Questions regarding host services

[Umang Jain]

Hi all,

I have a train of thoughts regarding a particular issue. I would like
to get community input and suggestions.

Perspective:

If you see the gnome-photos manifest [1],  it needs to talk to
org.freedesktop.Tracker1* services which is basically how gnome-photos
displays images from the filesystem using tracker-sparql queries.
Ubuntu has Tracker disabled therefore https://imgur.com/a/Ww3B0 ,
which brings me to

* Host services not available

Can flatpak ensure that requested services in manifest (--talk-to) are
available on host side? If not, is there any way we can make the user
aware of this and mention what are the possibles steps required to run
the app as expected?

* Portals

I did a bit of learning under the hood and came across portals. They
are "Trusted services under your session" that your sandbox is allowed
to talk to; typically accessed by D-Bus.
I have few more questions here:
        * So, does the above perspective comes under this umbrella?
Tracker as portal?
        * Who/what determines safety of portals ? App-developer?
        * Will flatpak provide access to "all" available portals or
just a subset (as per mention in the manifest).
        * Is there any "custom" portal (I guess not because then it
breaks the whole idea of security and sandboxing, maybe?).

I also came across Alex mentioning that eventually, there will
android-like permission model. Users can (dis)allow or override the
permission but the app might not work as expected; But I think before
doing that, there should be some kind of mechanism to ensure that
services are available on host side( and then only ask for
permission).

So, finally I want to ask again that if XYZ service is required on the
host side by a flatpak:ed app, how to report/make aware the user that
it needs ABC service? If that's something possible(maybe in future)
and need someone with time and energy to implement and make it happen.
:)

Thank you for your time.

[1]: https://gitlab.gnome.org/GNOME/gnome-photos/blob/master/flatpak/org.gnome.Photos.json

Cheers,
  uajain