Use systemd-logind to discover active session on non-seat0 seats

Before, `seat_local_get_active_session` would always return `NULL` for
non-`seat0` seats.  This broke Wayland sessions on non-`seat0` seats:

  1.  User logged in to a Wayland session on `seat1`.
  2.  LightDM properly terminated the X server to allow Wayland to
      take over the seat's hardware.  (See the side note below.)
  3.  Wayland session started.
  4.  The X server termination triggered a call to
      `display_server_stopped_cb`.
  5.  `display_server_stopped_cb` called `seat_get_active_session` to
      see if it needed to start a greeter to replace a terminated
      session associated with the terminated display.
  6.  `seat_get_active_session` called
      `seat_local_get_active_session`.
  7.  `seat_local_get_active_session` erroneously probed the active VT
      even though VTs are only associated with `seat0`.
  8.  After finding no matching session associated with both `seat1`
      and the active VT, `seat_local_get_active_session` returned
      `NULL` when it should have returned the new Wayland session.
  9.  Due to the `NULL` response, `display_server_stopped_cb`
      erroneously arrived at the conclusion that a greeter must be
      started on `seat1`.
  10. LightDM started a new X server and greeter on `seat1`, stomping
      on the newly created Wayland session.

Side note: I don't think that terminating X to allow Wayland to take
over the seat's hardware is required because I believe X and
most?/all? Wayland compositors cooperatively share the devices via
systemd-logind or maybe libseat.  When switching sessions, logind uses
a hand-off protocol to smoothly change which process is allowed to
access the devices.  This makes session switching possible even
without virtual terminals.  For details, see
<https://dvdhrm.wordpress.com/2013/08/25/sane-session-switching/>.

The above sequence of events is apparent in the debug log from a
minimal script (the script is not included in this commit; the
relevant lines from the log are copied below, with annotations):

    #
    # User logs in to seat1 with a Wayland session:
    #
    GREETER-X-1 AUTHENTICATION-COMPLETE USERNAME=no-password2 AUTHENTICATED=TRUE
    *GREETER-X-1 START-SESSION
    [+0.32s] DEBUG: Seat seat1: Creating display server of type wayland
    [+0.32s] DEBUG: Seat seat1: Display server ready, running session
    [+0.32s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session0
    [+0.32s] DEBUG: Session pid=1309577: Running command /home/rhansen/floss/lightdm/tests/src/lightdm-session test-session
    #
    # LightDM starts shutting down X+greeter for seat1 while
    # concurrently activating the new Wayland session (c1 = the
    # stopping seat1 greeter session, c2 = the starting Wayland
    # session).
    #
    [+0.33s] DEBUG: Seat seat1: Stopping greeter
    [+0.33s] DEBUG: Terminating login1 session c1
    [+0.33s] DEBUG: Session pid=1309572: Sending SIGTERM
    [+0.33s] DEBUG: Activating login1 session c2
    GREETER-X-1 TERMINATE SIGNAL=15
    LOGIN1 ACTIVATE-SESSION SESSION=c2
    #
    # The greeter session and X exit.
    #
    [+0.33s] DEBUG: Session pid=1309572: Exited with return value 0
    [+0.33s] DEBUG: Seat seat1: Session stopped
    [+0.33s] DEBUG: Seat seat1: Stopping display server, no sessions require it
    [+0.33s] DEBUG: Sending signal 15 to process 1309569
    XSERVER-1 TERMINATE SIGNAL=15
    [+0.33s] DEBUG: Process 1309569 exited with return value 0
    [+0.33s] DEBUG: XServer 1: X server stopped
    [+0.33s] DEBUG: Seat seat1: Display server stopped
    #
    # The start of the problem: LightDM should not restart the greeter
    # until the Wayland session terminates:
    #
    [+0.33s] DEBUG: Seat seat1: Active display server stopped, starting greeter
    [+0.33s] DEBUG: Seat seat1: Creating greeter session
    [+0.33s] DEBUG: Seat seat1: Creating display server of type x
    [+0.33s] DEBUG: Seat seat1: Starting local X display
    [+0.33s] DEBUG: XServer 1: Launching X Server
    [+0.33s] DEBUG: Launching process 1309583: /home/rhansen/floss/lightdm/tests/src/X :1 -seat seat1 -auth /var/run/lightdm/root/:1 -nolisten tcp
    [+0.33s] DEBUG: XServer 1: Waiting for ready signal from X server :1
    XSERVER-1 START SEAT=seat1
    #
    # The Wayland session has started concurrently with the
    # replacement X+greeter.
    #
    SESSION-WAYLAND START XDG_SEAT=seat1 XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/no-password2 XDG_SESSION_TYPE=wayland XDG_SESSION_DESKTOP=wayland USER=no-password2
    *XSERVER-1 INDICATE-READY
    XSERVER-1 INDICATE-READY
    [+0.34s] DEBUG: Got signal 10 from process 1309583
    [+0.34s] DEBUG: XServer 1: Got signal from X server :1
    [+0.34s] DEBUG: XServer 1: Connecting to XServer :1
    XSERVER-1 ACCEPT-CONNECT
    [+0.34s] DEBUG: Seat seat1: Display server ready, starting session authentication
    [+0.34s] DEBUG: Session pid=1309587: Started with service 'lightdm-greeter', username 'lightdm'
    [+0.40s] DEBUG: Session pid=1309587: Running command /home/rhansen/floss/lightdm/tests/src/.libs/test-gobject-greeter
    #
    # Finally, the replacement greeter stomps on the new Wayland
    # session.
    #
    [+0.40s] DEBUG: Locking login1 session c2
    LOGIN1 LOCK-SESSION SESSION=c2

