xdg-shell: Add invalid_parent error to xdg_popup

Send an invalid_parent error when the client tries to create a popup
with a paren that is neither a xdg_surface nor a xdg_popup.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

desktop-shell/shell.c

@@ -4092,12 +4092,21 @@ create_xdg_popup(struct shell_client *owner, void *shell,
 		 uint32_t serial,
 		 int32_t x, int32_t y)
 {
-	struct shell_surface *shsurf;
+	struct shell_surface *shsurf, *parent_shsurf;
 
 	/* Verify that we are creating the top most popup when mapping,
 	 * as its not until then we know whether it was mapped as most
 	 * top level or not. */
 
+	parent_shsurf = get_shell_surface(parent);
+	if (!shell_surface_is_xdg_popup(parent_shsurf) &&
+	    !shell_surface_is_xdg_surface(parent_shsurf)) {
+		wl_resource_post_error(owner->resource,
+				       XDG_POPUP_ERROR_INVALID_PARENT,
+				       "xdg_popup parent was invalid");
+		return NULL;
+	}
+
 	shsurf = create_common_surface(owner, shell, surface, client);
 	if (!shsurf)
 		return NULL;

protocol/xdg-shell.xml

@@ -403,6 +403,7 @@
 	These errors can be emitted in response to xdg_popup requests.
       </description>
       <entry name="not_the_topmost_popup" value="0" summary="The client tried to map or destroy a non-topmost popup"/>
+      <entry name="invalid_parent" value="1" summary="The client specified an invalid parent surface"/>
     </enum>
 
     <request name="destroy" type="destructor">