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.

1 files changed, 7 insertions, 1 deletions

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;
 	}
 }