Serve will do its job even if stop was called before

[kasium]

❓ I'm submitting a ...

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

🐞 Describe the bug. What is the current behavior?
If stopped is called on a preparing server, the server won't stop if it's reaches the serve state.

❓ What is the motivation / use case for changing the behavior?
I need a cheroot server which starts in another thread. Sometime it can happen that a server hangs during the prepare, e.g. due to a socket error. Therefore the "parent" thread will then stop the server and retry later.

💡 To Reproduce
Of course this will not happen in the real world like this. I used events to showcase the issue in a reproducible way:

from flask import Flask
from cheroot.wsgi import Server
from threading import Thread, Event

class WaitServer(Server):
    def bind_unix_socket(self, *args, **kwargs):
        print("Set bind_called_event")
        bind_called_event.set()
        print("Wait for stopped_event")
        stopped_event.wait()
        print("Exec bind_unix_socket")
        super().bind_unix_socket(*args, **kwargs)

def _run():
    print("[T] Start server")
    server.start()

app = Flask(__name__)
server = WaitServer(bind_addr="localhost", wsgi_app=app, numthreads=1)
thread = Thread(target=_run)
bind_called_event = Event()
stopped_event = Event()

print("Starting thread")
thread.start()
print("Wait for bind_called_event")
bind_called_event.wait()

server.stop()
print("Set stopped_event")
stopped_event.set()
print("Wait for thread")
thread.join()
print("Stopped everything")

💡 Expected behavior
When stopped is called, the server stops as soon as possible

📋 Details
The issue can be also be fixed, by using interrupt instead of stop.

from flask import Flask
from cheroot.wsgi import Server
from threading import Thread, Event

class WaitServer(Server):
    def bind_unix_socket(self, *args, **kwargs):
        print("Set bind_called_event")
        bind_called_event.set()
        print("Wait for stopped_event")
        stopped_event.wait()
        print("Exec bind_unix_socket")
        super().bind_unix_socket(*args, **kwargs)

def _run():
    print("[T] Start server")
    try:
        server.start()
    except Exception:
        pass


app = Flask(__name__)
server = WaitServer(bind_addr="localhost", wsgi_app=app, numthreads=1)
thread = Thread(target=_run)
bind_called_event = Event()
stopped_event = Event()

print("Starting thread")
thread.start()
print("Wait for bind_called_event")
bind_called_event.wait()

server.interrupt = Exception("stopping")
print("Set stopped_event")
stopped_event.set()
print("Wait for thread")
thread.join()
server.stop()
print("Stopped everything")

However, the code get's more complicated:

• try/except when usign start
• an exception needs to be used in the first place
• server.stop needs to be called again to shutdown workers

If the last server.stop is not used, the program is stucked again. E.g. with py-spy:

Process 12334: python bar.py
Python v3.7.1 (/usr/bin/python3.7)

Thread 12334 (idle): "MainThread"
    _wait_for_tstate_lock (threading.py:1048)
    join (threading.py:1032)
    _shutdown (threading.py:1273)
Thread 12346 (idle): "CP Server Thread-2"
    wait (threading.py:296)
    get (queue.py:170)
    run (cheroot/workers/threadpool.py:110)
    _bootstrap_inner (threading.py:917)
    _bootstrap (threading.py:885)

📋 Environment

• Cheroot version: 9.0.0
• CherryPy version: n/a
• Python version: 3.7.1
• OS: Linux
• Browser: n/a