[Telepathy] "unknown" presence

[Robert McQueen]

Whilst we're talking about tweaking the presence definition, what do
people think about adding an "unknown" well-known presence value to the
dictionary defined by GetStatuses? This would always have the type 0, ie
unset, and would never be valid for setting on oneself. In a way any CM
could arrange this anyway, but defining a well-known value would set a
precedent and suggest to clients they should be able to deal with and
display unknown presence if presented by a CM.

The rationale is that on XMPP, and on many other protocols, we actually
know whose presence we're expecting to see[1]: the people on our roster,
and the people who are in channels with us. Everyone else we don't know
their presence at all, and we know that we don't know it. Currently when
you do GetPresence, Gabble will default to saying they are offline,
making it harder in UIs to present presence information. If they wanted
to distinguish at the moment between offline (we know it) and offline
(we're not subscribed), the UI has to make presumptions about the
protocol's link between subscription states and availability of presence
information. The CM is the right place for this information to be
reported from.

Comments?

Regards,
Rob

1: The exception is XMPP directed presence, but the presence cache in
Gabble already has a way of deciding who is "interesting", so in this
case if we're talking to someone and they decide to send us directed
presence to reveal the fact they're online, and maybe also send us a
nickname and caps, they will appear online until e.g. we close the
channel, when they will revert to unknown.