Add `.trionics.start_or_cancel()` test suite

Resolve #474 with a new `tests/trionics/test_taskc.py` (9
tests) covering the `trio.Nursery.start()` wrapper landed in
PR #464, incl. the `modden.runtime.progman.open_wks()` use
case dug out as a minimal repro.

Deats,
- the lossy `RuntimeError('child exited without calling
  task_status.started()')` only fires when the child absorbs
  its ambient cancel pre-`.started()` (graceful-teardown
  pattern); a well-behaved child surfaces `Cancelled` direct
  from `.start()` on `trio` 0.29 - verified empirically 1st.
- `test_sibling_err_not_masked_by_startup_rte`: the `modden`
  case; ONLY the root-cause sibling `ValueError` escapes the
  nursery with the wrapper vs. bare-`.start()`'s lossy
  riding-along startup-RTE noise.
- `test_pure_oob_cancel_not_morphed_to_rte`: plain ancestor
  `cs.cancel()` exits clean vs. bare's eg-wrapped RTE.
- `test_genuine_startup_rte_still_raised`: sans cancellation
  the protocol-bug RTE re-raises same as bare.
- `test_childs_own_rte_never_demoted_to_cancel`: exact-msg +
  `isinstance`-guard regression cover; a child's own
  `RuntimeError('never got started!')`/`RuntimeError(1234)`
  never demotes to a `Cancelled`.
- `test_started_value_and_args_passthru`: positional args,
  `name=` and `.started()`-value forwarding.

Also,
- `use_start_or_cancel=False` params pin upstream `trio`'s
  current lossy behaviour as wart-documentation: a break on
  a `trio` upgrade likely means new upstream porcelain and
  the wrapper deserves a re-audit.
- verified 0 flakes over 50 hammer runs + 2 impl mutations
  each caught by exactly the targeted tests.

Prompt-IO: ai/prompt-io/claude/20260702T161624Z_65bf9df5_prompt_io.md
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

ai/prompt-io/claude/20260702T161624Z_65bf9df5_prompt_io.md

@@ -0,0 +1,80 @@
+---
+model: claude-fable-5
+service: claude
+session: f4da6654-63f9-4641-b567-f200da46d697
+timestamp: 2026-07-02T16:16:24Z
+git_ref: 65bf9df5
+scope: tests
+substantive: true
+raw_file: 20260702T161624Z_65bf9df5_prompt_io.raw.md
+---
+
+## Prompt
+
+> attempt to resolve
+> https://github.com/goodboy/tractor/issues/474
+> do it with /open-wkt.
+
+Issue #474: "We need test suite for
+`.trionics.start_or_cancel()`" — dig out the use case
+from `modden` as a unit test (and provide motivation
+for upstream `trio` to possibly provide better
+porcelain around this case). The wrapper landed via
+PR #464 with no tests.
+
+## Response summary
+
+New `tests/trionics/test_taskc.py` (9 tests) covering
+`start_or_cancel()`, authored in wkt
+`start_or_cancel_tests_474`.
+
+Behaviour was first probed empirically (trio 0.29):
+the lossy `RuntimeError('child exited without calling
+task_status.started()')` only fires when the child
+exits pre-`.started()` WITHOUT propagating the ambient
+`Cancelled` — i.e. when the child (or lib code it
+calls) absorbs the cancel in a graceful-teardown
+pattern; a well-behaved child surfaces `Cancelled`
+straight out of `.start()`. The `modden`
+`progman.open_wks()` use case was reconstructed from
+`modden/runtime/progman.py` accordingly.
+
+Tests (each `use_start_or_cancel` parametrization also
+pins upstream trio's current lossy behaviour as
+wart-documentation):
+
+- `test_sibling_err_not_masked_by_startup_rte` — the
+  `modden` case: sibling error OOB-cancels the shared
+  nursery scope; with the wrapper ONLY the root-cause
+  `ValueError` escapes; bare `.start()` adds the lossy
+  RTE alongside.
+- `test_pure_oob_cancel_not_morphed_to_rte` — plain
+  ancestor `cs.cancel()`: wrapper → clean exit; bare
+  → eg-wrapped RTE.
+- `test_genuine_startup_rte_still_raised` — no
+  cancellation → protocol-bug RTE re-raised same as
+  bare.
+- `test_childs_own_rte_never_demoted_to_cancel` — a
+  child's own `RuntimeError('never got started!')` /
+  `RuntimeError(1234)` under ambient cancel is never
+  demoted to `Cancelled` (exact-msg-match + str-guard
+  regression cover).
+- `test_started_value_and_args_passthru` — happy path:
+  positional args, `name=`, `.started()` value.
+
+Verified: 9/9 pass; 0 flakes across 50 hammer runs;
+two impl mutations (checkpoint removed; guard relaxed
+to substring match) each caught by exactly the
+targeted tests; `tests/trionics/` +
+`tests/test_trioisms.py` subset green (23 passed,
+5 xfailed); ruff clean; 69-col style.
+
+## Files changed
+
+- `tests/trionics/test_taskc.py` — new
+  `start_or_cancel()` unit-test suite (gh #474).
+
+## Human edits
+
+Pending review — session paused pre-commit per user
+deadline; nothing committed as of this entry.

