fix(drivingDistance): raise error on negative distance input

pgtap/dijkstra/driving_distance/edge_cases/issue_3091.pg

@@ -0,0 +1,25 @@
+/* :file: This file is part of the pgRouting project.
+:copyright: Copyright (c) 2026 pgRouting developers
+:license: Creative Commons Attribution-Share Alike 3.0 https://creativecommons.org/licenses/by-sa/3.0 */
+
+/* Issue #3091: pgr_drivingDistance accepts negative distance without raising an error */
+
+BEGIN;
+
+SELECT plan(2);
+
+-- Negative distance should raise an error, not silently return empty result.
+SELECT throws_ok(
+    $$SELECT * FROM pgr_drivingDistance('SELECT id, source, target, cost, reverse_cost FROM edges', 1, -1.0)$$,
+    'Negative value found on ''distance''',
+    '1: pgr_drivingDistance with negative distance raises error'
+);
+
+-- Distance of 0 is valid (returns only the origin node).
+SELECT ok(
+    (SELECT count(*) = 1 FROM pgr_drivingDistance('SELECT id, source, target, cost, reverse_cost FROM edges', 1, 0.0)),
+    '2: pgr_drivingDistance with distance=0 returns only origin'
+);
+
+SELECT * FROM finish();
+ROLLBACK;

src/driving_distance/driving_distance_driver.cpp

@@ -74,6 +74,12 @@ pgr_do_drivingDistance(
         pgassert(*return_count == 0);
         pgassert((*return_tuples) == NULL);
 
+        if (distance < 0) {
+            *err_msg = to_pg_msg("Negative value found on 'distance'");
+            *log_msg = to_pg_msg("distance must be non-negative");
+            return;
+        }
+
         auto roots = get_intSet(starts);
 
         hint = edges_sql;

src/driving_distance/driving_distance_withPoints_driver.cpp

@@ -87,6 +87,12 @@ pgr_do_withPointsDD(
         pgassert(!(*return_tuples));
         pgassert(*return_count == 0);
 
+        if (distance < 0) {
+            *err_msg = to_pg_msg("Negative value found on 'distance'");
+            *log_msg = to_pg_msg("distance must be non-negative");
+            return;
+        }
+
         auto roots = get_intSet(starts);
 
         hint = points_sql;