Revert "wayland: Drop Xwayland abstract socket"

This reverts commit e2123768f6.  Various
container/chroot (e.g. Snaps, pressure-vessel) systems still depend on
the presence of the abstract X11 socket.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1613
(cherry picked from commit ea2192c4899ac7897efca31e8e85c157895950ac)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1669>

src/wayland/meta-wayland-private.h

@@ -46,6 +46,7 @@ typedef struct
 {
   int display_index;
   char *lock_file;
+  int abstract_fd;
   int unix_fd;
   char *name;
 } MetaXWaylandConnection;

src/wayland/meta-xwayland.c

@@ -438,6 +438,51 @@ create_lock_file (int      display,
   return filename;
 }
 
+static int
+bind_to_abstract_socket (int        display,
+                         gboolean  *fatal,
+                         GError   **error)
+{
+  struct sockaddr_un addr;
+  socklen_t size, name_size;
+  int fd;
+
+  fd = socket (PF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);
+  if (fd < 0)
+    {
+      *fatal = TRUE;
+      g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno),
+                   "Failed to create socket: %s", g_strerror (errno));
+      return -1;
+    }
+
+  addr.sun_family = AF_LOCAL;
+  name_size = snprintf (addr.sun_path, sizeof addr.sun_path,
+                        "%c/tmp/.X11-unix/X%d", 0, display);
+  size = offsetof (struct sockaddr_un, sun_path) + name_size;
+  if (bind (fd, (struct sockaddr *) &addr, size) < 0)
+    {
+      *fatal = errno != EADDRINUSE;
+      g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno),
+                   "Failed to bind to %s: %s",
+                   addr.sun_path + 1, g_strerror (errno));
+      close (fd);
+      return -1;
+    }
+
+  if (listen (fd, 1) < 0)
+    {
+      *fatal = errno != EADDRINUSE;
+      g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno),
+                   "Failed to listen to %s: %s",
+                   addr.sun_path + 1, g_strerror (errno));
+      close (fd);
+      return -1;
+    }
+
+  return fd;
+}
+
 static int
 bind_to_unix_socket (int      display,
                      GError **error)
@@ -597,18 +642,29 @@ ensure_x11_unix_dir (GError **error)
 static gboolean
 open_display_sockets (MetaXWaylandManager  *manager,
                       int                   display_index,
+                      int                  *abstract_fd_out,
                       int                  *unix_fd_out,
+                      gboolean             *fatal,
                       GError              **error)
 {
-  int unix_fd;
+  int abstract_fd, unix_fd;
 
   if (!ensure_x11_unix_dir (error))
     return FALSE;
 
-  unix_fd = bind_to_unix_socket (display_index, error);
+  abstract_fd = bind_to_abstract_socket (display_index, fatal, error);
-  if (unix_fd < 0)
+  if (abstract_fd < 0)
     return FALSE;
 
+  unix_fd = bind_to_unix_socket (display_index, error);
+  if (unix_fd < 0)
+    {
+      *fatal = FALSE;
+      close (abstract_fd);
+      return FALSE;
+    }
+
+  *abstract_fd_out = abstract_fd;
   *unix_fd_out = unix_fd;
 
   return TRUE;
@@ -621,6 +677,7 @@ choose_xdisplay (MetaXWaylandManager     *manager,
 {
   int display = 0;
   char *lock_file = NULL;
+  gboolean fatal = FALSE;
 
   if (display_number_override != -1)
     display = display_number_override;
@@ -640,12 +697,23 @@ choose_xdisplay (MetaXWaylandManager     *manager,
         }
 
       if (!open_display_sockets (manager, display,
+                                 &connection->abstract_fd,
                                  &connection->unix_fd,
+                                 &fatal,
                                  &local_error))
         {
           unlink (lock_file);
-          g_prefix_error (error, "Failed to bind X11 socket: ");
+
-          return FALSE;
+          if (!fatal)
+            {
+              display++;
+              continue;
+            }
+          else
+            {
+              g_warning ("Failed to bind X11 socket");
+              return FALSE;
+            }
         }
 
       break;
@@ -843,9 +911,10 @@ meta_xwayland_start_xserver (MetaXWaylandManager *manager,
   launcher = g_subprocess_launcher_new (flags);
 
   g_subprocess_launcher_take_fd (launcher, xwayland_client_fd[1], 3);
-  g_subprocess_launcher_take_fd (launcher, manager->public_connection.unix_fd, 4);
+  g_subprocess_launcher_take_fd (launcher, manager->public_connection.abstract_fd, 4);
-  g_subprocess_launcher_take_fd (launcher, displayfd[1], 5);
+  g_subprocess_launcher_take_fd (launcher, manager->public_connection.unix_fd, 5);
-  g_subprocess_launcher_take_fd (launcher, manager->private_connection.unix_fd, 6);
+  g_subprocess_launcher_take_fd (launcher, displayfd[1], 6);
+  g_subprocess_launcher_take_fd (launcher, manager->private_connection.abstract_fd, 7);
 
   g_subprocess_launcher_setenv (launcher, "WAYLAND_SOCKET", "3", TRUE);
 
@@ -860,14 +929,16 @@ meta_xwayland_start_xserver (MetaXWaylandManager *manager,
   args[i++] = manager->auth_file;
   args[i++] = "-listen";
   args[i++] = "4";
-  args[i++] = "-displayfd";
+  args[i++] = "-listen";
   args[i++] = "5";
+  args[i++] = "-displayfd";
+  args[i++] = "6";
 #ifdef HAVE_XWAYLAND_INITFD
   args[i++] = "-initfd";
-  args[i++] = "6";
+  args[i++] = "7";
 #else
   args[i++] = "-listen";
-  args[i++] = "6";
+  args[i++] = "7";
 #endif
   for (j = 0; j <  G_N_ELEMENTS (x11_extension_names); j++)
     {
@@ -987,6 +1058,7 @@ meta_xwayland_init (MetaXWaylandManager  *manager,
                     GError              **error)
 {
   MetaDisplayPolicy policy;
+  gboolean fatal;
 
   if (!manager->public_connection.name)
     {
@@ -1002,13 +1074,17 @@ meta_xwayland_init (MetaXWaylandManager  *manager,
     {
       if (!open_display_sockets (manager,
                                  manager->public_connection.display_index,
+                                 &manager->public_connection.abstract_fd,
                                  &manager->public_connection.unix_fd,
+                                 &fatal,
                                  error))
         return FALSE;
 
       if (!open_display_sockets (manager,
                                  manager->private_connection.display_index,
+                                 &manager->private_connection.abstract_fd,
                                  &manager->private_connection.unix_fd,
+                                 &fatal,
                                  error))
         return FALSE;
     }
@@ -1022,7 +1098,7 @@ meta_xwayland_init (MetaXWaylandManager  *manager,
 
   if (policy == META_DISPLAY_POLICY_ON_DEMAND)
     {
-      g_unix_fd_add (manager->public_connection.unix_fd, G_IO_IN,
+      g_unix_fd_add (manager->public_connection.abstract_fd, G_IO_IN,
                      xdisplay_connection_activity_cb, manager);
     }