Unsound DecimalArray Validation in Unreleased arrow-rs

[tustvold]

Describe the bug

The validation logic added in #1767 is incorrect. In particular

let values_buffer = &self.buffers[0];

for pos in 0..values_buffer.len() {
    let raw_val = unsafe {
        std::slice::from_raw_parts(
            values_buffer.as_ptr().add(pos),
            16_usize,
        )
    };
    let value = i128::from_le_bytes(raw_val.try_into().unwrap());
    validate_decimal_precision(value, *p)?;
}

This will read 16 byte slices at 1 byte intervals in the underlying array of decimal data, which will as a result:

• This will interpret data at non-value intervals (avoiding UB largely by accident)
• This will read beyond the bounds of values_buffer which is unsound
• It will not take into account any offset

To Reproduce

#[test]
fn test_decimal_validation() {
    let mut builder = DecimalBuilder::new(4, 10, 4);
    builder.append_value(10000).unwrap();
    builder.append_value(20000).unwrap();
    let array = builder.finish();

    array.data().validate_full().unwrap();
}

Fails with

Invalid argument error: 42535295865117307932921825928971026471 is too large to store in a Decimal of precision 10. Max is 9999999999"

Expected behavior

This should not incorrectly fail for valid data

Additional context

This was discovered by DataFusion's test suite for Decimals

[viirya]

Oops, I will fix it. Thanks @tustvold

[tustvold]

Already preparing a fix 😄

[alamb]

marked as development process to suppress from release notes, as this was not released