logind: listen actively for session devices

Session compositors need access to fbdev, DRM and evdev devices if they
control a session. To make logind pass them to sessions, we need to
listen for them actively.

However, we avoid creating new seats for non master-of-seat devices. Only
once a seat is created, we start remembering all other session devices. If
the last master-device is removed (even if there are other non-master
devices still available), we destroy the seat. This is the current
behavior, but we need to explicitly implement it now as there may be
non-master devices in the seat->devices list.

Unlike master devices, we don't care whether our list of non-master
devices is complete. We don't export this list but use it only as cache if
sessions request these devices. Hence, if a session requests a device that
is not in the list, we will simply look it up. However, once a session
requested a device, we must be notified of "remove" udev events. So we
must link the devices somehow into the device-list.

Regarding the implementation, we now sort the device list by the "master"
flag. This guarantees that master devices are at the front and non-master
devices at the tail of the list. Thus, we can easily test whether a seat
has a master device attached.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/login/logind-seat.h b/src/login/logind-seat.h
index c8ab17f7cf..bd5390f553 100644
--- a/src/login/logind-seat.h
+++ b/src/login/logind-seat.h
@@ -63,6 +63,7 @@ int seat_attach_session(Seat *s, Session *session);
 bool seat_is_vtconsole(Seat *s);
 bool seat_can_multi_session(Seat *s);
 bool seat_can_tty(Seat *s);
+bool seat_has_master_device(Seat *s);
 bool seat_can_graphical(Seat *s);
 
 int seat_get_idle_hint(Seat *s, dual_timestamp *t);