Fix crash in case of empty ring

include/mapnik/geometry/polygon_vertex_processor.hpp

@@ -47,7 +47,10 @@ struct polygon_vertex_processor
                     ring.emplace_back(p);
                     break;
                 case SEG_CLOSE:
+                    if (!ring.empty())
+                    {
                         ring.emplace_back(ring.front());
+                    }
                     if (exterior)
                     {
                         polygon_.exterior_ring = std::move(ring);

test/unit/geometry/polygon_vertex_processor.cpp

@@ -0,0 +1,65 @@
+#include "catch.hpp"
+#include "unit/vertex_adapter/fake_path.hpp"
+
+#include <mapnik/geometry/polygon_vertex_processor.hpp>
+
+TEST_CASE("polygon_vertex_processor") {
+
+SECTION("empty polygon") {
+
+    fake_path path = {};
+    mapnik::geometry::polygon_vertex_processor<double> proc;
+    proc.add_path(path);
+    CHECK(proc.polygon_.exterior_ring.size() == 0);
+    CHECK(proc.polygon_.interior_rings.size() == 0);
+}
+
+SECTION("empty exterior ring") {
+
+    fake_path path = {};
+    path.vertices_.emplace_back(0, 0, mapnik::SEG_CLOSE);
+    path.rewind(0);
+
+    mapnik::geometry::polygon_vertex_processor<double> proc;
+    proc.add_path(path);
+    CHECK(proc.polygon_.exterior_ring.size() == 0);
+    CHECK(proc.polygon_.interior_rings.size() == 0);
+}
+
+SECTION("empty interior ring") {
+
+    fake_path path = {};
+    path.vertices_.emplace_back(-1, -1, mapnik::SEG_MOVETO);
+    path.vertices_.emplace_back( 1, -1, mapnik::SEG_LINETO);
+    path.vertices_.emplace_back( 1,  1, mapnik::SEG_LINETO);
+    path.vertices_.emplace_back(-1,  1, mapnik::SEG_LINETO);
+    path.vertices_.emplace_back( 0,  0, mapnik::SEG_CLOSE);
+    path.vertices_.emplace_back( 0,  0, mapnik::SEG_CLOSE);
+    path.rewind(0);
+
+    mapnik::geometry::polygon_vertex_processor<double> proc;
+    proc.add_path(path);
+
+    REQUIRE(proc.polygon_.interior_rings.size() == 1);
+    REQUIRE(proc.polygon_.interior_rings.front().size() == 0);
+
+    auto const& outer_ring = proc.polygon_.exterior_ring;
+    REQUIRE(outer_ring.size() == 5);
+
+    CHECK(outer_ring[0].x == Approx(-1));
+    CHECK(outer_ring[0].y == Approx(-1));
+
+    CHECK(outer_ring[1].x == Approx( 1));
+    CHECK(outer_ring[1].y == Approx(-1));
+
+    CHECK(outer_ring[2].x == Approx( 1));
+    CHECK(outer_ring[2].y == Approx( 1));
+
+    CHECK(outer_ring[3].x == Approx(-1));
+    CHECK(outer_ring[3].y == Approx( 1));
+
+    CHECK(outer_ring[4].x == Approx(-1));
+    CHECK(outer_ring[4].y == Approx(-1));
+}
+
+}

test/unit/vertex_adapter/fake_path.hpp

@@ -0,0 +1,93 @@
+/*****************************************************************************
+ *
+ * This file is part of Mapnik (c++ mapping toolkit)
+ *
+ * Copyright (C) 2017 Artem Pavlenko
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ *****************************************************************************/
+
+// mapnik
+#include <mapnik/vertex.hpp>
+
+// stl
+#include <vector>
+#include <tuple>
+
+namespace detail
+{
+
+template <typename T>
+struct fake_path
+{
+    using coord_type = std::tuple<T, T, unsigned>;
+    using cont_type = std::vector<coord_type>;
+    cont_type vertices_;
+    typename cont_type::iterator itr_;
+
+    fake_path(std::initializer_list<T> l)
+        : fake_path(l.begin(), l.size())
+    {
+    }
+
+    fake_path(std::vector<T> const &v, bool make_invalid = false)
+        : fake_path(v.begin(), v.size(), make_invalid)
+    {
+    }
+
+    template <typename Itr>
+    fake_path(Itr itr, size_t sz, bool make_invalid = false)
+    {
+        size_t num_coords = sz >> 1;
+        vertices_.reserve(num_coords + (make_invalid ? 1 : 0));
+        if (make_invalid)
+        {
+            vertices_.push_back(std::make_tuple(0,0,mapnik::SEG_END));
+        }
+
+        for (size_t i = 0; i < num_coords; ++i)
+        {
+            T x = *itr++;
+            T y = *itr++;
+            unsigned cmd = (i == 0) ? mapnik::SEG_MOVETO : mapnik::SEG_LINETO;
+            vertices_.push_back(std::make_tuple(x, y, cmd));
+        }
+        itr_ = vertices_.begin();
+    }
+
+    unsigned vertex(T *x, T *y)
+    {
+        if (itr_ == vertices_.end())
+        {
+            return mapnik::SEG_END;
+        }
+        *x = std::get<0>(*itr_);
+        *y = std::get<1>(*itr_);
+        unsigned cmd = std::get<2>(*itr_);
+        ++itr_;
+        return cmd;
+    }
+
+    void rewind(unsigned)
+    {
+        itr_ = vertices_.begin();
+    }
+};
+
+}
+
+using fake_path = detail::fake_path<double>;
+