[tracker/wip/carlosg/zero-or-more-fix: 1/3] libtracker-sparql: Fix actual 0-path matches in ? and * path operators

[Carlos Garnacho <carlosg src gnome org>]

commit ad0f8ab4a83cb8ecd2bb55619d06cc6544e45f7d
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sun Nov 7 13:54:01 2021 +0100

    libtracker-sparql: Fix actual 0-path matches in ? and * path operators
    
    These operators ("zero or one" and "zero or more" respectively) so far have
    worked on the prerequisite that the contained property path has at least
    one match with the given constraints. E.g.:
    
      SELECT ?u { <URI> nie:interpretedAs? ?u }
    
    Would return no results if the given uri had no nie:interpretedAs, but here
    we still want to have the 0-length match, regardless of there being a match
    or not.
    
    This commit fixes this by making these common table expressions look for
    "self" in the rdfs:Resource table, this is ensured to always match as long
    as the resource exists.
    
    Fixes: https://gitlab.gnome.org/GNOME/tracker/-/issues/337

 src/libtracker-data/tracker-sparql.c | 43 +++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 6 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index d12786e45..f6d42228a 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -895,10 +895,42 @@ _prepend_path_element (TrackerSparql      *sparql,
 {
        TrackerStringBuilder *old;
        gchar *table_name, *graph_column;
+       gchar *zero_length_match = NULL;
 
        if (path_elem->op == TRACKER_PATH_OPERATOR_NONE &&
            tracker_token_is_empty (&sparql->current_state->graph)) {
                tracker_sparql_add_union_graph_subquery (sparql, path_elem->data.property);
+       } else if (path_elem->op == TRACKER_PATH_OPERATOR_ZEROORONE ||
+                  path_elem->op == TRACKER_PATH_OPERATOR_ZEROORMORE) {
+               const gchar *graph;
+
+               graph = tracker_token_get_idstring (&sparql->current_state->graph);
+
+               if (tracker_token_is_empty (&sparql->current_state->graph) ||
+                   tracker_token_get_variable (&sparql->current_state->graph)) {
+                       TrackerOntologies *ontologies;
+                       TrackerClass *rdfs_resource;
+
+                       ontologies = tracker_data_manager_get_ontologies (sparql->data_manager);
+                       rdfs_resource = tracker_ontologies_get_class_by_uri (ontologies, RDFS_NS "Resource");
+                       tracker_sparql_add_union_graph_subquery_for_class (sparql,
+                                                                          rdfs_resource);
+
+                       zero_length_match = g_strdup_printf ("SELECT ID, ID, graph, %d, %d "
+                                                            "FROM \"unionGraph_rdfs:Resource\"",
+                                                            TRACKER_PROPERTY_TYPE_RESOURCE,
+                                                            TRACKER_PROPERTY_TYPE_RESOURCE);
+               } else if (tracker_sparql_find_graph (sparql, graph)) {
+                       zero_length_match = g_strdup_printf ("SELECT ID, ID, %d, %d, %d "
+                                                            "FROM \"%s\".\"rdfs:Resource\"",
+                                                            tracker_sparql_find_graph (sparql, graph),
+                                                            TRACKER_PROPERTY_TYPE_RESOURCE,
+                                                            TRACKER_PROPERTY_TYPE_RESOURCE,
+                                                            graph);
+               } else {
+                       /* Graph does not exist, ensure to come back empty */
+                       zero_length_match = g_strdup ("SELECT * FROM (SELECT 0 AS ID, NULL, NULL, 0, 0 LIMIT 
0)");
+               }
        }
 
        old = tracker_sparql_swap_builder (sparql, sparql->current_state->with_clauses);
@@ -991,14 +1023,13 @@ _prepend_path_element (TrackerSparql      *sparql,
                                       "(SELECT ID, value, graph, ID_type, value_type "
                                       "FROM \"%s_helper\" "
                                       "UNION "
-                                      "SELECT ID, ID, graph, ID_type, ID_type "
-                                      "FROM \"%s\" "
+                                      "%s "
                                       "UNION "
                                       "SELECT value, value, graph, value_type, value_type "
                                       "FROM \"%s\") ",
                                       path_elem->name,
                                       path_elem->name,
-                                      path_elem->data.composite.child1->name,
+                                      zero_length_match,
                                       path_elem->data.composite.child1->name);
                break;
        case TRACKER_PATH_OPERATOR_ONEORMORE:
@@ -1018,8 +1049,7 @@ _prepend_path_element (TrackerSparql      *sparql,
        case TRACKER_PATH_OPERATOR_ZEROORONE:
                _append_string_printf (sparql,
                                       "\"%s\" (ID, value, graph, ID_type, value_type) AS "
-                                      "(SELECT ID, ID, graph, ID_type, ID_type "
-                                      "FROM \"%s\" "
+                                      "(%s "
                                       "UNION "
                                       "SELECT ID, value, graph, ID_type, value_type "
                                       "FROM \"%s\" "
@@ -1027,7 +1057,7 @@ _prepend_path_element (TrackerSparql      *sparql,
                                       "SELECT value, value, graph, value_type, value_type "
                                       "FROM \"%s\") ",
                                       path_elem->name,
-                                      path_elem->data.composite.child1->name,
+                                      zero_length_match,
                                       path_elem->data.composite.child1->name,
                                       path_elem->data.composite.child1->name);
                break;
@@ -1075,6 +1105,7 @@ _prepend_path_element (TrackerSparql      *sparql,
        }
 
        tracker_sparql_swap_builder (sparql, old);
+       g_free (zero_length_match);
 }
 
 static inline gchar *