tests: Add check for compositor state on XWayland startup

Check that the first X11 window started has a compositor defined.

See: https://gitlab.gnome.org/GNOME/mutter/-/issues/2472#note_1582262
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2970>

src/tests/meson.build

@@ -464,6 +464,20 @@ if have_native_tests
     },
   ]
   if have_xwayland
+    x11_compositor_checker = executable('x11-compositor-checker',
+      sources: ['x11-compositor-checker.c'],
+      include_directories: tests_includes,
+      c_args: [
+        tests_c_args,
+      ],
+      dependencies: [
+        x11_dep,
+      ],
+      install: have_installed_tests,
+      install_dir: mutter_installed_tests_libexecdir,
+      install_rpath: pkglibdir,
+    )
+
     test_cases += [
       {
         'name': 'wayland-x11-interop',
@@ -480,7 +494,10 @@ if have_native_tests
       {
         'name': 'xwayland',
         'suite': 'wayland',
-        'depends': [ test_client ],
+        'depends': [
+          test_client,
+          x11_compositor_checker,
+        ],
         'sources': [
           'xwayland-tests.c',
         ],

src/tests/x11-compositor-checker.c

@@ -0,0 +1,52 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+/*
+ * Copyright (C) 2022 Alan Jenkins.
+ * Copyright (C) 2023 Canonical Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include <stdio.h>
+#include <X11/Xlib.h>
+#include <X11/Xutil.h>
+
+static int
+has_compositor (Display *dpy,
+                int      screen)
+{
+  char prop_name[20];
+
+  snprintf (prop_name, 20, "_NET_WM_CM_S%d", screen);
+  return XGetSelectionOwner (dpy, XInternAtom (dpy, prop_name, False)) != None;
+}
+
+int
+main (void)
+{
+  Display *dpy = XOpenDisplay ("");
+
+  if (has_compositor (dpy, XDefaultScreen (dpy)))
+    {
+      printf ("X11 Compositor is available for display %s.%d\n",
+              DisplayString (dpy), XDefaultScreen (dpy));
+      return 0;
+    }
+
+  printf ("NO X11 Compositor is available for display %s:%d\n",
+          DisplayString (dpy), XDefaultScreen (dpy));
+
+  return 1;
+}

src/tests/xwayland-tests.c

@@ -247,9 +247,68 @@ meta_test_hammer_activate (void)
   meta_test_client_destroy (wayland_client);
 }
 
+static void
+compositor_check_proc_async (GObject      *source_object,
+                             GAsyncResult *res,
+                             gpointer      user_data)
+{
+  g_autoptr (GError) error = NULL;
+  GMainLoop *loop = user_data;
+
+  g_subprocess_wait_check_finish (G_SUBPROCESS (source_object), res, &error);
+  g_assert_no_error (error);
+  g_main_loop_quit (loop);
+}
+
+static void
+meta_test_xwayland_compositor_selection (void)
+{
+  g_autoptr (GError) error = NULL;
+  g_autoptr (GSubprocessLauncher) launcher = NULL;
+  g_autoptr (GSubprocess) subprocess = NULL;
+  g_autoptr (GMainLoop) loop = NULL;
+  MetaDisplay *display = meta_context_get_display (test_context);
+  MetaWaylandCompositor *compositor;
+  const char *x11_display_name;
+  const char *x11_compositor_checker;
+
+  g_assert_null (meta_display_get_x11_display (display));
+
+  g_assert (meta_is_wayland_compositor ());
+  compositor = meta_context_get_wayland_compositor (test_context);
+  x11_display_name = meta_wayland_get_public_xwayland_display_name (compositor);
+  g_assert_nonnull (x11_display_name);
+
+  launcher = g_subprocess_launcher_new (G_SUBPROCESS_FLAGS_NONE);
+  g_subprocess_launcher_setenv (launcher,
+                                "DISPLAY", x11_display_name,
+                                TRUE);
+
+  x11_compositor_checker = g_test_build_filename (G_TEST_BUILT,
+                                                  "src",
+                                                  "tests",
+                                                  "x11-compositor-checker",
+                                                  NULL);
+
+  subprocess = g_subprocess_launcher_spawn (launcher,
+                                            &error,
+                                            x11_compositor_checker,
+                                            NULL);
+  g_assert_no_error (error);
+
+  loop = g_main_loop_new (NULL, FALSE);
+  g_subprocess_wait_check_async (subprocess, NULL,
+                                 compositor_check_proc_async, loop);
+  g_main_loop_run (loop);
+
+  g_assert_nonnull (meta_display_get_x11_display (display));
+}
+
 static void
 init_tests (void)
 {
+  g_test_add_func ("/backends/xwayland/compositor/selection",
+                   meta_test_xwayland_compositor_selection);
   g_test_add_func ("/backends/xwayland/restart/selection",
                    meta_test_xwayland_restart_selection);
   g_test_add_func ("/backends/xwayland/crash/only-x11",