[tracker/wip/carlosg/zero-or-more-fix: 1/2] libtracker-sparql: Fix actual 0-path matches in ? and * path operators

[Carlos Garnacho <carlosg src gnome org>]

commit c9d4af0179c70de1f1b082165d98423dbf11bcc7
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 | 49 +++++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 6 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index d12786e45..ba4fdf585 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -895,10 +895,39 @@ _prepend_path_element (TrackerSparql      *sparql,
 {
        TrackerStringBuilder *old;
        gchar *table_name, *graph_column;
+       gchar *zero_length_match_table = NULL, *zero_length_match_graph = 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_table = g_strdup ("\"unionGraph_rdfs:Resource\"");
+                       zero_length_match_graph = g_strdup ("graph");
+               } else if (tracker_sparql_find_graph (sparql, graph)) {
+                       zero_length_match_table = g_strdup_printf ("\"%s\".\"rdfs:Resource\"",
+                                                                  graph);
+                       zero_length_match_graph = g_strdup_printf ("%d",
+                                                                  tracker_sparql_find_graph (sparql, graph));
+               } else {
+                       /* Graph does not exist, ensure to come back empty */
+                       zero_length_match_table = g_strdup ("(SELECT 0 AS ID, NULL, NULL, 0, 0 LIMIT 0)");
+                       zero_length_match_graph = g_strdup_printf ("0");
+               }
        }
 
        old = tracker_sparql_swap_builder (sparql, sparql->current_state->with_clauses);
@@ -991,14 +1020,17 @@ _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\" "
+                                      "SELECT ID, ID, %s, %d, %d "
+                                      "FROM %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_graph,
+                                      TRACKER_PROPERTY_TYPE_RESOURCE,
+                                      TRACKER_PROPERTY_TYPE_RESOURCE,
+                                      zero_length_match_table,
                                       path_elem->data.composite.child1->name);
                break;
        case TRACKER_PATH_OPERATOR_ONEORMORE:
@@ -1018,8 +1050,8 @@ _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\" "
+                                      "(SELECT ID, ID, %s, %d, %d "
+                                      "FROM %s "
                                       "UNION "
                                       "SELECT ID, value, graph, ID_type, value_type "
                                       "FROM \"%s\" "
@@ -1027,7 +1059,10 @@ _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_graph,
+                                      TRACKER_PROPERTY_TYPE_RESOURCE,
+                                      TRACKER_PROPERTY_TYPE_RESOURCE,
+                                      zero_length_match_table,
                                       path_elem->data.composite.child1->name,
                                       path_elem->data.composite.child1->name);
                break;
@@ -1075,6 +1110,8 @@ _prepend_path_element (TrackerSparql      *sparql,
        }
 
        tracker_sparql_swap_builder (sparql, old);
+       g_free (zero_length_match_table);
+       g_free (zero_length_match_graph);
 }
 
 static inline gchar *