ai/prompt-io/claude/20260702T161624Z_65bf9df5_prompt_io.raw.md

@@ -0,0 +1,107 @@
+---
+model: claude-fable-5
+service: claude
+timestamp: 2026-07-02T16:16:24Z
+git_ref: 65bf9df5
+diff_cmd: git diff main..wkt/start_or_cancel_tests_474
+---
+
+# Raw output — gh #474 `start_or_cancel()` test suite
+
+## Generated test code
+
+> `git diff main..wkt/start_or_cancel_tests_474 -- tests/trionics/test_taskc.py`
+
+Prose summary of the generated module
+(`tests/trionics/test_taskc.py`):
+
+- module docstring framing the `trio.Nursery.start()`
+  startup-cancellation wart, the wrapper's repair, and
+  the intent that `use_start_or_cancel=False` params
+  double as upstream-trio wart-documentation (break on
+  a trio upgrade → upstream may have shipped porcelain,
+  re-audit the wrapper); cites gh #474 / PR #464 and
+  `modden`'s `progman.open_wks()` as the source use
+  case.
+- shared children: `absorbs_cancel_pre_started()` (the
+  graceful-teardown cancel-absorber which triggers the
+  lossy RTE path) + `raise_val_err()` (fast-erroring
+  sibling).
+- `test_sibling_err_not_masked_by_startup_rte`
+  (parametrized `use_start_or_cancel`): asserts eg
+  contains exactly one `ValueError` and, wrapper-case,
+  NO residual RTE (`eg.split(ValueError)` remainder is
+  `None`); bare-case, the residual RTE carries trio's
+  exact "child exited without calling" wording.
+- `test_pure_oob_cancel_not_morphed_to_rte`
+  (parametrized): wrapper-case runs clean and asserts
+  `cs.cancelled_caught`; bare-case asserts the
+  eg-wrapped RTE.
+- `test_genuine_startup_rte_still_raised`
+  (parametrized): no-cancel protocol bug → RTE with
+  trio's wording from both call forms.
+- `test_childs_own_rte_never_demoted_to_cancel`
+  (parametrized `rte_arg` in `'never got started!'`,
+  `1234`): child cancels the ambient scope then raises
+  its own RTE synchronously (no checkpoint between →
+  deterministically under-cancellation at catch time);
+  asserts the RTE survives with `args[0]` intact.
+- `test_started_value_and_args_passthru`: `.started()`
+  value, positional args and the `name=` kwarg (via
+  `trio.lowlevel.current_task().name`) all forward.
+
+## Non-code output (verbatim highlights)
+
+Behaviour probe (trio 0.29, scratchpad scripts) — the
+decision basis for the test shapes:
+
+```
+== B-sibling-err use_soc=False
+  start raised: RuntimeError('child exited without
+    calling task_status.started()')
+  top-level: ExceptionGroup([ValueError('sibling blew
+    up!'), RuntimeError('child exited without calling
+    task_status.started()')])
+== B-cs-cancel use_soc=False
+  top-level: ExceptionGroup([RuntimeError('child
+    exited without calling task_status.started()')])
+== B-sibling-err use_soc=True
+  start raised: Cancelled()
+  top-level: ExceptionGroup([ValueError('sibling blew
+    up!')])
+== B-cs-cancel use_soc=True
+  start raised: Cancelled()
+  top-level: clean return
+== own-rte-under-cancel (both) -> RTE('never got
+  started!') propagates unchanged
+```
+
+Key finding: with a WELL-BEHAVED (non-absorbing) child
+an OOB ancestor cancel surfaces `Cancelled` directly
+from `.start()` on trio 0.29 — the lossy RTE requires
+the child to absorb its cancel pre-`.started()`, which
+is what `modden`'s `open_from_wks` teardown did. Trio's
+nursery-exit wait defers cancel delivery to children,
+so all tested shapes are deterministic (0 flakes / 50
+runs).
+
+Mutation verification:
+
+```
+mutation 1 (checkpoint_if_cancelled removed):
+  FAILED test_sibling_err_not_masked_by_startup_rte[True]
+  FAILED test_pure_oob_cancel_not_morphed_to_rte[True]
+mutation 2 (guard relaxed to 'started' substring,
+            isinstance dropped):
+  FAILED test_childs_own_rte_never_demoted_to_cancel[never got started!]
+  FAILED test_childs_own_rte_never_demoted_to_cancel[1234]
+```
+
+Final runs:
+
+```
+tests/trionics/test_taskc.py: 9 passed in 0.03s
+hammer: 0/50 runs failed
+tests/trionics/ + tests/test_trioisms.py:
+  23 passed, 5 xfailed in 3.02s
+```

ai/prompt-io/opencode/20260804T030309Z_65bf9df5_prompt_io.md

@@ -0,0 +1,39 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+session: moc-teardown-completion-20260804
+timestamp: 2026-08-04T03:03:09Z
+git_ref: 65bf9df5
+scope: code
+substantive: true
+raw_file: 20260804T030309Z_65bf9df5_prompt_io.raw.md
+---
+
+## Prompt
+
+Patch Tractor's `maybe_open_context()` so the final consumer waits for
+resource `__aexit__()` completion and receives cleanup errors. Reuse
+`outcome.Outcome` for the exit result; use the smaller mutable
+`_CtxExit` holder if that makes the implementation simpler. Add and run
+the relevant existing unit tests, but do not commit or push the patch.
+After reviewing the result, simplify `_CtxExit` back to an optional
+exception because the success outcome carries no useful value.
+
+## Response summary
+
+Added an exception-backed completion handshake between
+`_Cache.run_ctx()` and the final `maybe_open_context()` consumer.
+Serialized consumer registration and final teardown under the per-key
+lock, preserving that lock for queued entrants. Added deterministic
+regressions for normal exit, cleanup errors, cancellation interactions,
+service-nursery cancellation, and teardown re-entry.
+
+## Files changed
+
+- `tractor/trionics/_mngrs.py` - publish and unwrap cached exit outcomes.
+- `tests/test_resource_cache.py` - cover completion and cancellation.
+
+## Human edits
+
+The user directed the final simplification from `outcome.Outcome` to an
+optional exception field. The patch remains uncommitted.

