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GH-49817: [C++] Reject decimal strings that exceed the target precision #49832

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GH-49817: [C++] Reject decimal strings that exceed the target precision #49832

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7977042
GH-49817: [C++] Reject decimal strings that exceed the target precision
SAY-5 Apr 21, 2026
7c31288
GH-49817: [C++] Test max decimal value and clarify shift comments
SAY-5 Jun 11, 2026
7761dd6
Fix lint
pitrou Jun 16, 2026
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8 changes: 4 additions & 4 deletions cpp/src/arrow/array/array_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -3445,16 +3445,16 @@ TEST_P(Decimal256Test, WithNulls) {
std::vector<Decimal256> draw = {Decimal256(1), Decimal256(2), Decimal256(-1),
Decimal256(4), Decimal256(-1), Decimal256(1),
Decimal256(2)};
Decimal256 big; // (pow(2, 255) - 1) / pow(10, 38)
Decimal256 big; // trimmed to kMaxPrecision (76) digits
ASSERT_OK_AND_ASSIGN(big,
Decimal256::FromString("578960446186580977117854925043439539266."
"34992332820282019728792003956564819967"));
"3499233282028201972879200395656481996"));
draw.push_back(big);

Decimal256 big_negative; // -pow(2, 255) / pow(10, 38)
Decimal256 big_negative; // trimmed to kMaxPrecision (76) digits
ASSERT_OK_AND_ASSIGN(big_negative,
Decimal256::FromString("-578960446186580977117854925043439539266."
"34992332820282019728792003956564819968"));
"3499233282028201972879200395656481996"));
draw.push_back(big_negative);

std::vector<uint8_t> valid_bytes = {true, true, false, true, false,
Expand Down
5 changes: 3 additions & 2 deletions cpp/src/arrow/json/converter_test.cc
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Expand Up @@ -254,9 +254,10 @@ TEST(ConverterTest, Decimal128And256PrecisionError) {
std::shared_ptr<StructArray> parse_array;
ASSERT_OK(ParseFromString(options, json_source, &parse_array));

// FromString now rejects precision overflow before the JSON converter wraps
// the error, so match on the shared body only.
std::string error_msg =
"Invalid: Failed to convert JSON to " + decimal_type->ToString() +
": 123456789012345678901234567890.0123456789 requires precision 40";
"123456789012345678901234567890.0123456789 requires precision 40";
EXPECT_RAISES_WITH_MESSAGE_THAT(
Invalid, ::testing::HasSubstr(error_msg),
Convert(decimal_type, parse_array->GetFieldByName("")));
Expand Down
14 changes: 14 additions & 0 deletions cpp/src/arrow/util/decimal.cc
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Expand Up @@ -879,6 +879,13 @@ Status DecimalFromString(const char* type_name, std::string_view s, Decimal* out
}
int32_t parsed_precision = static_cast<int32_t>(significant_digits);

// Reject precisions that exceed the target decimal's max (GH-49817).
if (parsed_precision > Decimal::kMaxPrecision) {
return Status::Invalid(s, " requires precision ", parsed_precision,
" which exceeds the ", type_name, " maximum of ",
Decimal::kMaxPrecision);
}

int32_t parsed_scale = 0;
if (dec.has_exponent) {
auto adjusted_exponent = dec.exponent;
Expand Down Expand Up @@ -943,6 +950,13 @@ Status SimpleDecimalFromString(const char* type_name, std::string_view s,
}
int32_t parsed_precision = static_cast<int32_t>(significant_digits);

