[BUG] copy_if_else producing incorrect answers for structs

[revans2]

Describe the bug
When trying to implement Case/When and If/Else for Spark for Structs I ran into a test that was failing. I brought the size of the test data down a lot, but it looks like NULLs are being inserted into incorrect places for some result/

Steps/Code to reproduce bug

diff --git a/cpp/tests/copying/copy_if_else_nested_tests.cpp b/cpp/tests/copying/copy_if_else_nested_tests.cpp
index 9ac34a3044..7ad374ca3b 100644
--- a/cpp/tests/copying/copy_if_else_nested_tests.cpp
+++ b/cpp/tests/copying/copy_if_else_nested_tests.cpp
@@ -102,6 +102,66 @@ TYPED_TEST(TypedCopyIfElseNestedTest, StructsWithNulls)
   CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(result_column->view(), expected_result->view());
 }
 
+TYPED_TEST(TypedCopyIfElseNestedTest, LongerStructsWithNulls)
+{
+  using T = TypeParam;
+
+  using namespace cudf;
+  using namespace cudf::test;
+
+  using ints    = fixed_width_column_wrapper<T, int32_t>;
+  using structs = structs_column_wrapper;
+  using bools   = fixed_width_column_wrapper<bool, int32_t>;
+
+  auto selector_column = bools{
+      1, 1, 1, 1, 0, 1, 0, 1, 0, 0,
+      1, 0, 0, 0, 1, 0, 0, 1, 0, 1,
+      0, 0, 0, 0, 1, 1, 1, 1, 0, 0,
+      1, 0, 1, 1, 0, 0, 1, 0, 0, 1,
+      0, 0, 1, 0, 0, 1, 0, 0, 0, 1,
+      0, 0, 1, 0, 1, 0, 1, 1, 1, 1,
+      1, 0, 1, 1, 1, 0}.release();
+
+  std::cerr << "AT 1" << std::endl;
+  auto lhs_child_0 = ints{{
+       120,   16,   53, -121,  -15,   21,  -33,   72,   54,   90,
+       -67, -117,  -77,    0,  113,   15,  -87,   77, -128,    0,
+         0,    0,  115,   49, -107,   10,  -62,  -78,  -49,   73,
+        92,  -22,    0,  127,  -49,  101,  -82,    0,    6, -110,
+        69,  -29,  -63,  105,  -18,   84, -113,  -65,    0,   29,
+       -61,   53,   62,   87,  -84,  -49, -108,  -85,  -21,   21,
+        84,   53,   56,  -56,  -53,  -28
+      }, iterator_with_null_at(std::vector<size_type>{13, 19, 20, 21, 32, 37, 48})};
+
+  std::cerr << "AT 2" << std::endl;
+  auto lhs_child_1 = ints{{
+        27,  -80,  -24,   76,  -56,   42,    5,   13,  -69,  -77,
+        61,  -77,   72,    0,   31,  118,  -30,   86,  125,    0,
+         0,    0,   75,  -49,  125,   60,  116,  118,   64,   20,
+       -70,  -18,    0,  -25,   22,  -46,  -89,   -9,   27,  -56,
+       -77,  123,    0,  -90,   87, -113,  -37,   22,  -22,  -53,
+        73,   99,  113,   -2,  -24,  113,   75,    6,   82,  -58,
+       122, -123, -127,   19,  -62,  -24
+      }, iterator_with_null_at(std::vector<size_type>{13, 19, 20, 21, 32, 42})};
+
+  std::cerr << "AT 3" << std::endl;
+  auto lhs_structs_column = structs{{lhs_child_0, lhs_child_1}, 
+      iterator_with_null_at(std::vector<size_type>{13, 19, 20, 21, 32})}.release();
+
+  std::cerr << "AT 4" << std::endl;
+  cudf::test::print(*lhs_structs_column);
+
+  auto result_column =
+    copy_if_else(lhs_structs_column->view(), lhs_structs_column->view(), selector_column->view());
+
+  std::cerr << "AT 8" << std::endl;
+  cudf::test::print(*result_column);
+
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(result_column->view(), lhs_structs_column->view());
+}
+
+
+
 TYPED_TEST(TypedCopyIfElseNestedTest, Lists)
 {
   using T = TypeParam;

Expected behavior
The test should pass because the the lhs and rhs of the copy_if_else is the same thing so I don't see how we would ever get anything back except the lhs, but we get back a different answer (and some how it also crashes, which is separate)

[gerashegalov]

a smaller repro:

diff --git a/cpp/tests/copying/copy_if_else_nested_tests.cpp b/cpp/tests/copying/copy_if_else_nested_tests.cpp
index 9ac34a3044..0f7fc625e4 100644
--- a/cpp/tests/copying/copy_if_else_nested_tests.cpp
+++ b/cpp/tests/copying/copy_if_else_nested_tests.cpp
@@ -102,6 +102,34 @@ TYPED_TEST(TypedCopyIfElseNestedTest, StructsWithNulls)
   CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(result_column->view(), expected_result->view());
 }
 
+TYPED_TEST(TypedCopyIfElseNestedTest, ReflectiveStructsWithNulls)
+{
+  using T = TypeParam;
+
+  using namespace cudf;
+  using namespace cudf::test;
+
+  using ints    = fixed_width_column_wrapper<T, int32_t>;
+  using structs = structs_column_wrapper;
+  using bools   = fixed_width_column_wrapper<bool, int32_t>;
+
+  auto selector_column = bools{0, 1}.release();
+
+  auto lhs_child_0 = ints{{100, 101}, iterator_with_null_at(std::vector<size_type>{1})};
+
+  auto lhs_structs_column = structs{{lhs_child_0}}.release();
+
+  cudf::test::print(*lhs_structs_column);
+
+  auto result_column =
+    copy_if_else(lhs_structs_column->view(), lhs_structs_column->view(), selector_column->view());
+
+  std::cout << "result: " << std::endl;
+  cudf::test::print(*result_column);
+
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(result_column->view(), lhs_structs_column->view());
+}
+
 TYPED_TEST(TypedCopyIfElseNestedTest, Lists)
 {
   using T = TypeParam;

[gerashegalov]

As for SIGSEGV:

Thread 1 "COPYING_TEST" received signal SIGSEGV, Segmentation fault.
0x0000555556b76a48 in cudf::test::(anonymous namespace)::stringify_column_differences[abi:cxx11](cudf::device_span<int const, 18446744073709551615ul>, cudf::column_view const&, cudf::column_view const&, bool, int) ()

[gerashegalov]

Reduced repro using just scatter https://github.com/rapidsai/cudf/pull/8482/files#diff-f6145d416b9549eb6e1689fbd15439e02eaa66bf508cbce61869b32cfd9f1761R250-R268