From 5d8336bc97b04fa72ecbb902d265835811234831 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Sun, 25 Jul 2021 17:45:25 -0400
Subject: [PATCH 1/7] create cache scope following task tree discipline

---
 pyproject.toml                    |  1 +
 requirements/install.in           |  5 +-
 trio_parallel/_impl.py            | 97 +++++++++++++++++++++++++------
 trio_parallel/_tests/test_impl.py | 52 +++++++++++++++++
 4 files changed, 135 insertions(+), 20 deletions(-)

diff --git a/pyproject.toml b/pyproject.toml
index 0ce7b807..3673a48a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -37,6 +37,7 @@ dependencies = [
     "attrs >= 17.3.0",
     "cffi; os_name == 'nt' and implementation_name != 'pypy'",
     "tblib",
+    "tricycle >= 0.3.0"
 ]
 requires-python = ">=3.7"
 dynamic = ["version"]
diff --git a/requirements/install.in b/requirements/install.in
index 734b6685..fd5e71c3 100644
--- a/requirements/install.in
+++ b/requirements/install.in
@@ -1,6 +1,7 @@
-# keep me in sync with setup.cfg!
+# keep me in sync with pyproject.toml!
 trio >= 0.18.0
 outcome
 attrs >= 17.3.0
 cffi; os_name == 'nt' and implementation_name != 'pypy'
-tblib
\ No newline at end of file
+tblib
+tricycle >= 0.3.0
diff --git a/trio_parallel/_impl.py b/trio_parallel/_impl.py
index d052f883..099f748a 100644
--- a/trio_parallel/_impl.py
+++ b/trio_parallel/_impl.py
@@ -9,6 +9,7 @@
 from typing import Type, Callable, Any, TypeVar
 
 import attr
+import tricycle
 import trio
 
 from ._proc import WORKER_PROC_MAP
@@ -270,42 +271,44 @@ def configure_default_context(
         DEFAULT_CONTEXT = ctx
 
 
+async def close_at_run_end(ctx):
+    try:
+        await trio.sleep_forever()
+    finally:
+        await ctx._aclose()  # noqa: ASYNC102
+
+
+CACHE_SCOPE_TREEVAR = tricycle.TreeVar("tp_cache_scope")
 if sys.platform == "win32":
     DEFAULT_CONTEXT_RUNVAR = trio.lowlevel.RunVar("win32_ctx")
     DEFAULT_CONTEXT_PARAMS = {}
 
-    # TODO: intelligently test ki protection here such that CI fails if the
-    #  decorators disappear
-
-    @trio.lowlevel.enable_ki_protection
     def get_default_context():
+        try:
+            return CACHE_SCOPE_TREEVAR.get()
+        except LookupError:
+            pass
         try:
             ctx = DEFAULT_CONTEXT_RUNVAR.get()
         except LookupError:
             ctx = WorkerContext._create(**DEFAULT_CONTEXT_PARAMS)
-            DEFAULT_CONTEXT_RUNVAR.set(ctx)
-            # KeyboardInterrupt here could leak the context
             trio.lowlevel.spawn_system_task(close_at_run_end, ctx)
+            # set ctx last so as not to leak on KeyboardInterrupt
+            DEFAULT_CONTEXT_RUNVAR.set(ctx)
         return ctx
 
-    @trio.lowlevel.enable_ki_protection
-    async def close_at_run_end(ctx):
-        try:
-            await trio.sleep_forever()
-        finally:
-            # KeyboardInterrupt here could leak the context
-            await ctx._aclose()  # noqa: ASYNC102
-
 else:
 
     def get_default_context():
-        return DEFAULT_CONTEXT
+        try:
+            return CACHE_SCOPE_TREEVAR.get()
+        except LookupError:
+            return DEFAULT_CONTEXT
 
-    def graceful_default_shutdown(ctx):
+    @atexit.register
+    def graceful_default_shutdown(ctx=DEFAULT_CONTEXT):
         ctx._worker_cache.shutdown(ctx.grace_period)
 
