Multi-client location access

[Dan Williams]

On Mon, 2024-12-09 at 13:35 +0100, Teemu Ikonen wrote:
> Hi,
> 
> In Geoclue we're wondering
> (https://gitlab.freedesktop.org/geoclue/geoclue/-/merge_requests/176)
> how to deal with multiple clients using ModemManager for location
> info.
> 
> The straightforward approach of enabling location gathering on client
> startup and disabling it on exit does not work, because other clients
> will then stop receiving location updates. The MR above describes and
> implements a protocol for turning location gathering on and off and
> reacting to changes. I asked for feedback on this in MM issues
> (https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issue
> s/870) but got no replies. Any thoughts on this subject in this list?

Good question...

Out of curiosity, what other clients have you found using location info
alongside GeoClue?

In any case, Aleksander and I talked about this last week and didn't
think of a great solution. Instead we came up with a hybrid approach
that may meet the needs of both long-lived clients (GeoClue) and short-
lived ones (mmcli).

a) refcount location source enable/disable in MM based on D-Bus
connection. When the D-Bus client goes away or drops off the bus, drop
the reference. When all users have dropped off the bus, disable the
location source.

b) to ensure we don't break mmcli and other tools that depend on the
existing behavior that sources stay enabled over D-Bus connection
changes, add a new API method that has the existing non-refcounted
behavior, and switch mmcli to that. Would really use refcounting
underneath though, but based on API start/stop requests instead of D-
Bus client.

Yeah, in reality that does break tools like mmcli, but I'm not sure how
we can really fix this otherwise. Adding a new ref-counted API would
likely take years for all clients to switch over.

What do you think?

Thanks,
Dan