ai/prompt-io/opencode/20260804T030309Z_65bf9df5_prompt_io.raw.md

@@ -0,0 +1,36 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+timestamp: 2026-08-04T03:03:09Z
+git_ref: 65bf9df5
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+Implemented cached-context exit completion in
+`tractor.trionics.maybe_open_context()`.
+
+> `git diff HEAD~1..HEAD -- tractor/trionics/_mngrs.py`
+
+The generated implementation adds `_CtxExit`, whose `done` event
+publishes an `outcome.Outcome[None]`. `_Cache.run_ctx()` records either
+`Value(None)` or `Error(exc)` after the resource exit attempt. The final
+MOC consumer signals `no_more_users`, waits for completion under a
+shielded cancel scope, removes the per-key lock, and unwraps the outcome
+so ordinary cleanup failures are raised at the consumer boundary.
+`trio.Cancelled`, `KeyboardInterrupt`, and `SystemExit` continue through
+the service task rather than being converted into regular cleanup
+errors.
+
+> `git diff HEAD~1..HEAD -- tests/test_resource_cache.py`
+
+The generated regressions cover successful exit blocking, cleanup-error
+delivery, final-user cancellation, cancellation combined with a cleanup
+error, and service-nursery cancellation. The existing teardown re-entry
+test now uses explicit events instead of a ten-second cleanup sleep and
+asserts that the replacement resource is a fresh cache miss.
+
+Verification:
+
+`env PYTHONPATH="$PWD" /home/goodboy/repos/tractor/py313/bin/python -m pytest tests/test_resource_cache.py`
+
+Result: `16 passed in 8.37s`.

ai/prompt-io/opencode/README.md

@@ -0,0 +1,22 @@
+# AI Prompt I/O Log - OpenCode
+
+This directory tracks prompt inputs and model outputs for AI-assisted
+development using `opencode`.
+
+## Policy
+
+Prompt logging follows the [NLNet generative AI policy][nlnet-ai]. All
+substantive AI contributions are logged with:
+
+- Model name and version
+- Timestamps
+- The prompts that produced the output
+- Unedited model output (`.raw.md` files)
+
+[nlnet-ai]: https://nlnet.nl/foundation/policies/generativeAI/
+
+## Usage
+
+Entries are created by the prompt-io workflow. Human contributors remain
+accountable for all decisions. AI-generated content is never presented as
+human-authored work.

tests/test_log_sys.py

@@ -2,16 +2,19 @@
 `tractor.log`-wrapping unit tests.
 
 '''
+import logging
 from pathlib import Path
 import shutil
 from types import ModuleType
 
 import pytest
 import tractor
+import trio
 from tractor import (
     _code_load,
     log,
 )
+from tractor.ipc import _chan
 
 
 def test_root_pkg_not_duplicated_in_logger_name():
@@ -222,6 +225,88 @@ def test_add_log_level_pluggable():
             delattr(log.StackLevelAdapter, name.lower())
 
 
+@pytest.mark.parametrize(
+    'suppression',
+    [
+        'level',
+        'logger',
+        'global',
+    ],
+)
+def test_log_guard_skips_payload_formatting(
+    monkeypatch: pytest.MonkeyPatch,
+    suppression: str,
+):
+    '''
+    Suppressed transport logs must not render payloads.
+
+    The original hot-path guard compared only the effective logger
+    level. A logger disabled through its `Logger.disabled` flag or
+    the global `logging.disable()` threshold could therefore still
+    call `pformat()` before `Logger.isEnabledFor()` discarded the
+    record.
+
+    Exercise effective-level, per-logger, and global suppression
+    independently. A poisoned `_chan.pformat()` proves rendering is
+    skipped, while the fake transport proves `Channel.send()` still
+    transmits the original payload and traceback-hiding flag.
+
+    '''
+    sent: list[tuple[object, bool]] = []
+
+    class FakeTransport:
+        async def send(
+            self,
+            payload: object,
+            hide_tb: bool = False,
+        ) -> None:
+            sent.append((payload, hide_tb))
+
+    def fail_pformat(payload: object) -> str:
+        raise AssertionError(
+            f'suppressed log rendered payload: {payload!r}'
+        )
+
+    chan_log = log.get_logger(
+        name=f'guard_test.{suppression}',
+    )
+    std_log = chan_log.logger
+    orig_level: int = std_log.level
+    orig_disable: int = logging.root.manager.disable
+    transport_level: int = log.CUSTOM_LEVELS['TRANSPORT']
+
+    monkeypatch.setattr(_chan, 'log', chan_log)
+    monkeypatch.setattr(_chan, 'pformat', fail_pformat)
+    try:
+        logging.disable(logging.NOTSET)
+        std_log.setLevel(transport_level)
+
+        if suppression == 'level':
+            std_log.setLevel(logging.INFO)
+        elif suppression == 'logger':
+            monkeypatch.setattr(std_log, 'disabled', True)
+        else:
+            logging.disable(logging.CRITICAL)
+
+        assert not chan_log.isEnabledFor(transport_level)
+
+        transport = FakeTransport()
+        chan = _chan.Channel(transport=transport)
+        payload = object()
+
+        async def send_payload() -> None:
+            await chan.send(
+                payload,
+                hide_tb=True,
+            )
+
+        trio.run(send_payload)
+        assert sent == [(payload, True)]
+    finally:
+        std_log.setLevel(orig_level)
+        logging.disable(orig_disable)
+
+
 # TODO, moar tests against existing feats:
 # ------ - ------
 # - [ ] color settings?

tests/test_resource_cache.py

@@ -9,6 +9,7 @@ from typing import Awaitable
 
 import pytest
 import trio
+from trio.testing import wait_all_tasks_blocked
 import tractor
 from tractor.trionics import (
     maybe_open_context,
@@ -94,6 +95,232 @@ def test_resource_only_entered_once(key_on):
     trio.run(main)
 
