`PrimitiveArray::validate_decimal_precision` rejects every decimal array with a negative scale

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
backend

Research direction

Read arrow-array/src/array/primitive_array.rs around PrimitiveArray::validate_decimal_precision, then compare its scale guard with validate_decimal_precision_and_scale in arrow-array/src/types.rs. Add a regression test for a negative-scale Decimal128Array such as the issue’s precision 5, scale -2 case; done means that value validates successfully while existing positive-scale checks still pass.

Written by the indexing model from the issue text.

Description

Describe the bug

PrimitiveArray::validate_decimal_precision returns Decimal precision {precision} is less than scale {scale} for any array whose scale is negative, whatever its precision and values. Its guard compares precision < self.scale() as u8, and a negative scale cast to u8 wraps, so -2 becomes 254.

https://github.com/apache/arrow-rs/blob/cc58320c2da359b757814db5a9651397d829b601/arrow-array/src/array/primitive_array.rs#L1708-L1714

The type-level check validate_decimal_precision_and_scale guards the same comparison with scale > 0:

https://github.com/apache/arrow-rs/blob/cc58320c2da359b757814db5a9651397d829b601/arrow-array/src/types.rs#L1465-L1469

The guard came in with #8396, first released in 57.0.0.

To Reproduce

On main at cc58320c2d:

use arrow_array::Decimal128Array;

fn main() {
    // 1 at scale -2 is 100, which fits in precision 5
    let a = Decimal128Array::from(vec![1])
        .with_precision_and_scale(5, -2)
        .unwrap();
    println!("{:?}", a.validate_decimal_precision(5));
    // Err(InvalidArgumentError("Decimal precision 5 is less than scale -2"))

    // The same value at scale 2 passes
    let b = Decimal128Array::from(vec![1])
        .with_precision_and_scale(5, 2)
        .unwrap();
    println!("{:?}", b.validate_decimal_precision(5));
    // Ok(())
}

Expected behavior

Ok(()) for the first array, since its value fits precision 5. A guard of self.scale() > 0 && precision < self.scale() as u8 would match validate_decimal_precision_and_scale.

Additional context

Property-based tests found it on a generated Decimal128(5, -2) array. Checking each value with DecimalType::validate_decimal_precision works around it.

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