wayland/client: Add way to create indirectly launched clients

This API creates a "client" then later sets up a wl_client and returns a
file descriptor some Wayland client can connect to. It's meant to be
used as a method other than WAYLAND_SOCKET and process launching, e.g.
passing a file descriptor via a D-Bus API.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2810>

src/meson.build

@@ -576,6 +576,7 @@ if have_wayland
     'wayland/meta-wayland-buffer.h',
     'wayland/meta-wayland.c',
     'wayland/meta-wayland-client.c',
+    'wayland/meta-wayland-client-private.h',
     'wayland/meta-wayland-cursor-surface.c',
     'wayland/meta-wayland-cursor-surface.h',
     'wayland/meta-wayland-data-device.c',

src/wayland/meta-wayland-client-private.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2023 Red Hat
+ *
+ * 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, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#ifndef META_WAYLAND_CLIENT_PRIVATE_H
+#define META_WAYLAND_CLIENT_PRIVATE_H
+
+#include "core/util-private.h"
+#include "meta/meta-wayland-client.h"
+
+META_EXPORT_TEST
+MetaWaylandClient * meta_wayland_client_new_indirect (MetaContext  *context,
+                                                      GError      **error);
+
+META_EXPORT_TEST
+int meta_wayland_client_setup_fd (MetaWaylandClient  *client,
+                                  GError            **error);
+
+#endif /* META_WAYLAND_CLIENT_PRIVATE_H */

src/wayland/meta-wayland-client.c

@@ -2,6 +2,7 @@
 
 /*
  * Copyright 2019 Sergio Costas (rastersoft@gmail.com)
+ * Copyright 2023 Red Hat
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -29,7 +30,7 @@
 
 #include "config.h"
 
-#include "meta/meta-wayland-client.h"
+#include "wayland/meta-wayland-client-private.h"
 
 #include <gio/gio.h>
 #include <glib-object.h>
@@ -66,6 +67,10 @@ struct _MetaWaylandClient
     gboolean process_launched;
   } subprocess;
 
+  struct {
+    int fd;
+  } indirect;
+
   struct wl_client *wayland_client;
   struct wl_listener client_destroy_listener;
 };
@@ -125,6 +130,28 @@ child_setup (gpointer user_data)
   meta_context_restore_rlimit_nofile (context, NULL);
 }
 
+/**
+ * meta_wayland_client_new_indirect: (skip)
+ */
+MetaWaylandClient *
+meta_wayland_client_new_indirect (MetaContext  *context,
+                                  GError      **error)
+{
+  MetaWaylandClient *client;
+
+  if (!meta_is_wayland_compositor ())
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED,
+                   "MetaWaylandClient can be used only with Wayland.");
+      return NULL;
+    }
+
+  client = g_object_new (META_TYPE_WAYLAND_CLIENT, NULL);
+  client->context = context;
+
+  return client;
+}
+
 /**
  * meta_wayland_client_new:
  * @context: (not nullable): a #MetaContext
@@ -216,6 +243,34 @@ set_wayland_client (MetaWaylandClient *client,
                                   &client->client_destroy_listener);
 }
 
+/**
+ * meta_wayland_client_setup_fd: (skip)
+ * @client: a #MetaWaylandClient
+ *
+ * Initialize a wl_client that can be connected to via the returned file
+ * descriptor. May only be used with a #MetaWaylandClient created with
+ * meta_wayland_client_new_indirect().
+ *
+ * Returns: (transfer full): A new file descriptor
+ */
+int
+meta_wayland_client_setup_fd (MetaWaylandClient  *client,
+                              GError            **error)
+{
+  struct wl_client *wayland_client;
+  int fd;
+
+  g_return_val_if_fail (!client->wayland_client, -1);
+  g_return_val_if_fail (!client->subprocess.launcher, -1);
+
+  if (!init_wayland_client (client, &wayland_client, &fd, error))
+    return -1;
+
+  set_wayland_client (client, wayland_client);
+
+  return fd;
+}
+
 /**
  * meta_wayland_client_spawnv:
  * @client: a #MetaWaylandClient
@@ -247,6 +302,13 @@ meta_wayland_client_spawnv (MetaWaylandClient   *client,
                         argv[0][0] != '\0',
                         NULL);
 
+  if (!client->subprocess.launcher)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "This client can not be launched");
+      return NULL;
+    }
+
   if (client->subprocess.process_launched)
     {
       g_set_error (error,