Fix crash when loading poly data with no cells

vtkm/io/internal/VTKDataSetCells.h

@@ -210,6 +210,15 @@ inline void FixupCellSet(vtkm::cont::ArrayHandle<vtkm::Id>& connectivity,
 
 inline bool IsSingleShape(const vtkm::cont::ArrayHandle<vtkm::UInt8>& shapes)
 {
+  if (shapes.GetNumberOfValues() < 1)
+  {
+    // If the data has no cells, is it single shape? That would make sense, but having
+    // a single shape cell set requires you to slect a shape, and there are no cells to
+    // make that selection from. We could get around that, but it's easier just to treat
+    // it as a general explicit grid.
+    return false;
+  }
+
   auto shapesPortal = shapes.ReadPortal();
   vtkm::UInt8 shape0 = shapesPortal.Get(0);
   for (vtkm::Id i = 1; i < shapes.GetNumberOfValues(); ++i)

vtkm/io/testing/UnitTestVTKDataSetReader.cxx

@@ -62,6 +62,17 @@ void TestReadingPolyData(Format format)
                    "Incorrect cellset type");
 }
 
+void TestReadingPolyDataEmpty()
+{
+  vtkm::cont::DataSet data =
+    readVTKDataSet(vtkm::cont::testing::Testing::DataPath("unstructured/empty_poly.vtk"));
+
+  VTKM_TEST_ASSERT(data.GetNumberOfPoints() == 8);
+  VTKM_TEST_ASSERT(data.GetNumberOfCells() == 0);
+  VTKM_TEST_ASSERT(data.GetCellSet().GetNumberOfPoints() == 8);
+  VTKM_TEST_ASSERT(data.GetNumberOfFields() == 1);
+}
+
 void TestReadingStructuredPoints(Format format)
 {
   std::string testFileName = (format == FORMAT_ASCII)
@@ -114,6 +125,17 @@ void TestReadingUnstructuredGrid(Format format)
                    "Incorrect cellset type");
 }
 
+void TestReadingUnstructuredGridEmpty()
+{
+  vtkm::cont::DataSet data =
+    readVTKDataSet(vtkm::cont::testing::Testing::DataPath("unstructured/empty_unstructured.vtk"));
+
+  VTKM_TEST_ASSERT(data.GetNumberOfPoints() == 26);
+  VTKM_TEST_ASSERT(data.GetNumberOfCells() == 0);
+  VTKM_TEST_ASSERT(data.GetCellSet().GetNumberOfPoints() == 26);
+  VTKM_TEST_ASSERT(data.GetNumberOfFields() == 2);
+}
+
 void TestReadingUnstructuredGridVisIt(Format format)
 {
   VTKM_TEST_ASSERT(format == FORMAT_ASCII);
@@ -452,6 +474,8 @@ void TestReadingVTKDataSet()
   TestReadingPolyData(FORMAT_ASCII);
   std::cout << "Test reading VTK Polydata file in BINARY" << std::endl;
   TestReadingPolyData(FORMAT_BINARY);
+  std::cout << "Test reading VTK Polydata with no cells" << std::endl;
+  TestReadingPolyDataEmpty();
   std::cout << "Test reading VTK StructuredPoints file in ASCII" << std::endl;
   TestReadingStructuredPoints(FORMAT_ASCII);
 
@@ -461,6 +485,8 @@ void TestReadingVTKDataSet()
   TestReadingUnstructuredGrid(FORMAT_ASCII);
   std::cout << "Test reading VTK UnstructuredGrid file in BINARY" << std::endl;
   TestReadingUnstructuredGrid(FORMAT_BINARY);
+  std::cout << "Test reading VTK UnstructuredGrid with no cells" << std::endl;
+  TestReadingUnstructuredGridEmpty();
 
   std::cout << "Test reading VTK RectilinearGrid file in ASCII" << std::endl;
   TestReadingRectilinearGrid1(FORMAT_ASCII);