[Rust] BinaryArray filter replaces nulls with empty strings

[asfimport]

The filter implementation for BinaryArray discards nullness of data. BinaryArrays that are null (seem to) always return an empty string slice when getting a value, so the way filter works might be a bug depending on what Arrow developers' or users' intentions are.

I think we should either preserve nulls (and their count) or document this as intended behaviour.

Below is a test case that reproduces the bug.

#[test]
fn test_filter_binary_array_with_nulls() {
let mut a: BinaryBuilder = BinaryBuilder::new(100);
a.append_null().unwrap();
a.append_string("a string").unwrap();
a.append_null().unwrap();
a.append_string("with nulls").unwrap();
let array = a.finish();
let b = BooleanArray::from(vec![true, true, true, true]);
let c = filter(&array, &b).unwrap();
let d: &BinaryArray = c.as_any().downcast_ref::<BinaryArray>().unwrap();
// I didn't expect this behaviour
assert_eq!("", d.get_string(0));
// fails here
assert!(d.is_null(0));
assert_eq!(4, d.len());
// fails here
assert_eq!(2, d.null_count());
assert_eq!("a string", d.get_string(1));
// fails here
assert!(d.is_null(2));
assert_eq!("with nulls", d.get_string(3));
}

Reporter: Neville Dipale / @nevi-me

Related issues:

• [Rust][Arrow] Nulls are transformed into "" after filtering for StringArray (duplicates)

_{Note: This issue was originally created as ARROW-5352. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Will this be done in the next few days?

[asfimport]

Andrew Lamb / @alamb:
Dupe of ARROW-10136 – thanks [~nevi_me] for pointing that out