-    atexit.register(graceful_default_shutdown, DEFAULT_CONTEXT)
-
 
 def default_context_statistics():
     """Return the statistics corresponding to the default context.
@@ -377,6 +380,64 @@ async def open_worker_context(
         await ctx._aclose()  # noqa: ASYNC102
 
 
+@asynccontextmanager
+@trio.lowlevel.enable_ki_protection
+async def cache_scope(
+    idle_timeout=DEFAULT_CONTEXT.idle_timeout,
+    init=DEFAULT_CONTEXT.init,
+    retire=DEFAULT_CONTEXT.retire,
+    grace_period=DEFAULT_CONTEXT.grace_period,
+    worker_type=WorkerType.SPAWN,
+):
+    """
+    The context will automatically wait for any running workers to become idle when
+    exiting the scope. Since this wait cannot be cancelled, it is more convenient to
+    only pass the context object to tasks that cannot outlive the scope, for example,
+    by using a :class:`~trio.Nursery`.
+
+    Args:
+      idle_timeout (float): The time in seconds an idle worker will
+          wait for a CPU-bound job before shutting down and releasing its own
+          resources. Pass `math.inf` to wait forever. MUST be non-negative.
+      init (Callable[[], bool]):
+          An object to call within the worker before waiting for jobs.
+          This is suitable for initializing worker state so that such stateful logic
+          does not need to be included in functions passed to
+          :func:`WorkerContext.run_sync`. MUST be callable without arguments.
+      retire (Callable[[], bool]):
+          An object to call within the worker after executing a CPU-bound job.
+          The return value indicates whether worker should be retired (shut down.)
+          By default, workers are never retired.
+          The process-global environment is stable between calls. Among other things,
+          that means that storing state in global variables works.
+          MUST be callable without arguments.
+      grace_period (float): The time in seconds to wait in ``__aexit__`` for workers to
+          exit before issuing SIGKILL/TerminateProcess and raising `BrokenWorkerError`.
+          Pass `math.inf` to wait forever. MUST be non-negative.
+      worker_type (WorkerType): The kind of worker to create, see :class:`WorkerType`.
+
+    Raises:
+      ValueError | TypeError: if an invalid value is passed for an argument, such as a
+          negative timeout.
+      BrokenWorkerError: if a worker does not shut down cleanly when exiting the scope.
+
+    .. warning::
+
+       The callables passed to retire MUST not raise! Doing so will result in a
+       :class:`BrokenWorkerError` at an indeterminate future
+       :func:`WorkerContext.run_sync` call.
+    """
+
+    ctx = WorkerContext._create(idle_timeout, init, retire, grace_period, worker_type)
+
+    try:
+        token = CACHE_SCOPE_TREEVAR.set(ctx)
+        yield
+    finally:
+        CACHE_SCOPE_TREEVAR.reset(token)
+        await ctx._aclose()  # noqa: ASYNC102
+
+
 async def run_sync(
     sync_fn: Callable[..., T],
     *args,
diff --git a/trio_parallel/_tests/test_impl.py b/trio_parallel/_tests/test_impl.py
index 691049c7..74373b93 100644
--- a/trio_parallel/_tests/test_impl.py
+++ b/trio_parallel/_tests/test_impl.py
@@ -1,5 +1,6 @@
 """ Tests of public API with mocked-out workers ("collaboration" tests)"""
 
