logind: implement generic multi-session

This enables the multi-session capability for seats that don't have VTs.
For legacy seats with VTs, everything stays the same. However, all other
seats now also get the multi-session capability.

The only feature that was missing was session-switching. As logind can
force a session-switch and signal that via the "Active" property, we only
need a way to allow synchronized/delayed session switches. Compositors
need to cleanup some devices before acknowledging the session switch.
Therefore, we use the session-devices to give compositors a chance to
block a session-switch until they cleaned everything up.

If you activate a session on a seat without VTs, we send a PauseDevice
signal to the active session for every active device. Only once the
session acknowledged all these with a PauseDeviceComplete() call, we
perform the final session switch.

One important note is that delayed session-switching is meant for
backwards compatibility. New compositors or other sessions should really
try to deal correctly with forced session switches! They only need to
handle EACCES/EPERM from syscalls and treat them as "PauseDevice" signal.

Following logind patches will add a timeout to session-switches which
forces the switch if the active session does not react in a timely
fashion. Moreover, explicit ForceActivate() calls might also be supported.
Hence, sessions must not crash if their devices get paused.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/login/logind-seat.h b/src/login/logind-seat.h
index d3438b8495..be6db6eed1 100644
--- a/src/login/logind-seat.h
+++ b/src/login/logind-seat.h
@@ -38,6 +38,7 @@ struct Seat {
         LIST_HEAD(Device, devices);
 
         Session *active;
+        Session *pending_switch;
         LIST_HEAD(Session, sessions);
 
         bool in_gc_queue:1;
@@ -59,6 +60,7 @@ int seat_read_active_vt(Seat *s);
 int seat_preallocate_vts(Seat *s);
 
 int seat_attach_session(Seat *s, Session *session);
+void seat_complete_switch(Seat *s);
 
 bool seat_has_vts(Seat *s);
 bool seat_is_seat0(Seat *s);