[tracker] libtracker-data: Handle empty clauses before BIND

[Carlos Garnacho <carlosg src gnome org>]

commit de70127517b7e86ca13fcc5554cb6dde270a27fa
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Feb 23 12:26:44 2019 +0100

    libtracker-data: Handle empty clauses before BIND
    
    The way BIND works, we wrap the current statements with a SELECT
    where we can set the variable name. This however broke if there's
    no prior statements, eg:
    
      SELECT ?a { BIND (?a AS ... ) ... }
    
    Generate correct SQL for this case.

 src/libtracker-data/tracker-sparql.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index 143c16298..6abdffe28 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -3203,16 +3203,25 @@ translate_Bind (TrackerSparql  *sparql,
        TrackerVariable *variable;
        TrackerBinding *binding;
        TrackerPropertyType type;
+       gboolean is_empty;
 
        /* Bind ::= 'BIND' '(' Expression 'AS' Var ')'
         */
        _expect (sparql, RULE_TYPE_LITERAL, LITERAL_BIND);
        _expect (sparql, RULE_TYPE_LITERAL, LITERAL_OPEN_PARENS);
 
-       str = _prepend_placeholder (sparql);
-       old = tracker_sparql_swap_builder (sparql, str);
+       is_empty = tracker_string_builder_is_empty (sparql->current_state.sql);
+
+       if (!is_empty) {
+               str = _prepend_placeholder (sparql);
+               old = tracker_sparql_swap_builder (sparql, str);
+       }
+
+       _append_string (sparql, "SELECT ");
+
+       if (!is_empty)
+               _append_string (sparql, "*, ");
 
-       _append_string (sparql, "SELECT *, ");
        _call_rule (sparql, NAMED_RULE_Expression, error);
        type = sparql->current_state.expression_type;
 
@@ -3224,15 +3233,18 @@ translate_Bind (TrackerSparql  *sparql,
        if (tracker_variable_has_bindings (variable))
                _raise (PARSE, "Expected undefined variable", "BIND");
 
-       _append_string_printf (sparql, "AS %s FROM (",
+       _append_string_printf (sparql, "AS %s ",
                               tracker_variable_get_sql_expression (variable));
 
        binding = tracker_variable_binding_new (variable, NULL, NULL);
        tracker_binding_set_data_type (binding, type);
        tracker_variable_set_sample_binding (variable, TRACKER_VARIABLE_BINDING (binding));
 
-       tracker_sparql_swap_builder (sparql, old);
-       _append_string (sparql, ") ");
+       if (!is_empty) {
+               _append_string (sparql, "FROM (");
+               tracker_sparql_swap_builder (sparql, old);
+               _append_string (sparql, ") ");
+       }
 
        _expect (sparql, RULE_TYPE_LITERAL, LITERAL_CLOSE_PARENS);