+import os
 import sys
 from typing import Callable, Optional
 
@@ -185,3 +186,54 @@ async def test_worker_returning_none_can_be_cancelled():
 def test_cannot_instantiate_WorkerContext():
     with pytest.raises(TypeError):
         _impl.WorkerContext()
+
+
+async def _assert_worker_pid(pid, matches):
+    comparison = pid == await run_sync(os.getpid)
+    assert comparison == matches
+
+
+async def test_cache_scope_overrides_run_sync():
+    pid = await run_sync(os.getpid)
+
+    async with _impl.cache_scope():
+        await _assert_worker_pid(pid, False)
+
+
+async def test_cache_scope_overrides_nursery_task():
+    pid = await run_sync(os.getpid)
+
+    async def check_both_sides_of_task_status_started(pid, task_status):
+        await _assert_worker_pid(pid, True)
+        task_status.started()
+        await _assert_worker_pid(pid, True)
+
+    async with trio.open_nursery() as nursery:
+        async with _impl.cache_scope():
+            nursery.start_soon(_assert_worker_pid, pid, True)
+
+    async with trio.open_nursery() as nursery:
+        async with _impl.cache_scope():
+            await nursery.start(check_both_sides_of_task_status_started, pid)
+
+
+async def test_cache_scope_follows_task_tree_discipline():
+    shared_nursery: Optional[trio.Nursery] = None
+
+    async def make_a_cache_scope_around_nursery(task_status):
+        nonlocal shared_nursery
+        async with _impl.cache_scope(), trio.open_nursery() as shared_nursery:
+            await _assert_worker_pid(pid, False)
+            task_status.started()
+            await e.wait()
+
+    async def assert_elsewhere_in_task_tree():
+        await _assert_worker_pid(pid, False)
+        e.set()
+
+    pid = await run_sync(os.getpid)
+    e = trio.Event()
+    async with trio.open_nursery() as nursery:
+        await nursery.start(make_a_cache_scope_around_nursery)
+        # this line tests the main difference from contextvars vs treevars
+        shared_nursery.start_soon(assert_elsewhere_in_task_tree)

From 62efb349b27d437faaca61fabbfb1dc4d1d4967e Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Tue, 24 Dec 2024 21:22:36 -0500
Subject: [PATCH 2/7] bump ci deps

---
 requirements/coverage.txt |  2 +-
 requirements/docs.txt     | 14 +++++++-------
 requirements/install.txt  | 10 +++++++---
 requirements/lint.txt     |  8 ++++----
 requirements/test.txt     |  8 ++++----
 5 files changed, 23 insertions(+), 19 deletions(-)

