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, 28 insertions, 0 deletions

diff --git a/src/login/logind-session-device.c b/src/login/logind-session-device.c
index 80fd364139..e92bb54fff 100644
--- a/src/login/logind-session-device.c
+++ b/src/login/logind-session-device.c
@@ -414,10 +414,21 @@ void session_device_free(SessionDevice *sd) {
 }
 
 void session_device_complete_pause(SessionDevice *sd) {
+        SessionDevice *iter;
+        Iterator i;
+
         if (!sd->active)
                 return;
 
         session_device_stop(sd);
+
+        /* if not all devices are paused, wait for further completion events */
+        HASHMAP_FOREACH(iter, sd->session->devices, i)
+                if (iter->active)
+                        return;
+
+        /* complete any pending session switch */
+        seat_complete_switch(sd->session->seat);
 }
 
 void session_device_resume_all(Session *s) {
@@ -449,3 +460,20 @@ void session_device_pause_all(Session *s) {
                 }
         }
 }
+
+unsigned int session_device_try_pause_all(Session *s) {
+        SessionDevice *sd;
+        Iterator i;
+        unsigned int num_pending = 0;
+
+        assert(s);
+
+        HASHMAP_FOREACH(sd, s->devices, i) {
+                if (sd->active) {
+                        session_device_notify(sd, SESSION_DEVICE_TRY_PAUSE);
+                        ++num_pending;
+                }
+        }
+
+        return num_pending;
+}