[C++] arrow::json::DecimalConverter should rescale values based on the explicit_schema

[asfimport]

The C++ lib doesn't read JSON decimal values correctly based on the explicit_schema. This can be reproduced by this helloworld program: https://github.com/stiga-huang/arrow-helloworld/tree/d267862

The input JSON file has the following rows:

{"id":1,"str":"Some","price":"30.04"}
{"id":2,"str":"data","price":"1.234"} 

If we read the price column using decimal128(9, 2), the values are

      30.04,
      12.34

If we use decimal128(9, 3) instead, the values are

      3.004,
      1.234

The decimal type in the explicit_schema is set here: https://github.com/stiga-huang/arrow-helloworld/blob/d26786270e87d9ab847658ead96a96190461b98f/json_decimal_example.cc#L38

The cause is arrow::json::DecimalConverter doesn't rescale the value based on the out_type_:

  Status Convert(const std::shared_ptr<Array>& in, std::shared_ptr<Array>* out) override {
    if (in->type_id() == Type::NA) {
      return MakeArrayOfNull(out_type_, in->length(), pool_).Value(out);
    }
    const auto& dict_array = GetDictionaryArray(in);

    using Builder = typename TypeTraits<T>::BuilderType;
    Builder builder(out_type_, pool_);
    RETURN_NOT_OK(builder.Resize(dict_array.indices()->length()));

    auto visit_valid = [&builder](string_view repr) {
      ARROW_ASSIGN_OR_RAISE(value_type value,
                            TypeTraits<T>::BuilderType::ValueType::FromString(repr));
      //////////// Should rescale the value based on out_type_ here
      builder.UnsafeAppend(value);
      return Status::OK();
    };

    auto visit_null = [&builder]() {
      builder.UnsafeAppendNull();
      return Status::OK();
    };

    RETURN_NOT_OK(VisitDictionaryEntries(dict_array, visit_valid, visit_null));
    return builder.Finish(out);
  }

arrow/cpp/src/arrow/json/converter.cc

Lines 171 to 173 in cdd0fdf

	ARROW_ASSIGN_OR_RAISE(value_type value,
	TypeTraits<T>::BuilderType::ValueType::FromString(repr));
	builder.UnsafeAppend(value);

Reporter: Quanlong Huang / @stiga-huang
Assignee: Quanlong Huang / @stiga-huang

PRs and other links:

• GitHub Pull Request #14380

_{Note: This issue was originally created as ARROW-17995. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 14380
#14380

[asfimport]

Quanlong Huang / @stiga-huang:
@pitrou Can we back port this to older branches like 8.0.2 and 9.0.1 ? I think it's a clean cherry-pick.

[asfimport]

Antoine Pitrou / @pitrou:
We're unlikely to do a 9.0.1 as 10.0.0 is entering the release phase very soon. Sorry!