---
model: openai/gpt-5.6-sol
service: opencode
session: 76c5d31c-5a2f-4503-9b16-410ee7f4fab3
timestamp: 2026-08-21T04:08:03Z
git_ref: 3c1bbe73
scope: code
substantive: true
raw_file: 20260821T040803Z_3c1bbe73_prompt_io.raw.md
---

## Prompt

Continue PR #481 review cleanup one item at a time. Compare the
remaining child-registration/reaping experiment against PR #484,
identify the next valid behavior, and generate an exact commit plan for
that boundary without committing automatically.

## Response summary

Separated late-child admission from the broader bounded-reap rewrite.
Added one synchronous `ActorNursery._register_child()` publication step
that stores the child and its reap events before sampling nursery
cancellation. This closes both scheduler orderings around
`ActorNursery.cancel()`'s child snapshot.

The MP backend registers immediately before synchronous process startup
and refuses to start when cancellation already owns the child. The Trio
backend registers immediately after `open_process()` and kills that
already-created process when registration observes cancellation. An
early `start_actor()` guard rejects calls begun after cancellation is
already visible.

Deterministic tests cover the nursery registration ordering and the MP
no-start invariant. Comparison with PR #484 confirmed that its retained
generic nursery/backends do not close this race.

## Files changed

- `tractor/runtime/_supervise.py` - atomically publish child ownership
  and reject actor starts after nursery cancellation.
- `tractor/spawn/_mp.py` - register before synchronous process startup
  and abort a cancellation-owned child.
- `tractor/spawn/_trio.py` - register immediately after process creation,
  kill a cancellation-owned child, and remove its stale unused import.
- `tests/test_to_actor.py` - cover late registration and MP startup
  suppression.

## Human edits

The human required review extras to be handled one item and one
behavioral commit at a time, with each item compared against PR #484
before acceptance. That direction split this late-registration fix from
the original broad experiment's bounded post-ack reaping,
`ActorNursery.cancel()` hard-reap rewrite, and debugger/error behavior.
The human accepted the narrower late-registration boundary by requesting
its commit plan. No direct source-line edits were made by the human.