// Reject precisions that exceed the target decimal's max (GH-49817).
if (parsed_precision > DecimalClass::kMaxPrecision) {
return Status::Invalid(s, " requires precision ", parsed_precision,
" which exceeds the ", type_name, " maximum of ",
DecimalClass::kMaxPrecision);
}

int32_t parsed_scale = 0;
if (dec.has_exponent) {
auto adjusted_exponent = dec.exponent;
Expand Down
66 changes: 45 additions & 21 deletions cpp/src/arrow/util/decimal_test.cc
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Expand Up @@ -324,6 +324,10 @@ TEST(Decimal128Test, TestStringRoundTrip) {
Decimal128 decimal(high_bits, low_bits);
for (int32_t scale : kScales) {
std::string str = decimal.ToString(scale);
// skip values whose printed form exceeds kMaxPrecision (GH-49817)
auto digits = str.size() - (str[0] == '-' ? 1 : 0) -
(str.find('.') == std::string::npos ? 0 : 1);
if (static_cast<int>(digits) > Decimal128::kMaxPrecision) continue;
ASSERT_OK_AND_ASSIGN(Decimal128 result, Decimal128::FromString(str));
EXPECT_EQ(decimal, result);
}
Expand Down Expand Up @@ -708,17 +712,23 @@ TEST(Decimal128Test, PrintLargeNegativeValue) {
}

TEST(Decimal128Test, PrintMaxValue) {
const std::string string_value("170141183460469231731687303715884105727");
const Decimal128 value(string_value);
const std::string printed_value = value.ToIntegerString();
ASSERT_EQ(string_value, printed_value);
// 99999...9 is the largest value that fits in kMaxPrecision (38) digits; the
// truncated bit-pattern max is also exercised (GH-49817).
for (const auto& string_value :
{std::string("99999999999999999999999999999999999999"),
std::string("17014118346046923173168730371588410572")}) {
const Decimal128 value(string_value);
ASSERT_EQ(string_value, value.ToIntegerString());
}
}

TEST(Decimal128Test, PrintMinValue) {
const std::string string_value("-170141183460469231731687303715884105728");
const Decimal128 value(string_value);
const std::string printed_value = value.ToIntegerString();
ASSERT_EQ(string_value, printed_value);
for (const auto& string_value :
{std::string("-99999999999999999999999999999999999999"),
std::string("-17014118346046923173168730371588410572")}) {
const Decimal128 value(string_value);
ASSERT_EQ(string_value, value.ToIntegerString());
}
}

struct ToStringTestParam {
Expand Down Expand Up @@ -1740,9 +1750,11 @@ TYPED_TEST(TestBasicDecimalFunctionality, FitsInPrecision) {
ASSERT_TRUE(TypeParam(max_nines).FitsInPrecision(TypeParam::kMaxPrecision));
ASSERT_TRUE(TypeParam("-" + max_nines).FitsInPrecision(TypeParam::kMaxPrecision));

std::string max_zeros(TypeParam::kMaxPrecision, '0');
ASSERT_FALSE(TypeParam("1" + max_zeros).FitsInPrecision(TypeParam::kMaxPrecision));
ASSERT_FALSE(TypeParam("-1" + max_zeros).FitsInPrecision(TypeParam::kMaxPrecision));
// max_nines + 1 is 10^kMaxPrecision, the first value that no longer fits;
// build it arithmetically since the string form is rejected now (GH-49817)
TypeParam one_past_max = TypeParam(max_nines) + 1;
ASSERT_FALSE(one_past_max.FitsInPrecision(TypeParam::kMaxPrecision));
ASSERT_FALSE((-one_past_max).FitsInPrecision(TypeParam::kMaxPrecision));
}

TEST(Decimal32Test, LeftShift) {
Expand All @@ -1763,9 +1775,11 @@ TEST(Decimal32Test, LeftShift) {
check(123, 16);
check(123, 30);

ASSERT_EQ(Decimal32("1999999998"), Decimal32("999999999") << 1);
// operands picked so the expected results fit in kMaxPrecision (9) digits,
// since FromString now rejects wider literals (GH-49817)
ASSERT_EQ(Decimal32("999999998"), Decimal32("499999999") << 1);
ASSERT_EQ(Decimal32("12799872"), Decimal32("99999") << 7);
ASSERT_EQ(Decimal32("1638383616"), Decimal32("99999") << 14);
ASSERT_EQ(Decimal32("819191808"), Decimal32("99999") << 13);

ASSERT_EQ(Decimal32("123456789"), Decimal32("123456789") << 0);
ASSERT_EQ(Decimal32("246913578"), Decimal32("123456789") << 1);
Expand All @@ -1777,9 +1791,11 @@ TEST(Decimal32Test, LeftShift) {
check(-123, 16);
check(-123, 30);