 
+def test_last_moc_user_waits_for_resource_exit():
+    '''
+    Verify the final user cannot return before resource teardown.
+
+    Previously the final `maybe_open_context()` user only signalled
+    `_Cache.run_ctx()` through its `no_more_users` event. The user
+    then returned while the service task was still running the
+    resource's `__aexit__()`, so callers could observe stale external
+    state immediately after their `async with` block.
+
+    The resource sets `exit_started` before blocking on
+    `allow_exit`. The user task must remain inside MOC until the test
+    releases that deterministic checkpoint and `__aexit__()` sets
+    `exit_finished`.
+
+    '''
+    async def main():
+        exit_started = trio.Event()
+        allow_exit = trio.Event()
+        exit_finished = trio.Event()
+        user_returned = trio.Event()
+
+        @acm
+        async def open_resource():
+            try:
+                yield
+            finally:
+                exit_started.set()
+                await allow_exit.wait()
+                exit_finished.set()
+
+        async def use_resource():
+            async with maybe_open_context(open_resource):
+                pass
+
+            assert exit_finished.is_set()
+            user_returned.set()
+
+        async with (
+            tractor.open_root_actor(),
+            trio.open_nursery() as tn,
+        ):
+            tn.start_soon(use_resource)
+            await exit_started.wait()
+            assert not user_returned.is_set()
+            allow_exit.set()
+            await user_returned.wait()
+
+    trio.run(main)
+
+
+def test_moc_delivers_resource_exit_error():
+    '''
+    Verify a resource exit error reaches the final MOC user.
+
+    Previously `_Cache.run_ctx()` executed the cached resource's
+    `__aexit__()` after the final user had returned. An exit failure
+    therefore surfaced later through the actor service nursery rather
+    than at the user's `async with maybe_open_context()` boundary.
+
+    This resource raises a unique `ResourceExitError` during exit.
+    Catching that exact instance around MOC proves the service task
+    delivered the failure to the final user without replacing it.
+
+    '''
+    class ResourceExitError(Exception):
+        pass
+
+    exit_error = ResourceExitError('resource exit failed')
+
+    async def main():
+        @acm
+        async def open_resource():
+            yield
+            raise exit_error
+
+        async with tractor.open_root_actor():
+            with pytest.raises(ResourceExitError) as exc_info:
+                async with maybe_open_context(open_resource):
+                    pass
+
+            assert exc_info.value is exit_error
+
+    trio.run(main)
+
+
+def test_moc_final_user_cancellation_waits_for_exit():
+    '''
+    Verify final-user cancellation still waits for successful exit.
+
+    Previously cancellation escaped the final MOC user immediately
+    after it signalled `_Cache.run_ctx()`, leaving resource exit to
+    finish later in the actor service task. This violated the context
+    manager boundary even when cleanup itself succeeded.
+
+    The consumer cancels its own scope while holding the sole cached
+    resource. The resource sets `exit_finished` from its `finally`
+    block, and the consumer checks that event immediately after its
+    cancel scope catches `trio.Cancelled`. This proves MOC's
+    completion wait is shielded without suppressing the original
+    cancellation.
+
+    '''
+    async def main():
+        exit_finished = trio.Event()
+
+        @acm
+        async def open_resource():
+            try:
+                yield
+            finally:
+                exit_finished.set()
+
+        async with tractor.open_root_actor():
+            with trio.CancelScope() as cs:
+                async with maybe_open_context(open_resource):
+                    cs.cancel()
+                    await trio.sleep_forever()
+
+            assert cs.cancelled_caught
+            assert exit_finished.is_set()
+
+    trio.run(main)
+
+
+def test_moc_exit_error_masks_final_user_cancellation():
+    '''
+    Verify cleanup errors survive final-user cancellation.
+
+    A cancelled final user previously signalled `no_more_users` and
+    propagated `trio.Cancelled` before `_Cache.run_ctx()` completed
+    resource exit. If `__aexit__()` then failed, its error was
+    detached from the API call which caused teardown.
+
+    The consumer cancels its own scope at a deterministic checkpoint
+    inside MOC. Resource exit raises `ResourceExitError`; observing
+    that exact error outside the cancel scope proves MOC shields the
+    completion wait and applies normal context-manager masking, where
+    a cleanup failure replaces the active cancellation.
+
+    '''
+    class ResourceExitError(Exception):
+        pass
+
+    exit_error = ResourceExitError('resource exit failed')
+
+    async def main():
+        @acm
+        async def open_resource():
+            yield
+            raise exit_error
+
+        async with tractor.open_root_actor():
+            with pytest.raises(ResourceExitError) as exc_info:
+                with trio.CancelScope() as cs:
+                    async with maybe_open_context(open_resource):
+                        cs.cancel()
+                        await trio.sleep_forever()
+
+            assert exc_info.value is exit_error
+
+    trio.run(main)
+
+
+def test_moc_service_nursery_cancellation_completes_exit():
+    '''
+    Verify service-nursery cancellation cannot strand a final user.
+
+    `_Cache.run_ctx()` and an MOC consumer may share a
+    caller-provided service nursery. Cancelling that nursery
+    interrupts the service task's `no_more_users` wait and the
+    consumer body together. A shielded final-user wait would deadlock
+    if `run_ctx()` failed to publish completion while propagating its
+    own `trio.Cancelled`.
+
+    The outer task waits for resource entry, then cancels the exact
+    nursery containing both tasks. The resource shields one cleanup
+    checkpoint and sets `exit_finished`; observing both it and
+    `service_finished` proves cancellation propagated normally while
+    MOC's completion handshake terminated deterministically.
+
+    '''
+    async def main():
+        resource_entered = trio.Event()
+        exit_finished = trio.Event()
+        service_finished = trio.Event()
+        service_tn: trio.Nursery|None = None
+
+        @acm
+        async def open_resource():
+            try:
+                resource_entered.set()
+                yield
+            finally:
+                with trio.CancelScope(shield=True):
+                    await trio.lowlevel.checkpoint()
+                    exit_finished.set()
+
+        async def use_resource(tn: trio.Nursery):
+            async with maybe_open_context(
+                open_resource,
+                tn=tn,
+            ):
+                await trio.sleep_forever()
+
+        async def run_service():
+            nonlocal service_tn
+
+            async with trio.open_nursery() as tn:
+                service_tn = tn
+                tn.start_soon(use_resource, tn)
+
+            service_finished.set()
+
+        async with trio.open_nursery() as outer_tn:
+            outer_tn.start_soon(run_service)
+            await resource_entered.wait()
+            assert service_tn is not None
+            service_tn.cancel_scope.cancel()
+            await service_finished.wait()
+
+        assert exit_finished.is_set()
+
+    trio.run(main)
+
+
 @tractor.context
 async def streamer(
     ctx: tractor.Context,
@@ -548,43 +775,52 @@ def test_moc_reentry_during_teardown(
     loglevel: str,
 ):
     '''
-    Reproduce the piker `open_cached_client('kraken')` race:
+    Reproduce re-entry while an identical cached context exits.
 
