• Part of [EPIC] [Parquet] Implement Variant type support in Parquet #6736
• From [Variant] Improve getter API for VariantList and VariantObject #7757 (comment)

Describe the bug

VariantObject::field_name(i:usize) returns the field name corresponding to its index. If index is out of bounds, the method should return None, but when the index is the length of the fields, it will return the first element. When the index is larger than the length of the fields, it starts to panic.

Steps to reproduce

Please use the following test:

This test builds up a VariantObject with three fields: {"active": true, "age": 42, "name": "hello"}

let res = variant_object.field_name(3); // 3 is out of bounds!

// the assertion below _will_ fail. We expect `res` to be `None`, but it returns the first field ("active"). 
assert_eq!(res.is_none()); 

// this call below will panic! This is because `field_name` internally calls
// `try_field_name`, which will err with the following: 
// validation error after construction: InvalidArgumentError("Tried to extract byte(s) 302..303 from 18-byte buffer")
let res = variant_object.field_name(300); // 300 is definitely out of bounds

Note, using the VariantBuilder to build the same object and then trying to access fields with an out of bounds index will err correctly:

#[test]
fn test_foo() {
    let mut variant_builder = VariantBuilder::new();
    let mut obj_builder = variant_builder.new_object();
    obj_builder.append_value("active", true);
    obj_builder.append_value("age", 42);
    obj_builder.append_value("name", "hello");
    obj_builder.finish();
    let (m, v) = variant_builder.finish();

    let variant = Variant::try_new(&m, &v).unwrap();
    let variant_obj = variant.as_object().unwrap();

    let res = variant_obj.field(3);
    assert!(res.is_none());

    let res = variant_obj.field(300);
    assert!(res.is_none());
}

You can obviously add a oob check since we know the number of elements in a VariantObject, but it seems like there is a bug in the underlying try_field_name logic.

cc @scovich