2 files changed, 23 insertions, 0 deletions

diff --git a/src/seat-local.c b/src/seat-local.c
index 90bb1455..65207145 100644
--- a/src/seat-local.c
+++ b/src/seat-local.c
@@ -261,6 +261,24 @@ seat_local_set_active_session (Seat *seat, Session *session)
 static Session *
 seat_local_get_active_session (Seat *seat)
 {
+    /*
+     * In the past, virtual terminal switching was the only way to switch
+     * between multiple sessions associated with a seat.  Due to operating
+     * system limitations, virtual terminal switching is limited to seat0, so
+     * the vt_* family of functions from vt.h must only be used with seat0.
+     *
+     * Nowadays, systemd-logind (via the org.freedesktop.login1 dbus interface)
+     * can be used to switch sessions.  logind supports multiple sessions even
+     * on non-seat0 seats.  Whenever logind switches sessions, a callback
+     * updates priv->active_session, so seat_get_expected_active_session should
+     * always return the currently active session.
+     *
+     * FIXME: Use seat_get_expected_active_session even for seat0, falling back
+     * to VT probing if the systemd-logind service is unavailable.
+     */
+    if (strcmp (seat_get_name (seat), "seat0") != 0)
+        return seat_get_expected_active_session (seat);
+
     gint vt = vt_get_active ();
     if (vt < 0)
         return NULL;
diff --git a/tests/scripts/multi-seat-wayland.conf b/tests/scripts/multi-seat-wayland.conf
index a9b6871c..a3356214 100644
--- a/tests/scripts/multi-seat-wayland.conf
+++ b/tests/scripts/multi-seat-wayland.conf
@@ -42,6 +42,11 @@ user-session=wayland
 #?SESSION-WAYLAND START XDG_SEAT=seat1 XDG_GREETER_DATA_DIR=.*/no-password2 XDG_SESSION_TYPE=wayland XDG_SESSION_DESKTOP=wayland USER=no-password2
 #?LOGIN1 ACTIVATE-SESSION SESSION=c2
 
+# Ensure that the X server and greeter for seat1 are not restarted until after
+# the user logs out.
+#?*WAIT
+#?*FENCE
+
 # Log out.  The X server and greeter for seat1 should start again.
 #?*SESSION-WAYLAND LOGOUT
 #?XSERVER-1 START SEAT=seat1