-    - same `acm_func`, NO kwargs (identical `ctx_key`)
+    - multiple tasks use the same `acm_func` with no kwargs,
-    - multiple tasks share the cached resource
+      producing an identical `ctx_key`;
-    - all users exit -> teardown starts
+    - all users leave and the final user starts resource teardown;
-    - a NEW task enters during `_Cache.run_ctx.__aexit__`
+    - `_Cache.run_ctx()` removes the cached value and resource entry
-    - `values[ctx_key]` is gone (popped in inner finally)
+      before entering the resource's blocking `__aexit__()` body;
-      but `resources[ctx_key]` still exists (outer finally
+    - a new task attempts to enter that same `ctx_key` during exit;
-      hasn't run yet bc the acm cleanup has checkpoints)
+    - the per-key lock keeps that entrant queued until exit
-    - old code: `assert not resources.get(ctx_key)` FIRES
+      completes;
+    - the entrant then receives a fresh cache miss and resource.
 
-    This models the real-world scenario where `brokerd.kraken`
+    Without teardown sharing the registration lock, re-entry could
-    tasks concurrently call `open_cached_client('kraken')`
+    race resource replacement while the prior generation was still
-    (same `acm_func`, empty kwargs, shared `ctx_key`) and
+    exiting. The final user could also return before that exit
-    the teardown/re-entry race triggers intermittently.
+    completed.
+
+    The first resource generation signals `in_aexit` and waits on
+    `allow_aexit`. The re-entry task signals `reentry_started` and
+    blocks inside MOC; only after `wait_all_tasks_blocked()` confirms
+    that ordering does the coordinator release cleanup. The entrant
+    must then receive a fresh cache miss. `first_done` additionally
+    proves the first MOC user observed completed teardown before
+    returning.
 
     '''
     async def main():
         in_aexit = trio.Event()
+        allow_aexit = trio.Event()
+        reentry_started = trio.Event()
+        generation: int = 0
 
         @acm
         async def cached_client():
             '''
-            Simulates `kraken.api.get_client()`:
+            Simulate a no-argument `kraken.api.get_client()`.
-            - no params (all callers share one `ctx_key`)
-            - slow-ish cleanup to widen the race window
-              between `values.pop()` and `resources.pop()`
-              inside `_Cache.run_ctx`.
 
             '''
+            nonlocal generation
+
+            generation += 1
+            resource_generation: int = generation
             yield 'the-client'
-            # Signal that we're in __aexit__ — at this
+            if resource_generation == 1:
-            # point `values` has already been popped by
-            # `run_ctx`'s inner finally, but `resources`
-            # is still alive (outer finally hasn't run).
                 in_aexit.set()
-            await trio.sleep(10)
+                await allow_aexit.wait()
 
         first_done = trio.Event()
 
@@ -598,16 +834,25 @@ def test_moc_reentry_during_teardown(
         async def reenter_during_teardown():
             '''
             Wait for the acm's `__aexit__` to start (meaning
-            `values` is popped but `resources` still exists),
+            the cached value is no longer available), then re-enter.
-            then re-enter — triggering the assert.
 
             '''
             await in_aexit.wait()
+
+            # Tell the coordinator this task is about to enter MOC.
+            # `Event.set()` is not a checkpoint. Though `async with`
+            # awaits MOC's `__aenter__()`, its async generator runs
+            # synchronously until the held per-key `lock.acquire()`
+            # actually suspends this task.
+            reentry_started.set()
             async with maybe_open_context(
                 cached_client,
             ) as (cache_hit, value):
+                assert not cache_hit
                 assert value == 'the-client'
 
+            await first_done.wait()
+
         with trio.fail_after(5):
             async with (
                 tractor.open_root_actor(
@@ -619,5 +864,15 @@ def test_moc_reentry_during_teardown(
             ):
                 tn.start_soon(use_and_exit)
                 tn.start_soon(reenter_during_teardown)
+                await reentry_started.wait()
+
+                # Wait until the re-entry task is queued on MOC's
+                # per-key lock while `_Cache.run_ctx()` remains
+                # blocked in the first generation's `__aexit__()`.
+                # Only then release cleanup, making the intended
+                # enter-during-sibling-exit ordering deterministic.
+                await wait_all_tasks_blocked()
+                assert not first_done.is_set()
+                allow_aexit.set()
 
     trio.run(main)

