[tracker] libtracker-data: Gracefully exclude NULL subject/pred/object from resultset

[Carlos Garnacho <carlosg src gnome org>]

commit 8433d3869a79e2e7bf2fdcf2952f86cfa6a73d22
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sun Dec 23 23:32:12 2018 +0100

    libtracker-data: Gracefully exclude NULL subject/pred/object from resultset
    
    This is mentioned in the spec, and was correctly handled in the previous
    parser (although the comment mentioned text from a similar restriction with
    CONSTRUCT). Solutions with unbound variables should simply be ignored,
    this was kinda the case but just detected through g_return_if_fail() later
    on.

 src/libtracker-data/tracker-sparql.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index ccbd2ca74..db60aa822 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -735,7 +735,8 @@ _init_token (TrackerToken      *token,
                        const gchar *value;
 
                        value = g_hash_table_lookup (sparql->solution_var_map, str);
-                       tracker_token_literal_init (token, value);
+                       if (value)
+                               tracker_token_literal_init (token, value);
                }
        } else if (tracker_grammar_rule_is_a (rule, RULE_TYPE_TERMINAL, TERMINAL_TYPE_PARAMETERIZED_VAR)) {
                tracker_token_parameter_init (token, str);
@@ -4385,10 +4386,8 @@ translate_GraphNode (TrackerSparql  *sparql,
         */
        if (_check_in_rule (sparql, NAMED_RULE_VarOrTerm)) {
                _call_rule (sparql, NAMED_RULE_VarOrTerm, error);
-               g_assert (!tracker_token_is_empty (&sparql->current_state.object));
        } else if (_check_in_rule (sparql, NAMED_RULE_TriplesNode)) {
                _call_rule (sparql, NAMED_RULE_TriplesNode, error);
-               g_assert (!tracker_token_is_empty (&sparql->current_state.object));
        } else if (_accept (sparql, RULE_TYPE_LITERAL, LITERAL_NULL)) {
                if (sparql->current_state.type != TRACKER_SPARQL_TYPE_UPDATE)
                        _raise (PARSE, "«NULL» literal is not allowed in this mode", "NULL");
@@ -4397,6 +4396,21 @@ translate_GraphNode (TrackerSparql  *sparql,
                g_assert_not_reached ();
        }
 
+       /* Quoting sparql11-update:
+        * If any solution produces a triple containing an unbound variable
+        * or an illegal RDF construct, such as a literal in a subject or
+        * predicate position, then that triple is not included when processing
+        * the operation: INSERT will not instantiate new data in the output
+        * graph, and DELETE will not remove anything.
+        *
+        * Updates are a Tracker extension and object may be explicitly NULL.
+        */
+       if (tracker_token_is_empty (&sparql->current_state.subject) ||
+           tracker_token_is_empty (&sparql->current_state.predicate) ||
+           (tracker_token_is_empty (&sparql->current_state.object) &&
+            sparql->current_state.type != TRACKER_SPARQL_TYPE_UPDATE))
+               return TRUE;
+
        switch (sparql->current_state.type) {
        case TRACKER_SPARQL_TYPE_SELECT:
                _add_quad (sparql,