[tracker/sam/2.x-export-data: 2/3] cli: Give clearer output when no data to export

[Sam Thursfield <sthursfield src gnome org>]

commit 134bb91611ba8ddc7c42104d218b136e281f2f60
Author: Sam Thursfield <sam afuera me uk>
Date:   Sun May 31 17:22:42 2020 +0200

    cli: Give clearer output when no data to export
    
    We would previously print a long list of `@prefix` statements and then
    exit. Now we print a comment like this:
    
            # No data to export

 src/tracker/tracker-export.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)
---
diff --git a/src/tracker/tracker-export.c b/src/tracker/tracker-export.c
index 740123205..87947788f 100644
--- a/src/tracker/tracker-export.c
+++ b/src/tracker/tracker-export.c
@@ -109,12 +109,17 @@ print_turtle (TrackerSparqlCursor *cursor,
        gchar *predicate;
        gchar *object;
 
+       if (!tracker_sparql_cursor_next (cursor, NULL, NULL)) {
+               g_print ("# No data to export\n");
+               return;
+       }
+
        if (show_prefixes) {
                g_hash_table_foreach (prefixes, (GHFunc) print_prefix, NULL);
                g_print ("\n");
        }
 
-       while (tracker_sparql_cursor_next (cursor, NULL, NULL)) {
+       do {
                const gchar *resource = tracker_sparql_cursor_get_string (cursor, 1, NULL);
                const gchar *key = tracker_sparql_cursor_get_string (cursor, 2, NULL);
                const gchar *value = tracker_sparql_cursor_get_string (cursor, 3, NULL);
@@ -147,7 +152,7 @@ print_turtle (TrackerSparqlCursor *cursor,
 
                g_free (predicate);
                g_free (object);
-       };
+       } while (tracker_sparql_cursor_next (cursor, NULL, NULL));
 }
 
 /* Print graphs and triples in TriG format */
@@ -162,12 +167,17 @@ print_trig (TrackerSparqlCursor *cursor,
        gchar *previous_graph = NULL;
        const gchar *graph;
 
+       if (!tracker_sparql_cursor_next (cursor, NULL, NULL)) {
+               g_print ("# No data to export\n");
+               return;
+       }
+
        if (show_prefixes) {
                g_hash_table_foreach (prefixes, (GHFunc) print_prefix, NULL);
                g_print ("\n");
        }
 
-       while (tracker_sparql_cursor_next (cursor, NULL, NULL)) {
+       do {
                graph = tracker_sparql_cursor_get_string (cursor, 0, NULL);
                const gchar *resource = tracker_sparql_cursor_get_string (cursor, 1, NULL);
                const gchar *key = tracker_sparql_cursor_get_string (cursor, 2, NULL);
@@ -211,7 +221,7 @@ print_trig (TrackerSparqlCursor *cursor,
 
                g_free (predicate);
                g_free (object);
-       };
+       } while (tracker_sparql_cursor_next (cursor, NULL, NULL));
 
        if (graph != NULL) {
                g_print ("}\n");
@@ -253,9 +263,6 @@ export_with_query (const gchar  *query,
                return FALSE;
        }
 
-       g_hash_table_foreach (prefixes, (GHFunc) print_prefix, NULL);
-       g_print ("\n");
-
        if (show_graphs) {
                print_trig (cursor, prefixes, FALSE, show_prefixes);
        } else {