[tracker/wip/carlosg/fixes-for-music] libtracker-bus: Make error clearer

[Carlos Garnacho <carlosg src gnome org>]

commit f81c4cd057ec3caf6bde242d7885f9561297d587
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Nov 17 11:39:50 2018 +0100

    libtracker-bus: Make error clearer
    
    The data != null check in get_string() really happens because of
    fetching values on a finished or not yet started cursor, so make
    it clearer to reason about.
    
    This is of course a programming error.

 src/libtracker-bus/tracker-bus-fd-cursor.vala | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/src/libtracker-bus/tracker-bus-fd-cursor.vala b/src/libtracker-bus/tracker-bus-fd-cursor.vala
index d42a6133f..751c65349 100644
--- a/src/libtracker-bus/tracker-bus-fd-cursor.vala
+++ b/src/libtracker-bus/tracker-bus-fd-cursor.vala
@@ -27,11 +27,13 @@ class Tracker.Bus.FDCursor : Tracker.Sparql.Cursor {
        internal int* types;
        internal char* data;
        internal string[] variable_names;
+       internal bool cursor_finished;
 
        public FDCursor (char* buffer, ulong buffer_size, string[] variable_names) {
                this.buffer = buffer;
                this.buffer_size = buffer_size;
                this.variable_names = variable_names;
+               this.cursor_finished = true;
                _n_columns = variable_names.length;
        }
 
@@ -63,7 +65,7 @@ class Tracker.Bus.FDCursor : Tracker.Sparql.Cursor {
        }
 
        public override unowned string? get_string (int column, out long length = null)
-       requires (data != null) {
+       requires (cursor_finished == false) {
                unowned string str = null;
 
                if (column >= n_columns) {
@@ -96,6 +98,8 @@ class Tracker.Bus.FDCursor : Tracker.Sparql.Cursor {
                }
 
                if (buffer_index >= buffer_size) {
+                       cursor_finished = true;
+                       data = null;
                        return false;
                }
 
@@ -118,6 +122,7 @@ class Tracker.Bus.FDCursor : Tracker.Sparql.Cursor {
                last_offset = buffer_read_int ();
 
                data = buffer + buffer_index;
+               cursor_finished = false;
 
                buffer_index += last_offset + 1;
 
@@ -132,5 +137,6 @@ class Tracker.Bus.FDCursor : Tracker.Sparql.Cursor {
        public override void rewind () {
                buffer_index = 0;
                data = buffer;
+               cursor_finished = false;
        }
 }