[gnome-software] appstream: Correctly distinguish empty content ratings from missing ones

[Philip Withnall <pwithnall src gnome org>]

commit 17f428a089b076d58954fbead64ed074b032f2ef
Author: Philip Withnall <withnall endlessm com>
Date:   Wed Oct 2 15:19:07 2019 +0100

    appstream: Correctly distinguish empty content ratings from missing ones
    
    The OARS semantics require that if `<content_rating/>` is present, any
    attributes which are missing from it (which could be ‘all attributes’)
    are assumed to have a value of `none`. If `<content_rating/>` is
    missing, they are assumed to have a value of `unknown`.
    
    Previously, we were assuming a value of `unknown` for all of them, which
    was wrong.
    
    See the [OARS
    specification](https://github.com/hughsie/oars/blob/master/specification/oars-1.1.md).
    
    Signed-off-by: Philip Withnall <withnall endlessm com>

 plugins/core/gs-appstream.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)
---
diff --git a/plugins/core/gs-appstream.c b/plugins/core/gs-appstream.c
index def0001e..74e865dd 100644
--- a/plugins/core/gs-appstream.c
+++ b/plugins/core/gs-appstream.c
@@ -605,17 +605,23 @@ gs_appstream_refine_app_content_rating (GsPlugin *plugin,
        /* get kind */
        as_content_rating_set_kind (cr, xb_node_get_attr (content_rating, "type"));
 
-       /* get attributes */
+       /* get attributes; no attributes being found (i.e.
+        * `<content_rating type="*"/>`) is OK: it means that all attributes have
+        * value `none`, as per the
+        * [OARS semantics](https://github.com/hughsie/oars/blob/master/specification/oars-1.1.md) */
        content_attributes = xb_node_query (content_rating, "content_attribute", 0, &error_local);
-       if (content_attributes == NULL) {
-               if (g_error_matches (error_local, G_IO_ERROR, G_IO_ERROR_NOT_FOUND))
-                       return TRUE;
-               if (g_error_matches (error_local, G_IO_ERROR, G_IO_ERROR_INVALID_ARGUMENT))
-                       return TRUE;
+       if (content_attributes == NULL &&
+           g_error_matches (error_local, G_IO_ERROR, G_IO_ERROR_NOT_FOUND)) {
+               g_clear_error (&error_local);
+       } else if (content_attributes == NULL &&
+                g_error_matches (error_local, G_IO_ERROR, G_IO_ERROR_INVALID_ARGUMENT)) {
+               return TRUE;
+       } else if (content_attributes == NULL) {
                g_propagate_error (error, g_steal_pointer (&error_local));
                return FALSE;
        }
-       for (guint i = 0; i < content_attributes->len; i++) {
+
+       for (guint i = 0; content_attributes != NULL && i < content_attributes->len; i++) {
                XbNode *content_attribute = g_ptr_array_index (content_attributes, i);
                as_content_rating_add_attribute (cr,
                                                 xb_node_get_attr (content_attribute, "id"),