gevent.pywsgi fails to inform which random port it uses when given port 0

[hcs-catchscan]

Environment

• gevent version: 23.9.1 from pypi installed using pip in venv
• Python version: 3.11.4 installed from Ubuntu main repos
• Operating System: Ubuntu 23.04 on x86-64 running kernel 6.2.0-33-generic

Description:

I'm trying to accommodate the faily simple use case of printing which port a server is running on on startup. This works completely fine when you tell gevent which port to use, but fails when gevent picks a random port.
I suspect the reason is that gevent waits until the serve_forever call to call bind, meaning it's not available ahead of time.

When telling gevent to bind on port 0 and then asking what port it has bound to it reports 0, which is verifiably false. Running ss -putan | grep python after starting the server reveals that gevent is listening on a random high-range port (as it should).

WSGI	Provided Port	Random Port
Werkzeug	✔️	✔️
gevent	✔️	❌

What I've run:

The following snippet should highlight the issue. When running the script, note that you will have to issue keyboard interrupts to enumerate the 4 servers. (Press CTRL+C four times)

Requires: gevent flask

from flask import Flask
from gevent.pywsgi import WSGIServer
from werkzeug.serving import make_server


def __build_app() -> Flask:
    app = Flask(__name__)
    @app.route('/')
    def hello(): return 'Hello World'
    return app


def demo_werkzeug(port: int):
    server = make_server('127.0.0.1', port, __build_app())
    print(f'Werkzeug server running on {server.server_name}:{server.server_port}')
    server.serve_forever()


def demo_gevent(port: int):
    server = WSGIServer(('127.0.0.1', port), __build_app())
    print(f'gevent server running on {server.server_host}:{server.server_port}')
    try:
        server.serve_forever()
    except KeyboardInterrupt:
        pass


if __name__ == '__main__':
    demo_werkzeug(8080)  # Works
    demo_gevent(8080)  # Works
    demo_werkzeug(0)  # Works
    demo_gevent(0)  # Doesnt work

The console output should look something like this:

Werkzeug server running on localhost:8080
gevent server running on 127.0.0.1:8080
Werkzeug server running on localhost:37619
gevent server running on 127.0.0.1:0