@AdrianVovk yes, everything from @modal is the reason I kinda ranted against focusing on server-based services :) Sure, actually having that handled in the session-level service would be nice.

I think background services are very necessary either way, for music playback, VPNs, whatever else. And yes the service manager can enforce limits, and do usage accounting that would be reported in gnome-control-center.

I have thought about the "pass file descriptor" thing, that's one of the first things that comes to mind but— what happens when the socket becomes readable? Just "wake the app" that still has the same socket open— well, this is just regular background activity with extra steps, just to be careful about apps' CPU time, which sounds kinda "meh" to me