Make Path::Polyline::GetContourPointBounds safe for OOB (flutter#60)

Author: dnfield

impeller/geometry/geometry_unittests.cc

@@ -853,5 +853,12 @@ TEST(GeometryTest, PolylineGetContourPointBoundsReturnsCorrectRanges) {
   ASSERT_EQ(b2, 6u);
 }
 
+TEST(GeometryTest, PolylineGetContourOutOfBoundsAborts) {
+  Path::Polyline polyline =
+      PathBuilder{}.AddLine({100, 100}, {200, 100}).TakePath().CreatePolyline();
+  ASSERT_EQ(polyline.GetContourPointBounds(0), std::make_tuple(2u, 2u));
+  ASSERT_EQ(polyline.GetContourPointBounds(14), std::make_tuple(2u, 2u));
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/geometry/path.cc

@@ -18,10 +18,13 @@ Path::~Path() = default;
 
 std::tuple<size_t, size_t> Path::Polyline::GetContourPointBounds(
     size_t contour_index) const {
-  const size_t start_index = contours[contour_index].start_index;
+  if (contour_index >= contours.size()) {
+    return {points.size(), points.size()};
+  }
+  const size_t start_index = contours.at(contour_index).start_index;
   const size_t end_index = (contour_index >= contours.size() - 1)
                                ? points.size()
-                               : contours[contour_index + 1].start_index;
+                               : contours.at(contour_index + 1).start_index;
   return std::make_tuple(start_index, end_index);
 }
 

impeller/geometry/path.h

@@ -61,6 +61,8 @@ class Path {
 
     /// Convenience method to compute the start (inclusive) and end (exclusive)
     /// point of the given contour index.
+    ///
+    /// The contour_index parameter is clamped to contours.size().
     std::tuple<size_t, size_t> GetContourPointBounds(
         size_t contour_index) const;
   };