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.

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);
 

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);

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;
+}

src/login/logind-session-device.h

@@ -57,3 +57,4 @@ void session_device_complete_pause(SessionDevice *sd);
 
 void session_device_resume_all(Session *s);
 void session_device_pause_all(Session *s);
+unsigned int session_device_try_pause_all(Session *s);

src/login/logind-session.c

@@ -100,6 +100,8 @@ void session_free(Session *s) {
         if (s->seat) {
                 if (s->seat->active == s)
                         s->seat->active = NULL;
+                if (s->seat->pending_switch == s)
+                        s->seat->pending_switch = NULL;
 
                 LIST_REMOVE(Session, sessions_by_seat, s->seat->sessions, s);
         }
@@ -375,21 +377,40 @@ int session_load(Session *s) {
 }
 
 int session_activate(Session *s) {
+        unsigned int num_pending;
+
         assert(s);
         assert(s->user);
 
-        if (s->vtnr <= 0)
-                return -ENOTSUP;
-
         if (!s->seat)
                 return -ENOTSUP;
 
         if (s->seat->active == s)
                 return 0;
 
-        assert(seat_has_vts(s->seat));
+        /* on seats with VTs, we let VTs manage session-switching */
+        if (seat_has_vts(s->seat)) {
+                if (s->vtnr <= 0)
+                        return -ENOTSUP;
 
-        return chvt(s->vtnr);
+                return chvt(s->vtnr);
+        }
+
+        /* On seats without VTs, we implement session-switching in logind. We
+         * try to pause all session-devices and wait until the session
+         * controller acknowledged them. Once all devices are asleep, we simply
+         * switch the active session and be done.
+         * We save the session we want to switch to in seat->pending_switch and
+         * seat_complete_switch() will perform the final switch. */
+
+        s->seat->pending_switch = s;
+
+        /* if no devices are running, immediately perform the session switch */
+        num_pending = session_device_try_pause_all(s);
+        if (!num_pending)
+                seat_complete_switch(s->seat);
+
+        return 0;
 }
 
 static int session_link_x11_socket(Session *s) {