Description of problem:
Due to the unordered nature of sets, it is possible for signal handlers like _gracefull_shutdown() to run first and clear the _signal_handlers before other registered handlers have run.


Version-Release number of selected component (if applicable):
17.1.3

How reproducible:
Randomly reproducible with a service launched with ServiceLauncher-launched service that calls SignalHandler.add_handler()

Steps to Reproduce:

import eventlet; eventlet.monkey_patch()

import os
import signal
import sys
import time

from oslo_config import cfg
from oslo_service import service


def one(*args):
    print("one")


def two(*args):
    print("two")


def three(*args):
    print("three")


class MyService(service.Service):
    def start(self):
        sh = service.SignalHandler()
        sh.add_handler("SIGTERM", one)
        sh.add_handler("SIGTERM", two)
        sh.add_handler("SIGTERM", three)

    def wait(self):
        os.kill(os.getpid(), signal.SIGTERM)


class MySL(service.ServiceLauncher):
    def _graceful_shutdown(self, *args, **kwargs):
        print("graceful")
        res = super()._graceful_shutdown(*args, **kwargs)
        return res


s = MyService()
l = MySL(cfg.CONF)
l.launch_service(s)
l.wait()




Actual results:
Signals are executed in an undefined order.

Expected results:
Signals are executed in a defined order, LIFO. (three, two, one, graceful)

Additional info: