[tracker/wip/carlosg/fix-minus: 2/3] libtracker-data: Fix MINUS support

[Carlos Garnacho <carlosg src gnome org>]

commit f6f4da9c9e7839c8b30a5063c8a2f319ec43e228
Author: Carlos Garnacho <carlosg gnome org>
Date:   Fri Mar 6 19:59:41 2020 +0100

    libtracker-data: Fix MINUS support
    
    We implement MINUS through sqlite's EXCEPT, which requires SELECTS on
    both sides of the operation to have the same variables in the same
    order. Ensure this is what happens.
    
    One thing to note is that we do not bother checking whether new
    variables are created on the right hand, as per MINUS semantics those
    cannot count in the result set.
    
    Closes: https://gitlab.gnome.org/GNOME/tracker/issues/190

 src/libtracker-data/tracker-sparql.c | 68 ++++++++++++++++++++++++++++++++++--
 1 file changed, 66 insertions(+), 2 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index bcdcd52e7..51fe37acb 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -5005,16 +5005,80 @@ append_subquery_select_vars (TrackerSparql  *sparql,
        _append_string (sparql, "FROM (");
 }
 
+static GList *
+intersect_var_set (GHashTable *ht1,
+                  GHashTable *ht2)
+{
+       GHashTableIter iter;
+       GList *intersection = NULL;
+       gpointer key;
+
+       g_hash_table_iter_init (&iter, ht1);
+
+       while (g_hash_table_iter_next (&iter, &key, NULL)) {
+               if (g_hash_table_contains (ht2, key))
+                       intersection = g_list_prepend (intersection, key);
+       }
+
+       return intersection;
+}
+
+
 static gboolean
 translate_MinusGraphPattern (TrackerSparql  *sparql,
                              GError        **error)
 {
+       TrackerStringBuilder *pre, *post, *cur;
+       TrackerContext *cur_context, *context;
+       GList *intersection, *l;
+
+       cur_context = sparql->current_state.context;
+
        /* MinusGraphPattern ::= 'MINUS' GroupGraphPattern
         */
        _expect (sparql, RULE_TYPE_LITERAL, LITERAL_MINUS);
-       _prepend_string (sparql, "SELECT * FROM (");
-       _append_string (sparql, ") EXCEPT ");
+
+       pre = _prepend_placeholder (sparql);
+       post = _append_placeholder (sparql);
+
+       context = tracker_context_new ();
+       tracker_sparql_push_context (sparql, context);
        _call_rule (sparql, NAMED_RULE_GroupGraphPattern, error);
+       tracker_sparql_pop_context (sparql, FALSE);
+
+       intersection = intersect_var_set (cur_context->variable_set, context->variable_set);
+
+       cur = tracker_sparql_swap_builder (sparql, pre);
+       append_subquery_select_vars (sparql, cur_context, intersection);
+       tracker_sparql_swap_builder (sparql, cur);
+
+       if (intersection) {
+               cur = tracker_sparql_swap_builder (sparql, post);
+               _append_string (sparql, ") WHERE (");
+               for (l = intersection; l; l = l->next) {
+                       if (l != intersection)
+                               _append_string (sparql, ", ");
+                       _append_string_printf (sparql, "%s ",
+                                              tracker_variable_get_sql_expression (l->data));
+               }
+
+               _append_string (sparql, ") NOT IN (");
+               append_subquery_select_vars (sparql, context, intersection);
+
+               tracker_sparql_swap_builder (sparql, cur);
+               _append_string (sparql, ")) ");
+       } else {
+               GList *vars;
+
+               cur = tracker_sparql_swap_builder (sparql, post);
+               _append_string (sparql, ") EXCEPT ");
+               vars = g_hash_table_get_keys (cur_context->variable_set);
+               append_subquery_select_vars (sparql, context, vars);
+               g_list_free (vars);
+
+               tracker_sparql_swap_builder (sparql, cur);
+               _append_string (sparql, ") ");
+       }
 
        return TRUE;
 }