ARROW-17995: [C++] Fix json decimals not being rescaled based on the explicit schema

cpp/src/arrow/json/converter.cc

@@ -166,10 +166,28 @@ class DecimalConverter : public PrimitiveConverter {
     using Builder = typename TypeTraits<T>::BuilderType;
     Builder builder(out_type_, pool_);
     RETURN_NOT_OK(builder.Resize(dict_array.indices()->length()));
+    const auto& decimal_type(checked_cast<const DecimalType&>(*out_type_));
+    int32_t out_precision = decimal_type.precision();
+    int32_t out_scale = decimal_type.scale();
 
-    auto visit_valid = [&builder](string_view repr) {
-      ARROW_ASSIGN_OR_RAISE(value_type value,
-                            TypeTraits<T>::BuilderType::ValueType::FromString(repr));
+    auto visit_valid = [&](string_view repr) {
+      value_type value;
+      int32_t precision, scale;
+      RETURN_NOT_OK(TypeTraits<T>::BuilderType::ValueType::FromString(
+          repr, &value, &precision, &scale));
+      if (precision > out_precision) {
+        return GenericConversionError(*out_type_, ": ", repr, " requires precision ",
+                                      precision);
+      }
+      if (scale != out_scale) {
+        auto result = value.Rescale(scale, out_scale);
+        if (ARROW_PREDICT_FALSE(!result.ok())) {
+          return GenericConversionError(*out_type_, ": ", repr, " requires scale ",
+                                        scale);
+        } else {
+          value = result.MoveValueUnsafe();
+        }
+      }
       builder.UnsafeAppend(value);
       return Status::OK();
     };

cpp/src/arrow/json/converter_test.cc

@@ -17,6 +17,7 @@
 
 #include "arrow/json/converter.h"
 
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 #include <string>
@@ -190,9 +191,14 @@ TEST(ConverterTest, Decimal128And256) {
     options.explicit_schema = schema({field("", decimal_type)});
 
     std::string json_source = R"(
-    {"" : "02.0000000000"}
-    {"" : "30.0000000000"}
-  )";
+      {"" : "02.0000000000"}
+      {"" : "30.0000000000"}
+      {"" : "30.0123456789"}
+      {"" : "30.012345678900"}
+      {"" : "30.0123"}
+      {"" : "0.012345678"}
+      {"" : "1234567890123456789012345678.0123456789"}
+    )";
 
     std::shared_ptr<StructArray> parse_array;
     ASSERT_OK(ParseFromString(options, json_source, &parse_array));
@@ -204,11 +210,59 @@ TEST(ConverterTest, Decimal128And256) {
     // assert equality
     auto expected = ArrayFromJSON(decimal_type, R"([
           "02.0000000000",
-          "30.0000000000"])");
+          "30.0000000000",
+          "30.0123456789",
+          "30.0123456789",
+          "30.0123000000",
+          "0.0123456780",
+          "1234567890123456789012345678.0123456789"
+    ])");
 
     AssertArraysEqual(*expected, *converted);
   }
 }
 
+TEST(ConverterTest, Decimal128And256ScaleError) {
+  for (auto decimal_type : {decimal128(38, 10), decimal256(38, 10)}) {
+    ParseOptions options;
+    options.explicit_schema = schema({field("", decimal_type)});
+
+    std::string json_source = R"(
+      {"" : "30.0123456789001"}
+    )";
+
+    std::shared_ptr<StructArray> parse_array;
+    ASSERT_OK(ParseFromString(options, json_source, &parse_array));
+
+    std::string error_msg = "Failed of conversion of JSON to " +
+                            decimal_type->ToString() +
+                            ": 30.0123456789001 requires scale 13";
+    EXPECT_RAISES_WITH_MESSAGE_THAT(
+        Invalid, ::testing::HasSubstr(error_msg),
+        Convert(decimal_type, parse_array->GetFieldByName("")));
+  }
+}
+
+TEST(ConverterTest, Decimal128And256PrecisionError) {
+  for (auto decimal_type : {decimal128(38, 10), decimal256(38, 10)}) {
+    ParseOptions options;
+    options.explicit_schema = schema({field("", decimal_type)});
+
+    std::string json_source = R"(
+      {"" : "123456789012345678901234567890.0123456789"}
+    )";
+
+    std::shared_ptr<StructArray> parse_array;
+    ASSERT_OK(ParseFromString(options, json_source, &parse_array));
+
+    std::string error_msg =
+        "Invalid: Failed of conversion of JSON to " + decimal_type->ToString() +
+        ": 123456789012345678901234567890.0123456789 requires precision 40";
+    EXPECT_RAISES_WITH_MESSAGE_THAT(
+        Invalid, ::testing::HasSubstr(error_msg),
+        Convert(decimal_type, parse_array->GetFieldByName("")));
+  }
+}
+
 }  // namespace json
 }  // namespace arrow

python/pyarrow/tests/test_json.py

@@ -16,6 +16,7 @@
 # under the License.
 
 from collections import OrderedDict
+from decimal import Decimal
 import io
 import itertools
 import json
@@ -243,6 +244,21 @@ def test_reconcile_accross_blocks(self):
             # Check that the issue was exercised
             assert table.column("a").num_chunks > 1
 
+    def test_explicit_schema_decimal(self):
+        rows = (b'{"a": 1}\n'
+                b'{"a": 1.45}\n'
+                b'{"a": -23.456}\n'
+                b'{}\n')
+        expected = {
+            'a': [Decimal("1"), Decimal("1.45"), Decimal("-23.456"), None],
+        }
+        for type_factory in (pa.decimal128, pa.decimal256):
+            schema = pa.schema([('a', type_factory(9, 4))])
+            opts = ParseOptions(explicit_schema=schema)
+            table = self.read_bytes(rows, parse_options=opts)
+            assert table.schema == schema
+            assert table.to_pydict() == expected
+
     def test_explicit_schema_with_unexpected_behaviour(self):
         # infer by default
         rows = (b'{"foo": "bar", "num": 0}\n'