libtracker-data: Detect remote queries with no projected parameters

Querying a service that does not use any variable in its graph pattern
is fairly pointless. There are no variables to project into the rest
of the query, thus there's no way to match anything with the service
graph pattern. In pure SPARQL, that is basically just:

  SELECT * { SERVICE <...> { } }

But since in Tracker we are more lenient wrt giving each projected
variable a name, we also have cases like:

  SELECT * { SERVICE <...> { SELECT 2 { } } }

That are anyways pointless to execute. Detect all these cases and
send an empty query string instead, the service vtable will behave
as expected.

1 files changed, 20 insertions, 11 deletions

diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index ce33a5ca2..10e04bfd1 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -5196,7 +5196,7 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 	GList *variable_rules = NULL, *l;
 	GList *join_vars = NULL;
 	TrackerToken service;
-	GString *service_sparql;
+	GString *service_sparql = NULL;
 	gboolean silent = FALSE, do_join;
 	gint i = 0;
 
@@ -5241,7 +5241,6 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 
 	pattern = _skip_rule (sparql, NAMED_RULE_GroupGraphPattern);
 	_append_string (sparql, "SELECT ");
-	service_sparql = g_string_new ("SELECT ");
 
 	variable_rules = extract_variables (sparql, pattern);
 
@@ -5269,6 +5268,9 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 		if (i > 0)
 			_append_string (sparql, ", ");
 
+		if (!service_sparql)
+			service_sparql = g_string_new ("SELECT ");
+
 		/* Variable was used before in the graph pattern, preserve
 		 * for later so we join on it properly.
 		 */
@@ -5289,6 +5291,9 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 		i++;
 	}
 
+	if (variable_rules == NULL)
+		_append_string (sparql, "* ");
+
 	if (tracker_token_get_variable (&service)) {
 		if (variable_rules != NULL)
 			_append_string (sparql, ", ");
@@ -5298,16 +5303,18 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 		join_vars = g_list_prepend (join_vars, tracker_token_get_variable (&service));
 	}
 
-	tracker_parser_node_get_extents (pattern, &pattern_start, &pattern_end);
-	pattern_str = g_strndup (&sparql->sparql[pattern_start], pattern_end - pattern_start);
-	escaped_str = _escape_sql_string (pattern_str, '"');
-	g_string_append (service_sparql, escaped_str);
-	g_list_free (variables);
-	g_free (pattern_str);
-	g_free (escaped_str);
+	if (service_sparql) {
+		tracker_parser_node_get_extents (pattern, &pattern_start, &pattern_end);
+		pattern_str = g_strndup (&sparql->sparql[pattern_start], pattern_end - pattern_start);
+		escaped_str = _escape_sql_string (pattern_str, '"');
+		g_string_append (service_sparql, escaped_str);
+		g_list_free (variables);
+		g_free (pattern_str);
+		g_free (escaped_str);
+	}
 
 	_append_string_printf (sparql, "FROM tracker_service WHERE query=\"%s\" AND silent=%d ",
-			       service_sparql->str,
+	                       service_sparql ? service_sparql->str : "",
 			       silent);
 
 	if (!tracker_token_get_variable (&service)) {
@@ -5315,6 +5322,9 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 		                       tracker_token_get_idstring (&service));
 	}
 
+	if (service_sparql)
+		g_string_free (service_sparql, TRUE);
+
 	i = 0;
 
 	/* Proxy parameters to the virtual table */
@@ -5344,7 +5354,6 @@ translate_ServiceGraphPattern (TrackerSparql  *sparql,
 
 	tracker_token_unset (&service);
 	tracker_sparql_pop_context (sparql, TRUE);
-	g_string_free (service_sparql, TRUE);
 	g_list_free (variable_rules);
 
 	if (do_join) {