Comment 25 for bug 1654116

Re: comment #21 and gating; if this is fully async, then gating is only going to tighten the race, but not eliminate it. i.e. code like:

    if is_leader():
        leader_set(...)

can still error as the leadership could be lost between the two calls. The only safe way of doing leadership code is probably something like:

    if is_leader():
        try:
            # do things you have to do before calling leader_set()
            leader_set(...) # if this passes, we are the leader
            # do other things that depend on being the leader
        except NotImplementedError: # because that's what bizarrely is returned by charmhelpers
            # we're not the leader after all (or something else ... can't really tell)
            if not is_leader():
                # we're not the leader any more, undo any damage
            else:
                raise # something else went wrong
        except OtherErrorsTheCodeMayHaveThrownIfWeAreInterested:
            # etc

Anyone spot any problems? If this is a pattern, we should try to get it into charmhelpers as a context manager or similar.