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, 9 insertions, 2 deletions

diff --git a/src/login/logind-seat.c b/src/login/logind-seat.c
index 470d08bc05..2c60b8ae51 100644
--- a/src/login/logind-seat.c
+++ b/src/login/logind-seat.c
@@ -448,10 +448,17 @@ bool seat_can_tty(Seat *s) {
         return seat_is_vtconsole(s);
 }
 
+bool seat_has_master_device(Seat *s) {
+        assert(s);
+
+        /* device list is ordered by "master" flag */
+        return !!s->devices && s->devices->master;
+}
+
 bool seat_can_graphical(Seat *s) {
         assert(s);
 
-        return !!s->devices;
+        return seat_has_master_device(s);
 }
 
 int seat_get_idle_hint(Seat *s, dual_timestamp *t) {
@@ -499,7 +506,7 @@ int seat_check_gc(Seat *s, bool drop_not_started) {
         if (seat_is_vtconsole(s))
                 return 1;
 
-        return !!s->devices;
+        return seat_has_master_device(s);
 }
 
 void seat_add_to_gc_queue(Seat *s) {