Assert paired `ActorNursery` reap state

`._mark_child_reaped()` previously discarded the reap-request event
without checking that its completion-event peer existed. A one-sided
entry would silently lose process-reap synchronization.

Capture both pops and assert paired presence while allowing the valid
both-absent startup-failure path. Keep an unset request valid because
backend cancellation can reap immediately after registration.

Extend graceful and failed-cancel-ack runtime tests to require all
child and reap mappings empty before `to_actor.run()` returns.

Review: PR #481 (goodboy)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-5012942328

Prompt-IO: ai/prompt-io/opencode/20260824T223614Z_88d538e3_prompt_io.md

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

ai/prompt-io/opencode/20260824T223614Z_88d538e3_prompt_io.md

@@ -0,0 +1,35 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+session: d9d7df2c-7044-463f-8768-ec024718eac9
+timestamp: 2026-08-24T22:36:14Z
+git_ref: 88d538e3
+scope: code
+substantive: true
+raw_file: 20260824T223614Z_88d538e3_prompt_io.raw.md
+---
+
+## Prompt
+
+Continue PR #481 review remediation after committing the shared
+`Context.cancel()` deadline fix. The human accepted the proposed
+child-reap bookkeeping invariant, asking only that the first fix receive
+its own commit plan and commit before this update began.
+
+## Response summary
+
+Check that `ActorNursery` removes its paired reap-coordination entries
+together while preserving valid pre-registration and immediate-cancel
+paths. Extend the existing real-runtime reap tests to prove all three
+child bookkeeping mappings are empty before `to_actor.run()` returns.
+
+## Files changed
+
+- `tractor/runtime/_supervise.py` - assert paired reap-map cleanup.
+- `tests/test_to_actor.py` - verify graceful and hard-reap bookkeeping.
+
+## Human edits
+
+The human explicitly accepted this invariant update but directed the
+preceding cancellation fix to be planned and committed as a separate
+boundary first. No direct source-line edits were made by the human.

ai/prompt-io/opencode/20260824T223614Z_88d538e3_prompt_io.raw.md

@@ -0,0 +1,26 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+timestamp: 2026-08-24T22:36:14Z
+git_ref: 88d538e3
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+Implement the approved PR #481 child-reap bookkeeping update after the
+preceding `Context.cancel()` fix was committed separately.
+
+> `git diff HEAD~1..HEAD -- tractor/runtime/_supervise.py`
+
+`ActorNursery._mark_child_reaped()` captures both reap-coordination
+entries and asserts that they are either both present or both absent.
+It intentionally does not require the reap-request event to be set,
+because backend cancellation can reap immediately after registration.
+
+> `git diff HEAD~1..HEAD -- tests/test_to_actor.py`
+
+Existing real-runtime graceful and hard-reap tests verify that
+`ActorNursery._children`, `ActorNursery._child_reap_requests`, and
+`ActorNursery._child_reaped` are all empty before the one-shot call
+returns.
+
+Run focused bookkeeping and real-runtime reap tests after the edit.

tests/test_to_actor.py

@@ -201,10 +201,10 @@ async def test_spawn_from_caller_nursery(
 
     Previously `to_actor.run()` treated an actor-runtime cancel ack
     as process reaping, so the call returned while the child monitor
-    and its `ActorNursery._children` record remained alive until the
-    entire nursery exited. The assertion inside the still-open
-    nursery proves child-process joining and record removal now
-    complete before the one-shot call returns.
+    and its `ActorNursery` child/reap bookkeeping remained alive until
+    the entire nursery exited. The assertions inside the still-open
+    nursery prove child-process joining and removal from all three
+    mappings complete before the one-shot call returns.
 
     '''
     async with tractor.open_nursery() as an:
@@ -214,6 +214,8 @@ async def test_spawn_from_caller_nursery(
             an=an,
         ) == 11
         assert not an._children
+        assert not an._child_reap_requests
+        assert not an._child_reaped
 
 
 @tractor_test
@@ -231,7 +233,7 @@ async def test_cancel_ack_failure_hard_reaps_child(
     gate and then waited forever for a still-running process. This
     test forces that exact result without cancelling the actor, caps
     the call to detect the former hang and verifies the child monitor
-    removes its `ActorNursery._children` record before returning.
+    removes every `ActorNursery` child/reap entry before returning.
 
     '''
     async def cancel_without_ack(
@@ -256,6 +258,8 @@ async def test_cancel_ack_failure_hard_reaps_child(
                 an=an,
             ) == 21
         assert not an._children
+        assert not an._child_reap_requests
+        assert not an._child_reaped
 
 
 def test_cancel_actor_timeout_closes_blocked_send():

tractor/runtime/_supervise.py

@@ -370,11 +370,18 @@ class ActorNursery:
 
         '''
         self._children.pop(uid, None)
+        reap_request: trio.Event|None = (
             self._child_reap_requests.pop(uid, None)
+        )
         reaped: trio.Event|None = self._child_reaped.pop(
             uid,
             None,
         )
+        assert (
+            (reap_request is None)
+            ==
+            (reaped is None)
+        )
         if reaped is not None:
             reaped.set()