supplement open--port/close-port with ensure-these-and-only-these-ports?

[Aaron Bentley]

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Hi all,

I take a stateful approach to writing charms.  That is, my charms
determine what the target state is, then take whatever actions are
needed to get the unit into that state, paying as little attention to
the current state as is possible.

open-port / close-port require knowledge of the current state; if I
know that I want only port 314 open, then I need to know whether any
other ports are open and close them.  In most cases, a charm only
opens specific ports, so I know which ports to close.

Right now, I'm writing an update to the Apache2 charm that would allow
the user to specify which ports to serve http on, which means that
when a user changes the port, I may need to close the old port and
open the new one.  If I want to use close-port / open-port, I need to
track what ports are open.  But juju already knows this, so I
shouldn't have to track it separately-- that violates DRY.

The smallest change would be to provide a way to list the open ports,
so that charms can close any open ports they no longer want open.  But
that leaves a bunch of work for a stateful charm author.  What they
actually want is a command that ensures specific ports are open and
closes all others.

ensure-these-and-only-these-ports was the first thing I thought of,
but we could extend open-port instead.  open-port would need to accept
multiple ports, not just ranges, and it would need to accept a
- --close-all-others flag, that would close all open ports not listed.

Does that seem like a sensible change?

Aaron
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