Security: cap the perspective grid's line-loop bounds at 121 a way

crates/tools/src/distort/perspective.rs

@@ -562,14 +562,17 @@ impl PerspectiveGrid {
             s
         };
         let (su, sv) = (step(w), step(h_len));
-        // Counted, not summed: a count in whole lines (at most 121 a way).
-        for i in 0..=((w + 1e-9) / su).floor() as usize {
+        // Counted, not summed: a count in whole lines (at most 121 a way). The fixed 1e-9
+        // tolerance can inflate the count when the whole domain is far smaller than it, so the
+        // bound is capped on its own, before any loop runs.
+        let count = |len: f64, s: f64| ((len + 1e-9) / s).floor().clamp(0.0, 120.0) as usize;
+        for i in 0..=count(w, su) {
             let u = i as f64 * su;
             if let (Some(a), Some(b)) = (h.apply(Point::new(u, 0.0)), h.apply(Point::new(u, h_len))) {
                 out.push((a, b));
             }
         }
-        for i in 0..=((h_len + 1e-9) / sv).floor() as usize {
+        for i in 0..=count(h_len, sv) {
             let v = i as f64 * sv;
             if let (Some(a), Some(b)) = (h.apply(Point::new(0.0, v)), h.apply(Point::new(w, v))) {
                 out.push((a, b));

crates/tools/src/distort/perspective/view.rs

@@ -123,6 +123,16 @@ mod tests {
         assert_eq!(g.snap_offset(b, Vec2::new(18.0, 0.0)), Vec2::new(18.0, 0.0), "nothing within reach");
     }
 
+    #[test]
+    fn a_tiny_stored_grid_draws_a_bounded_number_of_lines() {
+        // A stored grid can carry extreme values: the loop bound must not depend on a fixed
+        // tolerance swallowing the whole domain.
+        let g = PerspectiveGrid { cell: 1e-18, extent: 1e-18, height: 1e-18, ..PerspectiveGrid::normal(2, Rect::new(0.0, 0.0, 800.0, 600.0)) };
+        for pl in Plane::ALL {
+            assert!(g.lines(pl).len() <= 121 + 121, "at most 121 lines a way on {pl:?}");
+        }
+    }
+
     #[test]
     fn colours_and_rulers_come_from_the_definition() {
         let mut g = grid();