ARROW-7740: [C++] Fix StructArray::Flatten corruption

Flatten would modify in-place the data_->child_data ArrayData if the currently flattened array is a slice. The parent's child_data values would also be modified (and corrupted with wrong offset/length).

Closes #6792 from fsaintjacques/ARROW-7740

Lead-authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Co-authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

cpp/src/arrow/array.cc

@@ -796,7 +796,9 @@ Status StructArray::Flatten(MemoryPool* pool, ArrayVector* out) const {
   flattened.reserve(data_->child_data.size());
   std::shared_ptr<Buffer> null_bitmap = data_->buffers[0];
 
-  for (auto& child_data : data_->child_data) {
+  for (const auto& child_data_ptr : data_->child_data) {
+    auto child_data = child_data_ptr->Copy();
+
     std::shared_ptr<Buffer> flattened_null_bitmap;
     int64_t flattened_null_count = kUnknownNullCount;
 

cpp/src/arrow/array_struct_test.cc

@@ -195,6 +195,25 @@ TEST(StructArray, Validate) {
   ASSERT_RAISES(Invalid, arr->ValidateFull());
 }
 
+/// ARROW-7740: Flattening a slice shouldn't affect the parent array.
+TEST(StructArray, FlattenOfSlice) {
+  auto a = ArrayFromJSON(int32(), "[4, 5]");
+  auto type = struct_({field("a", int32())});
+  auto children = std::vector<std::shared_ptr<Array>>{a};
+
+  auto arr = std::make_shared<StructArray>(type, 2, children);
+  ASSERT_OK(arr->ValidateFull());
+
+  auto slice = internal::checked_pointer_cast<StructArray>(arr->Slice(0, 1));
+  ASSERT_OK(slice->ValidateFull());
+
+  ArrayVector flattened;
+  ASSERT_OK(slice->Flatten(default_memory_pool(), &flattened));
+
+  ASSERT_OK(slice->ValidateFull());
+  ASSERT_OK(arr->ValidateFull());
+}
+
 // ----------------------------------------------------------------------------------
 // Struct test
 class TestStructBuilder : public TestBuilder {

r/R/table.R

@@ -163,6 +163,14 @@ Table <- R6Class("Table", inherit = ArrowObject,
 
     Equals = function(other, check_metadata = TRUE, ...) {
       inherits(other, "Table") && Table__Equals(self, other, isTRUE(check_metadata))
+    },
+
+    Validate = function() {
+      Table__Validate(self)
+    },
+
+    ValidateFull = function() {
+      Table__ValidateFull(self)
     }
   ),
 

r/src/array_to_vector.cpp

@@ -575,8 +575,7 @@ class Converter_List : public Converter {
     auto values_array = list_array->values();
 
     auto ingest_one = [&](R_xlen_t i) {
-      auto slice =
-          values_array->Slice(list_array->value_offset(i), list_array->value_length(i));
+      auto slice = list_array->value_slice(i);
       SET_VECTOR_ELT(data, i + start, Array__as_vector(slice));
     };
 

r/src/table.cpp

@@ -97,6 +97,18 @@ bool Table__Equals(const std::shared_ptr<arrow::Table>& lhs,
   return lhs->Equals(*rhs.get(), check_metadata);
 }
 
+// [[arrow::export]]
+bool Table__Validate(const std::shared_ptr<arrow::Table>& table) {
+  STOP_IF_NOT_OK(table->Validate());
+  return true;
+}
+
+// [[arrow::export]]
+bool Table__ValidateFull(const std::shared_ptr<arrow::Table>& table) {
+  STOP_IF_NOT_OK(table->ValidateFull());
+  return true;
+}
+
 // [[arrow::export]]
 std::shared_ptr<arrow::ChunkedArray> Table__GetColumnByName(
     const std::shared_ptr<arrow::Table>& table, const std::string& name) {

r/tests/testthat/test-json.R

@@ -147,3 +147,16 @@ test_that("Can read json file with nested columns (ARROW-5503)", {
     )
   )
 })
+
+test_that("Can read json file with list<struct<T...>> nested columns (ARROW-7740)", {
+  tf <- tempfile()
+  on.exit(unlink(tf))
+  writeLines('
+    {"a":[{"b":1.0},{"b":2.0}]}
+    {"a":[{"b":1.0},{"b":2.0}]}
+  ', tf)
+
+  one <- tibble::tibble(b = c(1, 2))
+  expected <- tibble::tibble(a = c(list(one), list(one)))
+  expect_equivalent(arrow::read_json_arrow(tf), expected)
+})