Clarify provisional child registration

Child monitors register before their IPC handshake so cancellation
owns every started process, but the bare `None` portal arg obscured
that `Portal(chan)` replaces the provisional entry after connection.

Document that transition and name all `_register_child()` args in both
spawn backends. Replace the MP test's positional-only lambda with a
signature-accurate fake which asserts the provisional portal state.

Caught-during: review remediation
Found-via: `/run-tests` test_mp_late_registration_never_starts_process

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

Prompt-IO: ai/prompt-io/opencode/20260825T015742Z_e42ecb55_prompt_io.md

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

ai/prompt-io/opencode/20260825T015742Z_e42ecb55_prompt_io.md

@@ -0,0 +1,38 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+session: d9d7df2c-7044-463f-8768-ec024718eac9
+timestamp: 2026-08-25T01:57:42Z
+git_ref: e42ecb55
+scope: code
+substantive: true
+raw_file: 20260825T015742Z_e42ecb55_prompt_io.raw.md
+---
+
+## Prompt
+
+Continue PR #481 review remediation after committing the `Aid` reap-map
+migration. The human selected "keep" for comments explaining why both
+spawn backends provisionally register children with `portal=None`.
+
+## Response summary
+
+Document that a child has no `Portal` until its IPC handshake yields a
+`Channel`, and make `portal=None` explicit at both registration calls.
+Identify the later replacement of each provisional entry with
+`Portal(chan)`. Update the MP registration test double to accept and
+assert the explicit provisional portal state. Name every registration
+argument consistently in both backends.
+
+## Files changed
+
+- `tractor/spawn/_mp.py` - clarify provisional MP registration.
+- `tractor/spawn/_trio.py` - clarify provisional Trio registration.
+- `tests/test_to_actor.py` - model explicit provisional registration.
+
+## Human edits
+
+The human explicitly selected "keep" after receiving keep/defer/drop
+options for this paired clarification. During local review, the human
+then requested that `subactor` and `proc` also be passed by name in both
+backend calls. No direct source-line edits were made by the human.

ai/prompt-io/opencode/20260825T015742Z_e42ecb55_prompt_io.raw.md

@@ -0,0 +1,21 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+timestamp: 2026-08-25T01:57:42Z
+git_ref: e42ecb55
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+Implement the approved PR #481 clarification for provisional child
+registration in both process-spawn backends.
+
+> `git diff HEAD~1..HEAD -- tractor/spawn/_mp.py`
+
+> `git diff HEAD~1..HEAD -- tractor/spawn/_trio.py`
+
+Explain that `portal=None` is provisional because no `Portal` can exist
+until the child completes its IPC handshake and returns a `Channel`.
+Use an explicit keyword argument and identify the later replacement with
+`Portal(chan)`.
+
+Run lint and the full `to_actor` runtime suite.

tests/test_to_actor.py

@@ -456,6 +456,24 @@ def test_mp_late_registration_never_starts_process(
 
     process = FakeProcess()
 
+    def register_child(
+        subactor: object,
+        proc: object,
+        portal: object|None,
+    ) -> tuple[trio.Event, trio.Event, bool]:
+        '''
+        Simulate provisional MP registration before child startup.
+
+        '''
+        assert subactor
+        assert proc is process
+        assert portal is None
+        return (
+            trio.Event(),
+            trio.Event(),
+            True,
+        )
+
     class FakeContext:
         def get_start_method(self) -> str:
             return 'spawn'
@@ -465,11 +483,7 @@ def test_mp_late_registration_never_starts_process(
             return process
 
     nursery = SimpleNamespace(
-        _register_child=lambda *args: (
-            trio.Event(),
-            trio.Event(),
-            True,
-        ),
+        _register_child=register_child,
     )
     subactor = SimpleNamespace(
         aid=tractor.msg.Aid(

tractor/spawn/_mp.py

@@ -141,14 +141,16 @@ async def mp_proc(
     # `multiprocessing` only (since no async interface): publish the
     # process and its reap coordination before start so cancellation
     # can own every subsequently started child.
+    # No `Portal` exists until the IPC handshake returns `chan`.
+    # Replace this provisional entry with `Portal(chan)` below.
     (
         reap_request,
         _,
         cancel_during_registration,
     ) = actor_nursery._register_child(
-        subactor,
-        proc,
-        None,
+        subactor=subactor,
+        proc=proc,
+        portal=None,
     )
     if cancel_during_registration:
         raise RuntimeError(

tractor/spawn/_trio.py

@@ -130,14 +130,17 @@ async def trio_proc(
                 f' |_{proc}\n'
             )
 
+            # No `Portal` exists until the IPC handshake returns
+            # `chan`. Replace this provisional entry with
+            # `Portal(chan)` below.
             (
                 reap_request,
                 _,
                 cancel_during_registration,
             ) = actor_nursery._register_child(
-                subactor,
-                proc,
-                None,
+                subactor=subactor,
+                proc=proc,
+                portal=None,
             )
             if cancel_during_registration:
                 cancelled_during_spawn = True