[tracker/wip/carlosg/joining-services: 2/6] libtracker-data: Check binding type before printing variable in triples block

[Carlos Garnacho <carlosg src gnome org>]

commit 3cbd723603420cc71b2ef6b3ddf3ccf46683402f
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed May 13 01:27:18 2020 +0200

    libtracker-data: Check binding type before printing variable in triples block
    
    A "resource" binding might have been used previously in a services table query
    in which case it would hold a string. Play along in later triple blocks and
    use the same type.
    
    This makes this query work:
    
    SELECT ?u {
      SERVICE <dbus:org.freedesktop.Tracker3.Miner.Files> {
        ?u a rdfs:Resource
      }
      ?u tracker:indexed true
    }
    
    Fixes: https://gitlab.gnome.org/GNOME/tracker/-/issues/206

 src/libtracker-data/tracker-sparql.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index 111306033..be99b68c3 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -2054,7 +2054,7 @@ _end_triples_block (TrackerSparql  *sparql,
 
        /* Add select variables */
        while (g_hash_table_iter_next (&iter, (gpointer *) &var, NULL)) {
-               TrackerBinding *binding;
+               TrackerBinding *binding, *sample;
                GPtrArray *binding_list;
 
                binding_list = tracker_triple_context_get_variable_binding_list (triple_context,
@@ -2067,9 +2067,28 @@ _end_triples_block (TrackerSparql  *sparql,
 
                first = FALSE;
                binding = g_ptr_array_index (binding_list, 0);
-               _append_string_printf (sparql, "%s AS %s ",
-                                      tracker_binding_get_sql_expression (binding),
-                                      tracker_variable_get_sql_expression (var));
+               sample = TRACKER_BINDING (tracker_variable_get_sample_binding (var));
+
+               if (sample &&
+                   sample->data_type == TRACKER_PROPERTY_TYPE_STRING &&
+                   binding->data_type == TRACKER_PROPERTY_TYPE_RESOURCE) {
+                       TrackerStringBuilder *str, *old;
+
+                       str = _append_placeholder (sparql);
+                       old = tracker_sparql_swap_builder (sparql, str);
+
+                       _append_string_printf (sparql, "%s ",
+                                              tracker_binding_get_sql_expression (binding));
+
+                       convert_expression_to_string (sparql, binding->data_type);
+                       _append_string_printf (sparql, "AS %s ", tracker_variable_get_sql_expression (var));
+
+                       tracker_sparql_swap_builder (sparql, old);
+               } else {
+                       _append_string_printf (sparql, "%s AS %s ",
+                                              tracker_binding_get_sql_expression (binding),
+                                              tracker_variable_get_sql_expression (var));
+               }
        }
 
        if (first)