[gdm] user-switching: fix session migration

[Ray Strode <halfline src gnome org>]

commit fb6a3cf203c7fda3a9a33d3e3b4e7a7b868cb43e
Author: Ray Strode <rstrode redhat com>
Date:   Mon Mar 19 15:11:26 2012 -0400

    user-switching: fix session migration
    
    It wasn't properly jumping to the correct vt.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=655380

 daemon/gdm-slave.c |   30 ++++++++++++++++++++++++------
 1 files changed, 24 insertions(+), 6 deletions(-)
---
diff --git a/daemon/gdm-slave.c b/daemon/gdm-slave.c
index a1fded1..f1f4c47 100644
--- a/daemon/gdm-slave.c
+++ b/daemon/gdm-slave.c
@@ -1313,8 +1313,10 @@ gdm_slave_get_primary_session_id_for_user_from_systemd (GdmSlave   *slave,
         char  **sessions;
         uid_t   uid;
         char   *primary_ssid;
+        gboolean got_primary_ssid;
 
         primary_ssid = NULL;
+        got_primary_ssid = FALSE;
 
         res = sd_seat_can_multi_session (slave->priv->display_seat_id);
         if (res < 0) {
@@ -1342,16 +1344,23 @@ gdm_slave_get_primary_session_id_for_user_from_systemd (GdmSlave   *slave,
         }
 
         for (i = 0; sessions[i] != NULL; i++) {
+                gboolean is_active;
+                uid_t other;
 
-                if (primary_ssid == NULL) {
-                        uid_t other;
+                /* Always give preference to non-active sessions,
+                 * so we migrate when we can and don't when we can't
+                 */
+                is_active = sd_session_is_active (sessions[i]) > 0;
+
+                res = sd_session_get_uid (sessions[i], &other);
+                if (res == 0 && other == uid && !got_primary_ssid) {
+                        g_free (primary_ssid);
+                        primary_ssid = g_strdup (sessions[i]);
 
-                        res = sd_session_get_uid (sessions[i], &other);
-                        if (res == 0 && other == uid) {
-                                primary_ssid = g_strdup (sessions[i]);
+                        if (!is_active) {
+                                got_primary_ssid = TRUE;
                         }
                 }
-
                 free (sessions[i]);
         }
 
@@ -1496,6 +1505,15 @@ activate_session_id_for_systemd (GdmSlave   *slave,
         ret = FALSE;
         reply = NULL;
 
+        /* Can't activate what's already active. We want this
+         * to fail, because we don't want migration to succeed
+         * if the only active session is the one just created
+         * at the login screen.
+         */
+        if (sd_session_is_active (session_id) > 0) {
+                return FALSE;
+        }
+
         dbus_error_init (&local_error);
 
         message = dbus_message_new_method_call ("org.freedesktop.login1",