Repository: apache/arrow
File: cpp/src/parquet/arrow/schema.cc (C++)

Code before:
    const auto& list_group = static_cast<const GroupNode&>(list_node);
    if (list_group.field_count() > 1) {
      // The inner type of the list should be a struct when there are multiple fields
      // in the repeated group
      RETURN_NOT_OK(check_two_level_list_repeated(group));
      return GroupToStruct(list_group, current_levels, ctx, out, child_field);
    }
    if (list_group.field_count() == 0) {
      return Status::Invalid("Group must have at least one child.");
    }

    if (list_group.logical_type()->is_none() && HasListElementName(list_group, group)) {
      // Backward-compatibility rule 4
      RETURN_NOT_OK(check_two_level_list_repeated(group));
      return GroupToStruct(list_group, current_levels, ctx, out, child_field);
    }

    const auto& repeated_field = list_group.field(0);
    if (!list_group.logical_type()->is_none() || repeated_field->is_repeated()) {
      RETURN_NOT_OK(check_two_level_list_repeated(group));
      if (list_group.logical_type()->is_list()) {
        return ListToSchemaField(list_group, current_levels, ctx, out, child_field);
      } else if (list_group.logical_type()->is_map()) {
        return MapToSchemaField(list_group, current_levels, ctx, out, child_field);
      } else {
        current_levels.Increment(list_group);
        return GroupToStruct(list_group, current_levels, ctx, out, child_field);
      }
    }

    if (group.is_repeated()) {
      return Status::Invalid("LIST-annotated groups must not be repeated.");
    }

    return NodeToSchemaField(*repeated_field, current_levels, ctx, out, child_field);
  }

  RETURN_NOT_OK(check_two_level_list_repeated(group));
  const auto& primitive_node = static_cast<const PrimitiveNode&>(list_node);
  int column_index = ctx->schema->GetColumnIndex(primitive_node);
  ARROW_ASSIGN_OR_RAISE(std::shared_ptr<ArrowType> type,
                        GetTypeForNode(column_index, primitive_node, ctx));
  auto item_field = ::arrow::field(list_node.name(), type, /*nullable=*/false,
                                   FieldIdMetadata(list_node.field_id()));
  return PopulateLeaf(column_index, item_field, current_levels, ctx, out, child_field);
}

Status ListToSchemaField(const GroupNode& group, LevelInfo current_levels,
                         SchemaTreeContext* ctx, const SchemaField* parent,
                         SchemaField* out) {
  if (group.field_count() != 1) {

Review comment (bug):
In ResolveList(), `current_levels` has already been incremented for the repeated `list_node` (ListToSchemaField() calls `current_levels.IncrementRepeated()` before calling ResolveList). Since `list_group` is that same repeated node, calling `current_levels.Increment(list_group)` here will increment rep/def levels a second time and can produce incorrect LevelInfo (and therefore incorrect decoding) for LISTs whose repeated group is annotated with a non-(list/map) logical type or has a repeated child. Consider removing this increment (or only incrementing when there is an additional optional layer to account for) and pass the already-adjusted `current_levels` through to GroupToStruct().
```suggestion

```

Code after:
    return {};
  };

  SchemaField* child_field = &out->children[0];
  const Node& list_node = *group.field(0);
  if (!list_node.is_repeated()) {
    return Status::Invalid(
        "Non-repeated nodes in a LIST-annotated group are not supported.");
  }

  if (list_node.is_group()) {
    const auto& list_group = static_cast<const GroupNode&>(list_node);
    if (list_group.field_count() > 1) {
      // The inner type of the list should be a struct when there are multiple fields
      // in the repeated group
      RETURN_NOT_OK(check_two_level_list_repetition(group));
      return GroupToStruct(list_group, current_levels, ctx, out, child_field);
    }
    if (list_group.field_count() == 0) {
      return Status::Invalid("Group must have at least one child.");
    }

    if (list_group.logical_type()->is_none() && HasListElementName(list_group, group)) {
      // Rule 4 at
      // https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#backward-compatibility-rules
      //
      // required/optional group name=SOMETHING {
      //   repeated group name=array or $SOMETHING_tuple {
      //     required/optional TYPE item;
      //   }
      // }
      //
      // The inner type of the list should be a struct rather than a primitive value
      //
      // yields list<item: struct<item: TYPE ?nullable> not null> ?nullable
      RETURN_NOT_OK(check_two_level_list_repetition(group));
      return GroupToStruct(list_group, current_levels, ctx, out, child_field);
    }

    const auto& repeated_field = list_group.field(0);
    if (!list_group.logical_type()->is_none() || repeated_field->is_repeated()) {
      RETURN_NOT_OK(check_two_level_list_repetition(group));
      if (list_group.logical_type()->is_list()) {
        // Special case where the inner type might be a list with two-level encoding
        // like below:
        //
        // required/optional group name=SOMETHING (LIST) {
        //   repeated group array (LIST) {
        //     repeated TYPE item;
        //   }
        // }