Modify FixedSizeListBuilder behavior for null slots

js/src/builder/fixedsizelist.ts

@@ -24,10 +24,16 @@ export class FixedSizeListBuilder<T extends DataType = any, TNull = any> extends
     public setValue(index: number, value: T['TValue']) {
         const [child] = this.children;
         const start = index * this.stride;
-        for (let i = -1, n = value.length; ++i < n;) {
-            child.set(start + i, value[i]);
+        for (let i = -1, n = this.stride; ++i < n;) {
+            // Use either the actual value or a null (if the slot is null)
+            const finalVal = value ? value[i] : null;
+            child.set(start + i, finalVal);
         }
     }
+    public setValid(index: number, valid: boolean) {
+        this.length = this._nulls.set(index, +valid).length;
+        return true;
+    }
     public addChild(child: Builder<T>, name = '0') {
         if (this.numChildren > 0) {
             throw new Error('FixedSizeListBuilder can only have one child.');

js/test/unit/vector/vector-tests.ts

@@ -16,7 +16,7 @@
 // under the License.
 
 import {
-    Bool, DateDay, DateMillisecond, Dictionary, Float64, Int32, List, makeVector, Struct, Utf8, LargeUtf8, util, Vector, vectorFromArray, makeData
+    Bool, DateDay, DateMillisecond, Dictionary, Float64, Int32, List, makeVector, Struct, Utf8, LargeUtf8, util, Vector, vectorFromArray, makeData, FixedSizeList, Field,
 } from 'apache-arrow';
 
 describe(`makeVectorFromArray`, () => {
@@ -323,3 +323,40 @@ function basicVectorTests(vector: Vector, values: any[], extras: any[]) {
         }
     });
 }
+
+// GH-45862: Make sure vectorFromArray produces the correct result for
+// FixedSizeList with null slots
+describe(`vecorFromArray() with FixedSizeList<T> and null slots`, () => {
+    test(`correct child length with null slot first`, () => {
+        let vector = vectorFromArray(
+            [null, [1, 2, 3]],
+            new FixedSizeList(3, new Field('item', new Int32())),
+        );
+        let child = vector.getChildAt(0);
+
+        expect(child).toHaveLength(6);
+        expect(child?.nullCount).toBe(3);
+    });
+
+    test(`correct child length with null slot last`, () => {
+        let vector = vectorFromArray(
+            [[1, 2, 3], null],
+            new FixedSizeList(3, new Field('item', new Int32())),
+        );
+        let child = vector.getChildAt(0);
+
+        expect(child).toHaveLength(6);
+        expect(child?.nullCount).toBe(3);
+    });
+
+    test(`correct child length with null in the middle`, () => {
+        let vector = vectorFromArray(
+            [[1, 2, 3], null, [7, 8, 9]],
+            new FixedSizeList(3, new Field('item', new Int32())),
+        );
+        let child = vector.getChildAt(0);
+
+        expect(child).toHaveLength(9);
+        expect(child?.nullCount).toBe(3);
+    });
+});