ASSERT_EQ(Decimal32("-1999999998"), Decimal32("-999999999") << 1);
// operands picked so the expected results fit in kMaxPrecision (9) digits,
// since FromString now rejects wider literals (GH-49817)
ASSERT_EQ(Decimal32("-999999998"), Decimal32("-499999999") << 1);
ASSERT_EQ(Decimal32("-12799872"), Decimal32("-99999") << 7);
ASSERT_EQ(Decimal32("-1638383616"), Decimal32("-99999") << 14);
ASSERT_EQ(Decimal32("-819191808"), Decimal32("-99999") << 13);

ASSERT_EQ(Decimal32("-123456789"), Decimal32("-123456789") << 0);
ASSERT_EQ(Decimal32("-246913578"), Decimal32("-123456789") << 1);
Expand Down Expand Up @@ -1852,7 +1868,9 @@ TEST(Decimal64Test, LeftShift) {

ASSERT_EQ(Decimal64("1234567890123456"), Decimal64("1234567890123456") << 0);
ASSERT_EQ(Decimal64("2469135780246912"), Decimal64("1234567890123456") << 1);
ASSERT_EQ(Decimal64("6917529027641081856"), Decimal64("1234567890123456") << 55);
// shift amount picked so the expected result fits in kMaxPrecision (18
// digits), since FromString now rejects wider literals (GH-49817)
ASSERT_EQ(Decimal64("632098759743209472"), Decimal64("1234567890123456") << 9);

check(-123, 0);
check(-123, 1);
Expand All @@ -1866,7 +1884,9 @@ TEST(Decimal64Test, LeftShift) {

ASSERT_EQ(Decimal64("-1234567890123456"), Decimal64("-1234567890123456") << 0);
ASSERT_EQ(Decimal64("-2469135780246912"), Decimal64("-1234567890123456") << 1);
ASSERT_EQ(Decimal64("-6917529027641081856"), Decimal64("-1234567890123456") << 55);
// shift amount picked so the expected result fits in kMaxPrecision (18
// digits), since FromString now rejects wider literals (GH-49817)
ASSERT_EQ(Decimal64("-632098759743209472"), Decimal64("-1234567890123456") << 9);
}

TEST(Decimal64Test, RightShift) {
Expand Down Expand Up @@ -1932,8 +1952,10 @@ TEST(Decimal128Test, LeftShift) {

ASSERT_EQ(Decimal128("199999999999998"), Decimal128("99999999999999") << 1);
ASSERT_EQ(Decimal128("3435973836799965640261632"), Decimal128("99999999999999") << 35);
ASSERT_EQ(Decimal128("120892581961461708544797985370825293824"),
Decimal128("99999999999999") << 80);
// shift amount picked so the expected result fits in kMaxPrecision (38
// digits), since FromString now rejects wider literals (GH-49817)
ASSERT_EQ(Decimal128("60446290980730854272398992685412646912"),
Decimal128("99999999999999") << 79);

ASSERT_EQ(Decimal128("1234567890123456789012"), Decimal128("1234567890123456789012")
<< 0);
Expand All @@ -1951,8 +1973,10 @@ TEST(Decimal128Test, LeftShift) {
ASSERT_EQ(Decimal128("-199999999999998"), Decimal128("-99999999999999") << 1);
ASSERT_EQ(Decimal128("-3435973836799965640261632"), Decimal128("-99999999999999")
<< 35);
ASSERT_EQ(Decimal128("-120892581961461708544797985370825293824"),
Decimal128("-99999999999999") << 80);
// shift amount picked so the expected result fits in kMaxPrecision (38
// digits), since FromString now rejects wider literals (GH-49817)
ASSERT_EQ(Decimal128("-60446290980730854272398992685412646912"),
Decimal128("-99999999999999") << 79);

ASSERT_EQ(Decimal128("-1234567890123456789012"), Decimal128("-1234567890123456789012")
<< 0);
Expand Down
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