Let multiseat X users choose a desired X display number

common/configuration.c

@@ -354,6 +354,7 @@ config_init (Configuration *config)
     g_hash_table_insert (config->priv->seat_keys, "pam-greeter-service", GINT_TO_POINTER (KEY_SUPPORTED));
     g_hash_table_insert (config->priv->seat_keys, "xserver-backend", GINT_TO_POINTER (KEY_DEPRECATED));
     g_hash_table_insert (config->priv->seat_keys, "xserver-command", GINT_TO_POINTER (KEY_SUPPORTED));
+    g_hash_table_insert (config->priv->seat_keys, "desired-display-number", GINT_TO_POINTER (KEY_SUPPORTED));
     g_hash_table_insert (config->priv->seat_keys, "xmir-command", GINT_TO_POINTER (KEY_SUPPORTED));
     g_hash_table_insert (config->priv->seat_keys, "xserver-config", GINT_TO_POINTER (KEY_SUPPORTED));
     g_hash_table_insert (config->priv->seat_keys, "xserver-layout", GINT_TO_POINTER (KEY_SUPPORTED));

data/lightdm.conf

@@ -56,6 +56,7 @@
 # xserver-share = True if the X server is shared for both greeter and session
 # xserver-hostname = Hostname of X server (only for type=xremote)
 # xserver-display-number = Display number of X server (only for type=xremote)
+# desired-display-number = The display number you wish to have this seat have, if it is already used the next available will be used
 # xdmcp-manager = XDMCP manager to connect to (implies xserver-allow-tcp=true)
 # xdmcp-port = XDMCP UDP/IP port to communicate on
 # xdmcp-key = Authentication key to use for XDM-AUTHENTICATION-1 (stored in keys.conf)

src/seat-local.c

@@ -151,6 +151,9 @@ create_x_server (SeatLocal *seat)
 {
     g_autoptr(XServerLocal) x_server = x_server_local_new ();
 
+    gint desired_display_number = seat_get_integer_property_with_fallback (SEAT (seat), "desired-display-number", -1);
+    x_server_local_init_display_number (x_server, desired_display_number);
+
     gint vt = get_vt (seat, DISPLAY_SERVER (x_server));
     if (vt >= 0)
         x_server_local_set_vt (x_server, vt);

src/seat.c

@@ -180,10 +180,17 @@ seat_get_boolean_property (Seat *seat, const gchar *name)
 }
 
 gint
-seat_get_integer_property (Seat *seat, const gchar *name)
+seat_get_integer_property_with_fallback (Seat *seat, const gchar *name, gint fallback)
 {
     const gchar *value = seat_get_string_property (seat, name);
-    return value ? atoi (value) : 0;
+    return value ? atoi (value) : fallback;
+}
+
+
+gint
+seat_get_integer_property (Seat *seat, const gchar *name)
+{
+  return seat_get_integer_property_with_fallback (seat, name, 0);
 }
 
 const gchar *

src/seat.h

@@ -75,6 +75,8 @@ gchar **seat_get_string_list_property (Seat *seat, const gchar *name);
 
 gboolean seat_get_boolean_property (Seat *seat, const gchar *name);
 
+gint seat_get_integer_property_with_fallback (Seat *seat, const gchar *name, gint fallback);
+
 gint seat_get_integer_property (Seat *seat, const gchar *name);
 
 const gchar *seat_get_name (Seat *seat);

src/x-server-local.c

@@ -154,8 +154,16 @@ display_number_in_use (guint display_number)
 }
 
 static guint
-x_server_local_get_unused_display_number (void)
+x_server_local_get_unused_display_number (gint desired_display_number)
 {
+    // First attempt to use the requested display number
+    if (desired_display_number >= 0) {
+      if (!display_number_in_use (desired_display_number)) {
+	return desired_display_number;
+      }
+    }
+
+    // Else fall back
     guint number = config_get_integer (config_get_instance (), "LightDM", "minimum-display-number");
     while (display_number_in_use (number))
         number++;
@@ -194,6 +202,16 @@ x_server_local_set_command (XServerLocal *server, const gchar *command)
     priv->command = g_strdup (command);
 }
 
+void
+x_server_local_init_display_number (XServerLocal *server, gint number)
+{
+    XServerLocalPrivate *priv = x_server_local_get_instance_private (server);
+    g_return_if_fail (server != NULL);
+
+    priv->display_number = x_server_local_get_unused_display_number (number);
+    l_debug (server, "Requested display number %d, actual display number: %d", number, priv->display_number);
+}
+
 void
 x_server_local_set_vt (XServerLocal *server, gint vt)
 {
@@ -574,9 +592,10 @@ static void
 x_server_local_init (XServerLocal *server)
 {
     XServerLocalPrivate *priv = x_server_local_get_instance_private (server);
+
     priv->vt = -1;
     priv->command = g_strdup ("X");
-    priv->display_number = x_server_local_get_unused_display_number ();
+    priv->display_number = -1;
 }
 
 static void

src/x-server-local.h

@@ -45,6 +45,8 @@ XServerLocal *x_server_local_new (void);
 
 void x_server_local_set_command (XServerLocal *server, const gchar *command);
 
+void x_server_local_init_display_number (XServerLocal *server, gint number);
+
 void x_server_local_set_vt (XServerLocal *server, gint vt);
 
 void x_server_local_set_config (XServerLocal *server, const gchar *path);