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, 15 insertions, 0 deletions

diff --git a/src/login/logind-seat.c b/src/login/logind-seat.c
index f88738ab16..4a4d40adca 100644
--- a/src/login/logind-seat.c
+++ b/src/login/logind-seat.c
@@ -425,6 +425,21 @@ int seat_attach_session(Seat *s, Session *session) {
         return 0;
 }
 
+void seat_complete_switch(Seat *s) {
+        Session *session;
+
+        assert(s);
+
+        /* if no session-switch is pending or if it got canceled, do nothing */
+        if (!s->pending_switch)
+                return;
+
+        session = s->pending_switch;
+        s->pending_switch = NULL;
+
+        seat_set_active(s, session);
+}
+
 bool seat_has_vts(Seat *s) {
         assert(s);