Re: Turing the NM UI into an agent

[Dan Williams <dcbw redhat com>]

On Wed, 2008-03-26 at 19:17 +0100, Marcel Holtmann wrote:
> Hi Dan,
> 
> >> since you are re-working most of the D-Bus APIs based around Network
> >> Manager, I like to propose a radical change on how you handle the UI
> >> part. Personally I think that the current UI inside Network Manager
> >> (even with 0.7) has too much tasks to fulfill. I prefer having a
> >> "stupid" UI.
> >>
> >> So my main concern is that replacing the current UI is too
> >> complicated. One reason is that you need a well known bus name for  
> >> the
> >> UI on the system bus. This makes it impossible to install two UI
> >> components or split tasks between two applications. We did the same  
> >> in
> >> the early days of BlueZ and I think it is not a good design to hard-
> >> code a well known bus name into the daemon and have the UI use the
> >> same one. So for BlueZ we came up with the D-Bus agent concept. So  
> >> let
> >> me try to explain it.
> >>
> >> So the UI only gets a unique name (no need for a D-Bus security  
> >> file).
> >> Then it registers itself a object path (the actual path value is
> >> unimportant). After that it calls a RegisterAgent(object path) method
> >> inside the daemon. Now the daemon knows where to find an agent  
> >> (object
> >> path and the unique name from the message sender value) and can call
> >> into this object for additional information. It really serves as a
> >> callback interface and the only things to agree on is the interface
> >> description. The bus name and object path are kept random.
> >>
> >> This concept allows an easy way of monitoring agents. So you know the
> >> case when you have no agent (meaning no UI is running) or when an
> >> agent dies. No unneeded D-Bus calls that might fails etc. The other
> >> advantage is that in theory you can stack agents and call them in
> >> order. This is useful if you have a wizard or some small cases where
> >> the UI should look different from the general case. Or unregister and
> >> then re-register agents. For example for multi user scenarios.
> >
> > There's already a split between the UI bits and the configuration  
> > bits;
> > I think what you'd like to see is the User Settings service grab  
> > unique
> > bus names so that each user could provide their own user settings at  
> > the
> > same time to NetworkManager.  I'd like to go there in the future, but
> > then we need to figure out things that ConsoleKit will help with, but
> > which are still very much undefined, stuff like:
> >
> > - If user A has activated a connection on eth0, is user B allowed to
> > control eth0 and potentially tear down user A's connection?
> >
> > - If user A has activated eth0, is user B allowed to start a VPN
> > connection on eth0?
> >
> > Stuff like that.  There might be uses for this sort of thing in
> > virtualization actually, but it's kind of hard to see networking as  
> > the
> > same sort of thing as Multi-User X, for example, where different users
> > get exclusive access to individual mice, keyboards, displays, and USB
> > flash drives.
> 
> I wasn't talking on these kind of things. They are totally different.
> 
> > Right now the only thing that acquires a well-known bus name is the
> > settings service, which doesn't have to have an UI at all.  Up until  
> > NM
> > 0.5 it actually was a completely separate process from the applet.    
> > If
> > your user has permissions to do so at the D-Bus level (ie,  
> > at_console or
> > whatever), then any process can call
> > ActivateConnection/DeactivateConnection right now.  But only 1 process
> > can provide the configuration data, which limits the set of what
> > ActivateConnection can actually connect to.
> 
> I meant the settings service. Still need to get deeper into the  
> Network Manager terminology. So having a "static" settings service is  
> a bad design (from my perspective). Turning the settings service into  
> agent would be a good thing. From the logical point of view it is not  
> really different, but from a technical point of view it makes all the  
> difference.

Well, the point of having only one settings service was security.  You
don't want any service providing connections to NM, because then
anything can send "allowed" networks to NM.  This is made somewhat less
of an issue by PolicyKit, but still an issue.

Some trojan could start providing bogus networks to NetworkManager via
the settings service, which NM would then happily connect to.  With one
user settings service claiming the bus name, only one program can
provide those settings to NM, so the user is a lot more likely to notice
that something is wrong (unless the trojan duplicates the look & feel of
the user's expected applet).

I guess I simply don't see a case for more than one settings service
_per_user_ providing network connection information to NetworkManager.
There's a case for more than one in a fast-user-switching system though.

> So I see the UI as something totally "stupid" that the daemon tells it  
> what to do and the user can use it to influence settings inside the  
> daemon. Besides that both are loosely coupled.

The UI bits are somewhat decoupled already, it just happens that it's
convenient to have them in the same process right now.  When they were
split into nm-applet and NetworkManagerInfo, it wasn't very useful
abstraction.

> An example would be that the daemon needs a passphrase for a network.  
> So it calls a callback inside the UI agent. This call should always go  
> to the logged in user and thus to the applet run by that user. Any  
> other callback into the UI (or settings service) should be like this.

Well, that assumes only one user is logged in :)  Fast user switching is
a bit different here.  We need integration with ConsoleKit to figure out
where this stuff should go.  Besides, the password information is
provided by the settings service, which needs to grab that password from
the keyring.  No service _other_ than the settings service should be
providing information to NetworkManager, and NM should only be accepting
passwords and such from the settings service that provided the
connection which is now being activated.

> So big technical advantage here is that the daemon always knows if an  
> agent is present or not. And if not, it can do a proper fallback.  
> Calling a D-Bus method that fails is a bad thing. Also the fact that  
> you don't have to agree on a well known name and object path makes  
> this a lot cleaner. The D-Bus interface for the agent should be the  
> only contract between the UI and the daemon. The rest a variable  
> details.

I don't disagree in principle here.  Right now the UI and settings
service (which in the gnome applet are implemented in the nm-applet
process) are tied together for practical reasons, though it's not
extremely hard to split them.

> > Could you describe some of the scenarios that would involve stacking  
> > UI
> > agents?  I'm not sure I follow the use-cases here.
> 
> So you have the applet handling all security request (passphrase  
> etc.), but then you wanna do a wizard to setup a new network  
> connection. In that case you could re-direct the passphrase request to  
> the wizard (which is a different application from the applet). While  
> for WiFi this might be still limited in its usefulness. For all the  
> other wireless technologies like Bluetooth, WiMAX, UWB etc. it is  
> different.

I'm not sure I see the case for an external wizard; NM will be
controlling the network device itself and will be proxying the secrets
where necessary (for WiMAX, NM would be pushing the certificates to the
OMA-DM client or to the driver for example, and for BT NM would be
pushing the necessary bits to Bluez).  I'm not quite sure about UWB yet,
and I know you've done a ton of work there, so maybe that's what you're
thinking about.

For Bluetooth, I had planned on having whatever configuration would be
required for DUN & PAN to be implemented in the applet, since the
configuration UI can be shared between the applet, the connection
editor, and any password request dialogs.  I think it's fairly important
to have a consistent UI for configuring these things, and I haven't yet
seen the need for loadable modules on the applet/connection editor side,
since any new device types will certainly need support added in both NM
and the UI agents.  I'm not sure there's a big win by keeping the UI
bits for each device type separate at this time.

Dan