fix: validate negative distance in pgr_drivingDistance

Author: sakirr

pgtap/dijkstra/driving_distance/edge_cases/negative_distance.pg

@@ -0,0 +1,45 @@
+/* :file: This file is part of the pgRouting project.
+:copyright: Copyright (c) 2025-2026 pgRouting developers
+:license: Creative Commons Attribution-Share Alike 3.0 https://creativecommons.org/licenses/by-sa/3.0 */
+
+BEGIN;
+
+UPDATE edges SET cost = sign(cost), reverse_cost = sign(reverse_cost);
+SELECT CASE WHEN min_version('3.6.0') THEN plan(3) ELSE plan(1) END;
+
+CREATE OR REPLACE FUNCTION test_function()
+RETURNS SETOF TEXT AS
+$BODY$
+BEGIN
+    IF min_version('3.6.0') THEN
+        -- Test negative distance throws error
+        SELECT throws_ok(
+            $$SELECT * FROM pgr_drivingDistance('SELECT id, source, target, cost FROM edges', 1, -1.0)$$,
+            'XX000',
+            'Invalid value of ''distance''',
+            'SHOULD THROW because distance is negative'
+        );
+
+        -- Test zero distance throws error  
+        SELECT throws_ok(
+            $$SELECT * FROM pgr_drivingDistance('SELECT id, source, target, cost FROM edges', 1, 0.0)$$,
+            'XX000',
+            'Invalid value of ''distance''',
+            'SHOULD THROW because distance is zero'
+        );
+
+        -- Test positive distance works (regression test)
+        SELECT lives_ok(
+            $$SELECT * FROM pgr_drivingDistance('SELECT id, source, target, cost FROM edges', 1, 1.0)$$,
+            'SHOULD NOT THROW because distance is positive'
+        );
+    ELSE
+        SELECT skip(1, 'Function standardized on 3.6.0');
+    END IF;
+END;
+$BODY$
+LANGUAGE plpgsql;
+
+SELECT * FROM test_function();
+SELECT * FROM finish();
+ROLLBACK;

src/driving_distance/driving_distance.c

@@ -49,6 +49,10 @@ void process(
         bool equicost,
         MST_rt **result_tuples,
         size_t *result_count) {
+    if (distance <= 0) {
+        pgr_throw_error("Invalid value of 'distance'", "Valid values are greater than 0");
+    }
+
     pgr_SPI_connect();
     char* log_msg = NULL;
     char* notice_msg = NULL;