Multi-client location access

[Teemu Ikonen]

On 17/12/2024 15.57, Dan Williams wrote:

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

I've written Satellite (https://codeberg.org/tpikonen/satellite) and 
there are also apps using the gpsd API which can be made to work with 
something like gnss-share 
(https://gitlab.postmarketos.org/postmarketOS/gnss-share).

>> 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.

This sounds like a good plan. Geoclue does refcounting with its location 
sources and it works well.

Since implementation will no doubt take a long time, do you have any 
comments on the protocol described in the MR above? It's also 
implemented in the Satellite app, and I've ran it and patched Geoclue 
concurrently without problems. But, I'm not that familiar with MM 
internals, so I might be missing something.

If the concept is OK, ideally it could be mentioned or referenced in MM 
docs somewhere, until a proper multi-client implementation is ready.

Best,

Teemu