[jira] [Created] (ARROW-15651) Structs incorrectly initialise null values

[Alfred Mountfield (Jira)]

Alfred Mountfield created ARROW-15651:
-----------------------------------------

             Summary: Structs incorrectly initialise null values
                 Key: ARROW-15651
                 URL: https://issues.apache.org/jira/browse/ARROW-15651
             Project: Apache Arrow
          Issue Type: Bug
          Components: JavaScript
    Affects Versions: 7.0.0
            Reporter: Alfred Mountfield


Nullable StructArrays created with FixedSizeList fields seem to incorrectly 
initialise.
I've tried to create them using both the `Builder` and `makeVector` methods and 
I believe it breaks the specification.

I believe that the underlying arrays for the fields of a struct should be the 
length of the struct array.

However the `nullCount`s, `numChildren`s, and other methods all return 
different numbers. (And this is causing a problem when we try to read the same 
memory within Rust as the lengths and offsets differ)

Specifically this:
{code:JavaScript}
let list_field = new Field('n1', new FixedSizeList(2, Float64), true);
let struct_field = new Field('foo', new Struct([list_field]), true);

let builder = new Builder({
type:struct_field.type,
nullValues: [null, undefined],
});

builder.append(null);

console.log('Builder:' + JSON.stringify(builder));
console.log('numChildren: ' + builder.numChildren);
console.log('nullCount: ' + builder.nullCount);
console.log('length: ' + builder.length);

let vec1 = builder.toVector();
console.log('Vector from Builder:' + vec1);
console.log('numChildren: ' + vec1.numChildren);
console.log('nullCount: ' + vec1.nullCount);
console.log('length: ' + vec1.length);

let vec2 = makeVector({
data: [null],
type:struct_field.type,
nullable:true,
});

console.log('Vector from makeVector:' + JSON.stringify(vec2));
console.log('numChildren: ' + vec2.numChildren);
console.log('nullCount: ' + vec2.nullCount);
console.log('length: ' + vec2.length);
{code}

Results in (I've removed some fields for brevity)
{code:JavaScript}
Builder: 
\{"length":1,"finished":false,"type":{"children":[{"name":"n1","type":{"listSize":2,"children":[null]},"nullable":true,"metadata":{}}]},"children":[],"nullValues":[null,null],"stride":1,"_nulls":\{"buffer":
 ... ,"stride":0.125,"BYTES_PER_ELEMENT":1,"length":1,"numValid":0}}
numChildren: 0
nullCount: 1
length: 1

Vector from Builder:[]
numChildren: 1
nullCount: 1
length: 1

Vector from makeVector:[]
numChildren: 1
nullCount: 0
length: 0
{code}



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