On Tue, Apr 06, 2021 at 12:51:56PM +0100, Simon McVittie wrote: > On Fri, 02 Apr 2021 at 20:54:05 +0300, Arseny Maslennikov wrote: > > There's at least a use case to know if an active session owned by the > > UID is present on seat X: > > https://gitlab.gnome.org/GNOME/gvfs/-/issues/557 > > In short, if a USB storage drive is connected to a particular seat, > > we'd like to only try automounting it on that seat, and not on a random > > seat that wins a race condition. > > Consider this system: > > uid 1000 > \- login session 1 on seat 1 > \- GNOME > \- login session 2 on seat 2 > \- getty/bash > \- systemd --user > \- gvfsd process 11111 > uid 2000 > \- login session 3 on seat 3 > \- GNOME > \- login session 4 on seat 4 > \- getty/bash > \- systemd --user > \- gvfsd process 22222 > > (Note that this is already a bit silly because each of uid 1000 and uid > 2000 presumably represents a person, and at any given time, that person > is physically sitting in at most one seat...) At any given time one or both sessions might be locked (and thus inactive). Anyway, I wouldn't expect additional complexity that explicitly forbids this scenario. > > If gvfsd is a `systemd --user` service, then there are two instances of it: > one for uid 1000, and one for uid 2000. It is *not* the case that there are > four instances of it, one per seat. > > Now we plug a USB drive into seat 3, and another USB drive into seat 4. > Suppose both users have configured gvfsd to automount USB drives. What > we want is that uid 1000 does not mount either of the USB drives (even > if they have the necessary permissions), and uid 2000 mounts both of them. IOW, if we're a user service, query all the sessions and their seats and look for the right seat in there. I agree with you here, this is a solid approach. > > You seem to be suggesting that the right logic is for each gvfsd instance > to run this pseudocode: > > # Here get_login_session() is assumed to return non-null even if the > # process is a systemd --user service. Will it return 3 or 4 for > # uid 2000's gvfsd? It can only return one of those! > if this_process.get_login_session().seat == usb_drive.seat: > usb_drive.do_automount() > > I don't think that's right, though: assuming get_login_session() is > deterministic, we will only automount *either* the USB drive on seat 3 > *or* the one on seat 4, not both. > > I think the logic we want in gvfsd is probably more like this: > > for login_session in this_uid.get_login_sessions(): > if login_session.seat == usb_drive.seat: > usb_drive.do_automount() > break > > Or, perhaps even better: > > # This time we want this to return null if this process is a > # systemd --user service > login_session = this_process.get_login_session() > > if login_session: > # traditional D-Bus activation, or XDG autostart, or run in the > # foreground for development/debugging; responsible for only its > # own login session > if login_session.seat == usb_drive.seat: > usb_drive.do_automount() > else: > # systemd --user service, shared by all our login sessions > for login_session in this_uid.get_login_sessions(): > if login_session.seat == usb_drive.seat: > usb_drive.do_automount() > break This behaviour would make sense to me. > > either of which will do the right thing: uid 1000 will not automount either > of the USB sticks (even if they have permissions to do so), and uid 2000 > will automount both of them. > > smcv
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