tests/trionics/test_taskc.py

@@ -0,0 +1,312 @@
+'''
+`tractor.trionics._taskc.start_or_cancel()` unit tests.
+
+`trio.Nursery.start()` collapses an out-of-band (ancestor)
+cancellation into a lossy,
+
+  `RuntimeError('child exited without calling
+  task_status.started()')`
+
+whenever the started child exits pre-`.started()` WITHOUT
+propagating the ambient `trio.Cancelled`; a common outcome
+when the child (or any lib code it calls) runs a graceful
+teardown which absorbs the cancel and returns early. Our
+`start_or_cancel()` wrapper re-surfaces the real in-flight
+cancellation in that case so the true root error/cancel
+propagates to the `.start()` caller instead.
+
+These tests verify both that repair AND document upstream
+`trio`'s current lossy behaviour via the
+`use_start_or_cancel=False` parametrizations; if a `trio`
+upgrade breaks one of THOSE cases it likely means upstream
+shipped better startup-cancellation porcelain and our
+wrapper deserves a re-audit!
+
+The core use case was dug out of `modden`'s
+`progman.open_wks()` program-spawn machinery as per gh
+issue #474; the wrapper landed originally via gh PR #464.
+
+'''
+import pytest
+import trio
+from trio import TaskStatus
+
+from tractor.trionics import start_or_cancel
+
+
+async def absorbs_cancel_pre_started(
+    task_status: TaskStatus[None] = trio.TASK_STATUS_IGNORED,
+):
+    '''
+    Swallow the ambient (ancestor-scope) cancel and return
+    early, a naughty-but-realistic graceful-teardown pattern
+    and the exact shape which causes `trio.Nursery.start()`
+    to raise its lossy startup `RuntimeError` in place of
+    the real `trio.Cancelled`.
+
+    '''
+    try:
+        await trio.sleep(2)
+    except trio.Cancelled:
+        return
+    task_status.started()
+
+
+async def raise_val_err():
+    '''
+    Sibling task which blows up (fast) thus OOB-cancelling
+    the shared parent-nursery's cancel-scope.
+
+    '''
+    await trio.lowlevel.checkpoint()
+    raise ValueError('sibling blew up!')
+
+
+@pytest.mark.parametrize(
+    'use_start_or_cancel',
+    [
+        True,
+        False,
+    ],
+)
+def test_sibling_err_not_masked_by_startup_rte(
+    use_start_or_cancel: bool,
+):
+    '''
+    The `modden.runtime.progman` use case: a sibling task
+    errors while the `.start()`-ed child is still
+    pre-`.started()`, OOB-cancelling the shared nursery
+    scope; the child absorbs its cancel (graceful teardown)
+    and exits early.
+
+    - with `start_or_cancel()` the in-flight cancellation
+      is re-surfaced as the real `trio.Cancelled` (then
+      absorbed by the cancelled nursery scope) so ONLY the
+      root-cause sibling error escapes the nursery.
+
+    - with a bare `.start()`, upstream `trio` (currently)
+      also delivers its lossy startup `RuntimeError`
+      alongside, obscuring that the child was in fact
+      cancelled due to the sibling's error.
+
+    '''
+    async def main():
+        async with trio.open_nursery() as tn:
+            tn.start_soon(raise_val_err)
+            if use_start_or_cancel:
+                await start_or_cancel(
+                    tn,
+                    absorbs_cancel_pre_started,
+                )
+            else:
+                await tn.start(absorbs_cancel_pre_started)
+
+    with pytest.raises(ExceptionGroup) as excinfo:
+        trio.run(main)
+
+    eg: ExceptionGroup = excinfo.value
+    val_eg, rest_eg = eg.split(ValueError)
+    assert len(val_eg.exceptions) == 1
+
+    if use_start_or_cancel:
+        # the re-surfaced `Cancelled` is absorbed by the
+        # (sibling-error cancelled) nursery scope leaving
+        # NO startup-noise, just the root cause.
+        assert rest_eg is None
+    else:
+        # the `trio` wart: a lossy startup RTE rides along
+        # with (and distracts from) the root cause.
+        rte = rest_eg.exceptions[0]
+        assert isinstance(rte, RuntimeError)
+        assert 'child exited without calling' in rte.args[0]
+
+
+@pytest.mark.parametrize(
+    'use_start_or_cancel',
+    [
+        True,
+        False,
+    ],
+)
+def test_pure_oob_cancel_not_morphed_to_rte(
+    use_start_or_cancel: bool,
+):
+    '''
+    A plain (error-free) ancestor `CancelScope.cancel()`
+    fired while the (cancel-absorbing) child is still
+    pre-`.started()`:
+
+    - `start_or_cancel()` re-surfaces the `Cancelled` so
+      the cancelled scope exits CLEAN, no error at all.
+
+    - a bare `.start()` (currently) morphs the plain
+      cancel into an (eg-wrapped) startup `RuntimeError`.
+
+    '''
+    async def main():
+        with trio.CancelScope() as cs:
+            async with trio.open_nursery() as tn:
+
+                async def canceller():
+                    await trio.lowlevel.checkpoint()
+                    cs.cancel()
+
+                tn.start_soon(canceller)
+                if use_start_or_cancel:
+                    await start_or_cancel(
+                        tn,
+                        absorbs_cancel_pre_started,
+                    )
+                else:
+                    await tn.start(
+                        absorbs_cancel_pre_started,
+                    )
+
+        assert cs.cancelled_caught
+
+    if use_start_or_cancel:
+        trio.run(main)
+    else:
+        with pytest.raises(ExceptionGroup) as excinfo:
+            trio.run(main)
+
+        rte = excinfo.value.exceptions[0]
+        assert isinstance(rte, RuntimeError)
+        assert 'child exited without calling' in rte.args[0]
+
+
+@pytest.mark.parametrize(
+    'use_start_or_cancel',
+    [
+        True,
+        False,
+    ],
+)
+def test_genuine_startup_rte_still_raised(
+    use_start_or_cancel: bool,
+):
+    '''
+    Absent ANY in-flight cancellation, a child exiting
+    cleanly without calling `task_status.started()` is a
+    genuine startup-protocol bug; `start_or_cancel()` must
+    re-raise the resulting `RuntimeError` exactly like a
+    bare `.start()` does.
+
+    '''
+    async def exits_wo_started(
+        task_status: TaskStatus[None] = (
+            trio.TASK_STATUS_IGNORED
+        ),
+    ):
+        await trio.lowlevel.checkpoint()
+
+    async def main():
+        async with trio.open_nursery() as tn:
+            with pytest.raises(RuntimeError) as excinfo:
+                if use_start_or_cancel:
+                    await start_or_cancel(
+                        tn,
+                        exits_wo_started,
+                    )
+                else:
+                    await tn.start(exits_wo_started)
+
+            rte = excinfo.value
+            assert (
+                'child exited without calling'
+                in
+                rte.args[0]
+            )
+
+    trio.run(main)
+
+
+@pytest.mark.parametrize(
+    'rte_arg',
+    [
+        # a bare `'started' in args[0]` substring match
+        # would (wrongly) demote this one to a `Cancelled`
+        # under ambient cancellation.
+        'never got started!',
+        # non-`str` first-arg edge; must not `TypeError`
+        # inside the wrapper's msg-match guard.
+        1234,
+    ],
+)
+def test_childs_own_rte_never_demoted_to_cancel(
+    rte_arg: str|int,
+):
+    '''
+    A child's OWN `RuntimeError`, one which merely smells
+    like `trio`'s startup wording (or carries a non-`str`
+    first arg), raised under ambient cancellation must NOT
+    be demoted to a `trio.Cancelled` by the exact-msg-match
+    guard inside `start_or_cancel()`; the real error must
+    always propagate to the caller unchanged.
+
+    '''
+    async def cancels_cs_then_raises(
+        task_status: TaskStatus[None] = (
+            trio.TASK_STATUS_IGNORED
+        ),
+    ):
+        # cancel the ambient (ancestor) scope then raise
+        # sync-ly, no checkpoint between, so the child
+        # deterministically dies with ITS error while the
+        # caller is under effective cancellation.
+        cs.cancel()
+        raise RuntimeError(rte_arg)
+
+    cs = trio.CancelScope()
+
+    async def main():
+        with cs:
+            async with trio.open_nursery() as tn:
+                await start_or_cancel(
+                    tn,
+                    cancels_cs_then_raises,
+                )
+
+    with pytest.raises(ExceptionGroup) as excinfo:
+        trio.run(main)
+
+    rte = excinfo.value.exceptions[0]
+    assert isinstance(rte, RuntimeError)
+    assert rte.args[0] == rte_arg
+
+
+def test_started_value_and_args_passthru():
+    '''
+    Happy path: positional args, the `name=` kwarg and the
+    `.started(value)`-delivered value all pass through
+    `start_or_cancel()` identically to a bare `.start()`.
+
+    '''
+    async def echo_started(
+        *args,
+        task_status: TaskStatus[tuple] = (
+            trio.TASK_STATUS_IGNORED
+        ),
+    ):
+        task_name: str = trio.lowlevel.current_task().name
+        task_status.started((
+            args,
+            task_name,
+        ))
+
+    async def main():
+        async with trio.open_nursery() as tn:
+            (
+                args,
+                task_name,
+            ) = await start_or_cancel(
+                tn,
+                echo_started,
+                'chillin',
+                10,
+                name='doggy',
+            )
+            assert args == ('chillin', 10)
+            assert task_name == 'doggy'
+
+    trio.run(main)

