wayland: Avoid a race in wl_seat capabilities

The way wl_seat capabilities work, by notifying clients of capabilities
changes, and clients consequently requesting the relevant interface
objects (pointer, keyboard, touch) is inherently racy.

On quick VT changes for example, capabilities on the seat will be added
and removed, and by the time the client receives the capability change
notification and requests the relevant keyboard, pointer or touch,
another VT switch might have occurred and the wl_pointer, wl_keyboard or
wl_touch already destroyed, leading to a protocol error which kills the
client.

To avoid this, create the objects when requested regardless of the
capabilities.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1797
Related: https://bugzilla.gnome.org/show_bug.cgi?id=790932
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/77>

src/wayland/meta-wayland-pointer.c

@@ -109,7 +109,7 @@ meta_wayland_pointer_client_new (void)
 }
 
 static void
-meta_wayland_pointer_client_free (MetaWaylandPointerClient *pointer_client)
+meta_wayland_pointer_make_resources_inert (MetaWaylandPointerClient *pointer_client)
 {
   struct wl_resource *resource, *next;
 
@@ -141,10 +141,25 @@ meta_wayland_pointer_client_free (MetaWaylandPointerClient *pointer_client)
       wl_list_init (wl_resource_get_link (resource));
       wl_resource_set_user_data (resource, NULL);
     }
+}
 
+static void
+meta_wayland_pointer_client_free (MetaWaylandPointerClient *pointer_client)
+{
+  meta_wayland_pointer_make_resources_inert (pointer_client);
   g_free (pointer_client);
 }
 
+static void
+make_resources_inert_foreach (gpointer key,
+                              gpointer value,
+                              gpointer data)
+{
+  MetaWaylandPointerClient *pointer_client = value;
+
+  meta_wayland_pointer_make_resources_inert (pointer_client);
+}
+
 static gboolean
 meta_wayland_pointer_client_is_empty (MetaWaylandPointerClient *pointer_client)
 {
@@ -158,8 +173,6 @@ MetaWaylandPointerClient *
 meta_wayland_pointer_get_pointer_client (MetaWaylandPointer *pointer,
                                          struct wl_client   *client)
 {
-  if (!pointer->pointer_clients)
-    return NULL;
   return g_hash_table_lookup (pointer->pointer_clients, client);
 }
 
@@ -475,10 +488,6 @@ meta_wayland_pointer_enable (MetaWaylandPointer *pointer)
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
   ClutterSeat *clutter_seat;
 
-  pointer->pointer_clients =
-    g_hash_table_new_full (NULL, NULL, NULL,
-                           (GDestroyNotify) meta_wayland_pointer_client_free);
-
   pointer->cursor_surface = NULL;
 
   clutter_seat = clutter_backend_get_default_seat (clutter_get_default_backend ());
@@ -508,6 +517,10 @@ meta_wayland_pointer_disable (MetaWaylandPointer *pointer)
   ClutterBackend *clutter_backend = clutter_get_default_backend ();
   ClutterSeat *clutter_seat = clutter_backend_get_default_seat (clutter_backend);
 
+  g_hash_table_foreach (pointer->pointer_clients,
+                        make_resources_inert_foreach,
+                        NULL);
+
   g_signal_handlers_disconnect_by_func (cursor_tracker,
                                         (gpointer) meta_wayland_pointer_on_cursor_changed,
                                         pointer);
@@ -531,7 +544,6 @@ meta_wayland_pointer_disable (MetaWaylandPointer *pointer)
   meta_wayland_pointer_set_focus (pointer, NULL);
   meta_wayland_pointer_set_current (pointer, NULL);
 
-  g_clear_pointer (&pointer->pointer_clients, g_hash_table_unref);
   pointer->cursor_surface = NULL;
 }
 
@@ -1356,11 +1368,28 @@ meta_wayland_pointer_init (MetaWaylandPointer *pointer)
   pointer->default_grab.interface = &default_pointer_grab_interface;
   pointer->default_grab.pointer = pointer;
   pointer->grab = &pointer->default_grab;
+  pointer->pointer_clients =
+    g_hash_table_new_full (NULL, NULL, NULL,
+                           (GDestroyNotify) meta_wayland_pointer_client_free);
+}
+
+static void
+meta_wayland_pointer_finalize (GObject *object)
+{
+  MetaWaylandPointer *pointer = META_WAYLAND_POINTER (object);
+
+  g_clear_pointer (&pointer->pointer_clients, g_hash_table_unref);
+
+  G_OBJECT_CLASS (meta_wayland_pointer_parent_class)->finalize (object);
 }
 
 static void
 meta_wayland_pointer_class_init (MetaWaylandPointerClass *klass)
 {
+  GObjectClass *object_class = G_OBJECT_CLASS (klass);
+
+  object_class->finalize = meta_wayland_pointer_finalize;
+
   signals[FOCUS_SURFACE_CHANGED] = g_signal_new ("focus-surface-changed",
                                                  G_TYPE_FROM_CLASS (klass),
                                                  G_SIGNAL_RUN_LAST,

src/wayland/meta-wayland-seat.c

@@ -46,7 +46,6 @@ seat_get_pointer (struct wl_client *client,
   MetaWaylandSeat *seat = wl_resource_get_user_data (resource);
   MetaWaylandPointer *pointer = seat->pointer;
 
-  if (meta_wayland_seat_has_pointer (seat))
   meta_wayland_pointer_create_new_resource (pointer, client, resource, id);
 }
 
@@ -58,7 +57,6 @@ seat_get_keyboard (struct wl_client *client,
   MetaWaylandSeat *seat = wl_resource_get_user_data (resource);
   MetaWaylandKeyboard *keyboard = seat->keyboard;
 
-  if (meta_wayland_seat_has_keyboard (seat))
   meta_wayland_keyboard_create_new_resource (keyboard, client, resource, id);
 }
 
@@ -70,7 +68,6 @@ seat_get_touch (struct wl_client *client,
   MetaWaylandSeat *seat = wl_resource_get_user_data (resource);
   MetaWaylandTouch *touch = seat->touch;
 
-  if (meta_wayland_seat_has_touch (seat))
   meta_wayland_touch_create_new_resource (touch, client, resource, id);
 }
 

src/wayland/meta-wayland-touch.c

@@ -521,16 +521,8 @@ meta_wayland_touch_create_new_resource (MetaWaylandTouch   *touch,
                                         struct wl_resource *seat_resource,
                                         uint32_t            id)
 {
-  MetaWaylandSeat *seat = wl_resource_get_user_data (seat_resource);
   struct wl_resource *cr;
 
-  if (!meta_wayland_seat_has_touch (seat))
-    {
-      wl_resource_post_error (seat_resource, WL_DISPLAY_ERROR_INVALID_METHOD,
-                              "Cannot retrieve touch interface without touch capability");
-      return;
-    }
-
   cr = wl_resource_create (client, &wl_touch_interface, wl_resource_get_version (seat_resource), id);
   wl_resource_set_implementation (cr, &touch_interface, touch, unbind_resource);
   wl_list_insert (&touch->resource_list, wl_resource_get_link (cr));