        import webbrowser
                    
        class Application(tornado.web.Application):
            def listen(self, *args, **kw):
                sockets = tornado.netutil.bind_sockets(0, 'localhost')
                server = tornado.httpserver.HTTPServer(self)
                server.add_sockets(sockets)
                pairs = []
                for s in sockets:        
                    pairs.append(s.getsockname()[:2])
                pairs.sort()
                host, port = pairs[0]
                if ":" in host:    
                    host = f"[{host}]"    
                url = f"http://{host}:{port}"    
                log.info("Listening on %s", url)    
        
                async def open_browser():        
                    webbrowser.open_new_tab(url)     
 
                tornado.ioloop.IOLoop.current().add_callback(open_browser)    
                return server

        # Monkey patch to be able to start servers in a smoother way that the
        # defaults of livereload
        import livereload.server
        livereload.server.web.Application = Application
        server.serve()

Integrating this in livereload seems to require some refactoring of how the server is started, and I didn't engage on it. I hope that my monkeypatched code can be a useful reference.

Ideally, I would have liked to be able to subclass Server or the Application used by server, and have enough hooks to intervene in the listening bit.