tractor/ipc/_chan.py

@@ -198,9 +198,6 @@ class Channel:
         # assert transport.raddr == addr
         chan = Channel(transport=transport)
 
-        # ?TODO, compact this into adapter level-methods?
-        # -[ ] would avoid extra repr-calcs if level not active?
-        #   |_ how would the `calc_if_level` look though? func?
         if log.at_least_level('runtime'):
             from tractor.devx import (
                 pformat as _pformat,
@@ -325,6 +322,8 @@ class Channel:
         '''
         __tracebackhide__: bool = hide_tb
         try:
+            if log.at_least_level('transport'):
+                # don't materialize the payload repr if not necessary
                 log.transport(
                     '=> send IPC msg:\n\n'
                     f'{pformat(payload)}\n'

tractor/ipc/_transport.py

@@ -309,7 +309,10 @@ class MsgpackTransport(MsgTransport):
             log.transport(f'received header {size}')  # type: ignore
             msg_bytes: bytes = await self.recv_stream.receive_exactly(size)
 
-            log.transport(f"received {msg_bytes}")  # type: ignore
+            if log.at_least_level('transport'):
+                log.transport(  # type: ignore
+                    f'received {msg_bytes}'
+                )
             try:
                 # NOTE: lookup the `trio.Task.context`'s var for
                 # the current `MsgCodec`.

tractor/log.py

@@ -111,9 +111,7 @@ def at_least_level(
     if isinstance(level, str):
         level: int = CUSTOM_LEVELS[level.upper()]
 
-    if log.getEffectiveLevel() <= level:
+    return log.isEnabledFor(level)
-        return True
-    return False
 
 
 # TODO, compare with using a "filter" instead?

tractor/msg/_ops.py

@@ -306,12 +306,12 @@ class PldRx(Struct):
             ):
                 try:
                     pld: PayloadT = self._pld_dec.decode(pld)
+                    if log.at_least_level('runtime'):
+                        # don't materialize the payload repr if not necessary
                         log.runtime(
                             f'Decoded payload for\n'
-                        # f'\n'
+                            f'\n'
                             f'{msg}\n'
-                        # ^TODO?, ideally just render with `,
-                        # pld={decode}` in the `msg.pformat()`??
                             f'where, '
                             f'{type(msg).__name__}.pld={pld!r}\n'
                         )

tractor/runtime/_rpc.py

@@ -1003,17 +1003,15 @@ async def process_messages(
             task_status.started(loop_cs)
 
             async for msg in chan:
+                if log.at_least_level('transport'):
                     log.transport(   # type: ignore
                         f'IPC msg from peer\n'
                         f'<= {chan.aid.reprol()}\n\n'
 
-                    # TODO: use of the pprinting of structs is
+                        # TODO: pretty-printing structs is FRAGILE;
-                    # FRAGILE and should prolly not be
+                        # -[ ] add a non-raising log formatter with
-                    #
+                        #      native-repr fallback before using
-                    # avoid fmting depending on loglevel for perf?
+                        #      `.msg.pretty_struct` here.
-                    # -[ ] specifically `pretty_struct.pformat()` sub-call..?
-                    #   - how to only log-level-aware actually call this?
-                    # -[ ] use `.msg.pretty_struct` here now instead!
                         # f'{pretty_struct.pformat(msg)}\n'
                         f'{msg}\n'
                     )
@@ -1262,6 +1260,7 @@ async def process_messages(
                         log.exception(message)
                         raise RuntimeError(message)
 
+                if log.at_least_level('transport'):
                     log.transport(
                         'Waiting on next IPC msg from\n'
                         f'peer: {chan.aid.reprol()}\n'

tractor/trionics/_mngrs.py

@@ -198,6 +198,16 @@ async def gather_contexts(
 # Further potential examples of interest:
 # https://gist.github.com/njsmith/cf6fc0a97f53865f2c671659c88c1798#file-cache-py-L8
 
+class _CtxExit:
+    '''
+    Completion state for a cached context's shared exit.
+
+    '''
+    def __init__(self) -> None:
+        self.done = trio.Event()
+        self.error: Exception|None = None
+
+
 class _Cache:
     '''
     Globally (actor-processs scoped) cached, task access to
@@ -213,7 +223,11 @@ class _Cache:
     values: dict[Any,  Any] = {}
     resources: dict[
         Hashable,
-        tuple[trio.Nursery, trio.Event]
+        tuple[
+            trio.Nursery,
+            trio.Event,
+            _CtxExit,
+        ],
     ] = {}
     # nurseries: dict[int, trio.Nursery] = {}
     no_more_users: trio.Event|None = None
@@ -223,19 +237,39 @@ class _Cache:
         cls,
         mng,
         ctx_key: tuple,
+        ctx_exit: _CtxExit,
         task_status: trio.TaskStatus[T] = trio.TASK_STATUS_IGNORED,
 
