tests: Override X11 display number

As with the Wayland display name, to avoid clashes with already an
running Xwayland or Xorg instance, override the X11 display name to
something less likely to cause a clash.

https://gitlab.gnome.org/GNOME/mutter/issues/193

src/tests/test-utils.c

@@ -27,6 +27,7 @@
 #include "core/display-private.h"
 #include "core/window-private.h"
 #include "wayland/meta-wayland.h"
+#include "wayland/meta-xwayland.h"
 #include "x11/meta-x11-display-private.h"
 
 struct _TestClient {
@@ -94,6 +95,7 @@ test_init (int    *argc,
   ensure_test_client_path (*argc, *argv);
 
   meta_wayland_override_display_name ("mutter-test-display");
+  meta_xwayland_override_display_number (512);
 }
 
 AsyncWaiter *

src/wayland/meta-xwayland.c

@@ -59,6 +59,8 @@ G_DEFINE_TYPE (MetaWaylandSurfaceRoleXWayland,
                meta_wayland_surface_role_xwayland,
                META_TYPE_WAYLAND_ACTOR_SURFACE)
 
+static int display_number_override = -1;
+
 void
 meta_xwayland_associate_window_with_surface (MetaWindow          *window,
                                              MetaWaylandSurface  *surface)
@@ -371,6 +373,12 @@ x_io_error (Display *display)
   return 0;
 }
 
+void
+meta_xwayland_override_display_number (int number)
+{
+  display_number_override = number;
+}
+
 static gboolean
 choose_xdisplay (MetaXWaylandManager *manager)
 {
@@ -378,11 +386,9 @@ choose_xdisplay (MetaXWaylandManager *manager)
   char *lock_file = NULL;
   gboolean fatal = FALSE;
 
-  /* Hack to keep the unused Xwayland instance on
-   * the login screen from taking the prime :0 display
-   * number.
-   */
-  if (g_getenv ("RUNNING_UNDER_GDM") != NULL)
+  if (display_number_override != -1)
+    display = display_number_override;
+  else if (!g_getenv ("RUNNING_UNDER_GDM"))
     display = 1024;
 
   do

src/wayland/meta-xwayland.h

@@ -30,6 +30,9 @@
 #include "meta/types.h"
 #include "wayland/meta-wayland-types.h"
 
+void
+meta_xwayland_override_display_number (int number);
+
 void
 meta_xwayland_handle_wl_surface_id (MetaWindow *window,
                                     guint32     surface_id);