Cannot capture the exception in case the server port is already occupied

[rkachach]

I'm submitting a ...

• bug report
• feature request
• question about the decisions made in the repository

Do you want to request a feature or report a bug?
BUG:

[27/May/2022:15:26:08] ENGINE Bus STARTING
[27/May/2022:15:26:08] ENGINE Started monitor thread 'Autoreloader'.
[27/May/2022:15:26:09] ENGINE Error in 'start' listener <bound method Server.start of <cherrypy._cpserver.Server object at 0x7f731a28ea40>>
Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/portend.py", line 122, in free
    Checker(timeout=0.1).assert_free(host, port)
  File "/usr/lib/python3.10/site-packages/portend.py", line 71, in assert_free
    list(itertools.starmap(self._connect, info))
  File "/usr/lib/python3.10/site-packages/portend.py", line 87, in _connect
    raise PortNotFree(tmpl.format(**locals()))
portend.PortNotFree: Port 2000 is in use on 192.168.100.1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/cherrypy/process/wspbus.py", line 230, in publish
    output.append(listener(*args, **kwargs))
  File "/usr/lib/python3.10/site-packages/cherrypy/_cpserver.py", line 180, in start
    super(Server, self).start()
  File "/usr/lib/python3.10/site-packages/cherrypy/process/servers.py", line 177, in start
    portend.free(*self.bind_addr, timeout=Timeouts.free)
  File "/usr/lib/python3.10/site-packages/portend.py", line 126, in free
    raise Timeout("Port {port} not free on {host}.".format(**locals()))
portend.Timeout: Port 2000 not free on 192.168.100.1.

[27/May/2022:15:26:09] ENGINE Shutting down due to error in start listener:
Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/cherrypy/process/wspbus.py", line 268, in start
    self.publish('start')
  File "/usr/lib/python3.10/site-packages/cherrypy/process/wspbus.py", line 248, in publish
    raise exc
cherrypy.process.wspbus.ChannelFailures: Timeout('Port 2000 not free on 192.168.100.1.')

[27/May/2022:15:26:09] ENGINE Bus STOPPING
[27/May/2022:15:26:09] ENGINE HTTP Server cherrypy._cpwsgi_server.CPWSGIServer(('192.168.100.1', 2000)) already shut down
[27/May/2022:15:26:09] ENGINE Stopped thread 'Autoreloader'.
[27/May/2022:15:26:09] ENGINE Bus STOPPED
[27/May/2022:15:26:09] ENGINE Bus EXITING
[27/May/2022:15:26:09] ENGINE Bus EXITED

What is the current behavior?
It's not possible to capture the exception when trying to bind the server in a port which is already occupied

If the current behavior is a bug, please provide the steps to reproduce and if possible a screenshots and logs of the problem. If you can, show us your code.

Run the following code twice: the first one to reserver the port (2000 in this case) and in the second one the code will fail to capture the exception.

import cherrypy

class App1(object):
    @cherrypy.expose
    def index(self):
         return ('server1')

cherrypy.server.unsubscribe()

server2 = cherrypy._cpserver.Server()
server2.socket_port=2000
server2._socket_host="192.168.100.1"
server2.thread_pool=2
server2.subscribe()

try:
    cherrypy.engine.start()
    cherrypy.engine.block()
except Exception as e:
    print(f"Exception: {e}")

What is the expected behavior?
Be able to capture the exception

What is the motivation / use case for changing the behavior?
Detect the case when a port is already in use and try another one.

Please tell us about your environment:

• Cheroot version: 8.5.2
• CherryPy version: 18.6.1
• Python version: 3.6.X
• OS: Fedora 35