     ) -> None:
+        entered: bool = False
+        try:
             async with mng as value:
-            _, no_more_users = cls.resources[ctx_key]
+                entered = True
+                (
+                    _,
+                    no_more_users,
+                    _,
+                ) = cls.resources[ctx_key]
                 cls.values[ctx_key] = value
                 task_status.started(value)
                 try:
                     await no_more_users.wait()
                 finally:
-                value = cls.values.pop(ctx_key)
+                    cls.values.pop(ctx_key)
                     cls.resources.pop(ctx_key)
 
+        except Exception as exc:
+            if not entered:
+                raise
+
+            # Deliver regular `__aexit__()` failures to the final
+            # consumer instead of raising into the service nursery.
+            ctx_exit.error = exc
+
+        finally:
+            if entered:
+                ctx_exit.done.set()
+
 
 class _UnresolvedCtx:
     '''
@@ -281,9 +315,10 @@ async def maybe_open_context(
         )
 
     # yielded output
-    # sentinel = object()
     yielded: Any = _UnresolvedCtx
     user_registered: bool = False
+    ctx_exit: _CtxExit|None = None
+    exit_error: Exception|None = None
 
     # Lock resource acquisition around task racing  / ``trio``'s
     # scheduler protocol.
@@ -300,7 +335,6 @@ async def maybe_open_context(
         ] = trio.StrictFIFOLock()
         header: str = 'Allocated NEW lock for @acm_func,\n'
     else:
-        await trio.lowlevel.checkpoint()
         header: str = 'Reusing OLD lock for @acm_func,\n'
 
     log.debug(
@@ -368,7 +402,6 @@ async def maybe_open_context(
             resources = _Cache.resources
             entry: tuple|None = resources.get(ctx_key)
             if entry:
-                service_tn, ev = entry
                 raise RuntimeError(
                     f'Caching resources ALREADY exist?!\n'
                     f'ctx_key={ctx_key!r}\n'
@@ -376,12 +409,18 @@ async def maybe_open_context(
                     f'task: {task}\n'
                 )
 
-            resources[ctx_key] = (service_tn, trio.Event())
+            ctx_exit = _CtxExit()
+            resources[ctx_key] = (
+                service_tn,
+                trio.Event(),
+                ctx_exit,
+            )
             try:
                 yielded: Any = await service_tn.start(
                     _Cache.run_ctx,
                     mngr,
                     ctx_key,
+                    ctx_exit,
                 )
             except BaseException:
                 # If `run_ctx` (wrapping the acm's `__aenter__`)
@@ -427,6 +466,11 @@ async def maybe_open_context(
             raise taskc
     else:
         # XXX, cached-entry-path
+        (
+            _,
+            _,
+            ctx_exit,
+        ) = _Cache.resources[ctx_key]
         _Cache.users[ctx_key] += 1
         user_registered = True
         log.debug(
@@ -445,24 +489,17 @@ async def maybe_open_context(
         )
 
     finally:
-        if lock.locked():
-            stats: trio.LockStatistics = lock.statistics()
-            owner: trio.Task|None = stats.owner
-            log.error(
-                f'Lock never released by last owner={owner!r} !?\n'
-                f'{stats}\n'
-                f'\n'
-                f'task={task!r}\n'
-                f'ctx_key={ctx_key!r}\n'
-                f'acm_func={acm_func}\n'
-
-            )
-
         if user_registered:
+            # Serialize user registration and teardown under the same
+            # per-key lock so no entrant can acquire a resource after
+            # its final user has committed to exiting it.
+            with trio.CancelScope(shield=True):
+                await lock.acquire()
+                try:
                     _Cache.users[ctx_key] -= 1
 
-        if yielded is not _UnresolvedCtx:
+                    # If no consumers remain, keep entrants queued
-            # if no more consumers, teardown the client
+                    # until the cached context has completely exited.
                     if _Cache.users[ctx_key] <= 0:
                         log.debug(
                             f'De-allocating @acm-func entry\n'
@@ -470,19 +507,39 @@ async def maybe_open_context(
                             f'acm_func={acm_func!r}\n'
                         )
 
-                # XXX: if we're cancelled we the entry may have never
+                        # XXX: if we're cancelled, the entry may
-                # been entered since the nursery task was killed.
+                        # have never been entered since the nursery
-                # _, no_more_users = _Cache.resources[ctx_key]
+                        # task was killed.
                         entry = _Cache.resources.get(ctx_key)
                         if entry:
-                    _, no_more_users = entry
+                            (
+                                _,
+                                no_more_users,
+                                ctx_exit,
+                            ) = entry
                             no_more_users.set()
 
-                maybe_lock = _Cache.locks.pop(
+                        assert ctx_exit is not None
-                    ctx_key,
+                        await ctx_exit.done.wait()
-                    None,
+                        exit_error = ctx_exit.error
-                )
+
-                if maybe_lock is None:
+                        # A queued entrant already holds a reference
+                        # to this lock. Keep it registered until that
+                        # task has acquired and released it.
+                        stats = lock.statistics()
+                        if not stats.tasks_waiting:
+                            maybe_lock = _Cache.locks.get(ctx_key)
+                            if maybe_lock is lock:
+                                _Cache.locks.pop(ctx_key)
+                            else:
                                 log.error(
-                        f'Resource lock for {ctx_key} ALREADY POPPED?'
+                                    f'Resource lock for {ctx_key} '
+                                    f'was replaced before teardown?'
                                 )
+                finally:
+                    lock.release()
+
+        if exit_error is not None:
+            # Always re-raise a regular `__aexit__()` error at the
+            # final consumer's context boundary.
+            raise exit_error