Allocate a buffer of the correct length for ScalarValue::FixedSizeBinary in ScalarValue::to_array_of_size

[tobixdev]

Which issue does this PR close?

• Closes ScalarValue::to_array_of_size Does Not Correctly Allocate a Values Buffer for ScalarValue::FixedSizeBinary #18870.

We should open a follow-up issue that tracks re-use FixedSizeBinaryArray::new_null after upgrading arrow-rs.

With this PR, we could backport it to #18843 if we wish.

Alternatively, we could also wait for a fix by arrow-rs.

Rationale for this change

As we will have to wait for another arrow-rs update to get the fix for the issue apache/arrow-rs#8901 , we have a temporaray fix that basically calls the same operations from DataFusion's code. Once the fix is in the arrow-rs codebase, we can re-use FixedSizeBinaryArray::new_null.

What changes are included in this PR?

• A test. (assert_eq!(result.as_fixed_size_binary().values().len(), 10); returned 0 before)
• Directly allocate the values buffer in DataFusion.

Are these changes tested?

Yes

Are there any user-facing changes?

Yes, the buffer will now be allocated, zeroed, and set to the expected length. Same as in the arrow-rs fix.

[tobixdev]

Hmm seems this change causes problems with DataType::FixedSizeBinary(0) as it divides by zero in FixedSizeBinaryArray::try_new.

    /// Create a new [`FixedSizeBinaryArray`] from the provided parts, returning an error on failure
    ///
    /// # Errors
    ///
    /// * `size < 0`
    /// * `values.len() / size != nulls.len()`
    pub fn try_new(
        size: i32,
        values: Buffer,
        nulls: Option<NullBuffer>,
    ) -> Result<Self, ArrowError> {
        let data_type = DataType::FixedSizeBinary(size);
        let s = size.to_usize().ok_or_else(|| {
            ArrowError::InvalidArgumentError(format!("Size cannot be negative, got {size}"))
        })?;

        let len = values.len() / s; <--- crash

I don't see any way to create a new FixedSizeBinaryArray with zero length using try_new. Is this on-purpose? The comments says 0 should be fine. Is this a bug in arrow-rs?

We maybe could create a FixedSizeBinaryArray using the From<ArrayData> implementation.

Any preferences on how we should proceed? As I am unsure whether we even want to fix this issue on the DataFusion side (or wait for an arrow-rs update), I'll wait another opinion.

[Jefffrey]

I don't see any way to create a new FixedSizeBinaryArray with zero length using try_new. Is this on-purpose? The comments says 0 should be fine. Is this a bug in arrow-rs?

This does seem like a bug; for reference, pyarrow allows it:

>>> import pyarrow as pa
>>> pa.binary(0)
FixedSizeBinaryType(fixed_size_binary[0])
>>> pa.array([], type=pa.binary(0))
<pyarrow.lib.FixedSizeBinaryArray object at 0x102030b80>
[]
>>> pa.array([], type=pa.binary(0)).type
FixedSizeBinaryType(fixed_size_binary[0])

It would be nice to upstream a proper fix to arrow-rs and then can wait for it here; otherwise if we need a fix for DataFusion now we can do a workaround as you describe:

We maybe could create a FixedSizeBinaryArray using the From<ArrayData> implementation.

[tobixdev]

@Jefffrey Thanks for checking against pyarrow!

Here is a PR that fixes the issue in arrow-rs.

apache/arrow-rs#8927

I also found out that we can create zero-sized item arrays using the builder API. I'll give that a try.