https://review.openstack.org/#/c/223066/1 -- guru_meditation_report: Use
SIGUSR2 instead of SIGUSR1

[Daniel Berrange]

    I'd really be strongly against having applications pass in the
    required signal, because I think it is important that we have a
    consistent signal used across all openstack projects. If we have
    different signals in different projects, then you increase the
    complexity for admins by making them remember different signals for
    each process. This in turn *will* lead to admin mistakes where they
    send the wrong signal, which will lead to the app taking the wrong
    action.


[From Joshua Harlow]:

    What about something like:
    
    http://paste.openstack.org/show/465087/
    
      import signal
      
      def _try_set_signal(func, signum, *signums):
         sigs_to_try = [signum]
         sigs_to_try.extend(signums)
         for signum in sigs_to_try:
             sig_handler = signal.getsignal(signum)
             if sig_handler is None or sig_handler in (signal.SIG_DFL, signal.SIG_IGN):
                signal.signal(signum, func)
                return signum
         raise ValueError("Unable to register %s, tried %s" % (func, sigs_to_try))
      
      
      def a():
        pass

      signum_registered = _try_set_signal(...)
      LOG.info("YOUR SIGNAL IS %s", signum_registered)

    The _try_set_signal would be given [function, signum(provided by
    user if not None), SIGUSR1, SIGUSR2], if none of those work this
    function will raise a value error (which can be captured if wanted?
    or logged? or re-raised or something). That way the existing API
    seems to work as expected and if someone else is using the mentioned
    signals, then it will not register and the caller can know about
    that (vs overwriting existing handlers which seems sorta bad).

Related reading:
https://docs.python.org/2/library/signal.html#signal.getsignal


[From Julien Danjaou]:

I'm gonna propose my own way of doing it: having an oslo.config option
defined here that indicates the default value of SIGUSR2.

That means:

    It works with mod_wsgi by default (like the current patch)
    It can be changed by the operator as needed
    It's visible in the default generated configuration file
    All OpenStack projets have the same default

Bonus point if the actual config value is validated at runtime by
Joshua's code so the operator has a feedback that it's not going to
work. (Though I'm pretty sure there are corner cases where other part of
the stack could erase the signal defined by guru meditation, but let's
not think about it.)