update ... by: loses HAS_NULLS on the per-group gather — aggregates read null sentinels as values

[singaraiona]

Summary

ray_update's by: branch gathers each group's rows into a per-group sub-table with ray_vec_new + raw memcpy, but never propagates RAY_ATTR_HAS_NULLS from the source column. The per-group aggregate then sees raw sentinel payloads as values.

Repro

(set NN (table [k v] (list [1 2 1 2] (as 'I64 [10 0N 30 40]))))
(at (update {mn: (min v) by: k from: NN}) 'mn)  ;; [10 0Nl 10 0Nl]  — wrong, should be 10, 40
(at (update {sm: (sum v) by: k from: NN}) 'sm)  ;; [40 -9223372036854775768 ...] — sentinel summed in
(select {mn: (min v) from: NN by: k})            ;; 10, 40 — correct oracle

Notes

• Pre-existing on the atom-aggregate path (reachable on base via new columns); it became reachable on existing columns once fix(query): support update over parted tables and prevent column duplication by: #406 removed the duplicate-column bug, which is how it surfaced during that PR's review.
• Likely one-line fix adjacent to the gather in ray_update's by: branch: sub_col->attrs |= full_col->attrs & RAY_ATTR_HAS_NULLS; — plus a regression test mirroring the repro (min/sum over a nullable column under update by:, checked against the select by: oracle).

[singaraiona]

Confirmed on current dev at 9e1592a4 (v2.5.14 debug build).

The report's diagnosis is correct: the update by: per-group gather allocates each sub-column and raw-copies its payload, but does not preserve RAY_ATTR_HAS_NULLS. The copied I64 null sentinel is consequently treated as an ordinary value by min/sum.

I am implementing the fix with regression coverage for nullable grouped min and sum, checked against the corresponding select by: result.

[singaraiona]

Implemented and verified locally.

The update-by grouped gather now preserves RAY_ATTR_HAS_NULLS whenever any selected source row is null, so aggregate kernels no longer take the null-free fast path on gathered data. Added test/rfl/regress/issue_411.rfl covering grouped min and sum plus select/update agreement.

Validation: full debug/ASan/UBSan suite passes, 3701/3701. Pushed to dev in bfb5b380.