diff --git a/requirements/coverage.txt b/requirements/coverage.txt
index d361c9d3..2a0f418b 100644
--- a/requirements/coverage.txt
+++ b/requirements/coverage.txt
@@ -5,5 +5,5 @@
 #
 #    pip-compile-multi
 #
-coverage[toml]==7.6.3
+coverage[toml]==7.6.9
     # via -r requirements\coverage.in
diff --git a/requirements/docs.txt b/requirements/docs.txt
index 4d33e1d8..58140558 100644
--- a/requirements/docs.txt
+++ b/requirements/docs.txt
@@ -10,11 +10,11 @@ alabaster==1.0.0
     # via sphinx
 babel==2.16.0
     # via sphinx
-certifi==2024.8.30
+certifi==2024.12.14
     # via requests
-charset-normalizer==3.4.0
+charset-normalizer==3.4.1
     # via requests
-click==8.1.7
+click==8.1.8
     # via towncrier
 colorama==0.4.6
     # via
@@ -26,13 +26,13 @@ docutils==0.21.2
     #   sphinx-rtd-theme
 imagesize==1.4.1
     # via sphinx
-jinja2==3.1.4
+jinja2==3.1.5
     # via
     #   sphinx
     #   towncrier
 markupsafe==3.0.2
     # via jinja2
-packaging==24.1
+packaging==24.2
     # via sphinx
 pygments==2.18.0
     # via sphinx
@@ -46,7 +46,7 @@ sphinx==8.1.3
     #   sphinx-rtd-theme
     #   sphinxcontrib-jquery
     #   sphinxcontrib-trio
-sphinx-rtd-theme==3.0.1
+sphinx-rtd-theme==3.0.2
     # via -r requirements\docs.in
 sphinxcontrib-applehelp==2.0.0
     # via sphinx
@@ -66,5 +66,5 @@ sphinxcontrib-trio==1.1.2
     # via -r requirements\docs.in
 towncrier==24.8.0
     # via -r requirements\docs.in
-urllib3==2.2.3
+urllib3==2.3.0
     # via requests
diff --git a/requirements/install.txt b/requirements/install.txt
index 8ffed149..757ee7c2 100644
--- a/requirements/install.txt
+++ b/requirements/install.txt
@@ -1,11 +1,11 @@
-# SHA1:63e73656a123a857e40bd1da6cce5906958410ed
+# SHA1:ab818e2cd7a4dab60404b5f2cc3e40669cdb1c52
 #
 # This file is autogenerated by pip-compile-multi
 # To update, run:
 #
 #    pip-compile-multi
 #
-attrs==24.2.0
+attrs==24.3.0
     # via
     #   -r requirements\install.in
     #   outcome
@@ -28,5 +28,9 @@ sortedcontainers==2.4.0
     # via trio
 tblib==3.0.0
     # via -r requirements\install.in
-trio==0.27.0
+tricycle==0.4.1
     # via -r requirements\install.in
+trio==0.27.0
+    # via
+    #   -r requirements\install.in
+    #   tricycle
diff --git a/requirements/lint.txt b/requirements/lint.txt
index 7a6cb3b1..b2f31306 100644
--- a/requirements/lint.txt
+++ b/requirements/lint.txt
@@ -7,7 +7,7 @@
 #
 black==24.10.0
     # via -r requirements\lint.in
-click==8.1.7
+click==8.1.8
     # via black
 colorama==0.4.6
     # via click
@@ -15,15 +15,15 @@ flake8==7.1.1
     # via
     #   -r requirements\lint.in
     #   flake8-async
-flake8-async==24.9.5
+flake8-async==24.11.4
     # via -r requirements\lint.in
-libcst==1.5.0
+libcst==1.5.1
     # via flake8-async
 mccabe==0.7.0
     # via flake8
 mypy-extensions==1.0.0
     # via black
-packaging==24.1
+packaging==24.2
     # via black
 pathspec==0.12.1
     # via black
diff --git a/requirements/test.txt b/requirements/test.txt
index da91ad80..2b9655e5 100644
--- a/requirements/test.txt
+++ b/requirements/test.txt
@@ -8,7 +8,7 @@
 -r install.txt
 colorama==0.4.6
     # via pytest
-coverage[toml]==7.6.3
+coverage[toml]==7.6.9
     # via
     #   -r requirements\test.in
     #   pytest-cov
@@ -16,17 +16,17 @@ execnet==2.1.1
     # via pytest-xdist
 iniconfig==2.0.0
     # via pytest
-packaging==24.1
+packaging==24.2
     # via pytest
 pluggy==1.5.0
     # via pytest
-pytest==8.3.3
+pytest==8.3.4
     # via
     #   -r requirements\test.in
     #   pytest-cov
     #   pytest-trio
     #   pytest-xdist
-pytest-cov==5.0.0
+pytest-cov==6.0.0
     # via -r requirements\test.in
 pytest-trio==0.8.0
     # via -r requirements\test.in

From 83d4f0d3207554b303048ff98fc67ead46352df7 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Tue, 24 Dec 2024 21:27:23 -0500
Subject: [PATCH 3/7] bugfix posix

---
 trio_parallel/_impl.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/trio_parallel/_impl.py b/trio_parallel/_impl.py
index 099f748a..59187f61 100644
--- a/trio_parallel/_impl.py
+++ b/trio_parallel/_impl.py
@@ -302,7 +302,7 @@ def get_default_context():
     def get_default_context():
         try:
             return CACHE_SCOPE_TREEVAR.get()
-        except LookupError:
+        except (LookupError, RuntimeError):
             return DEFAULT_CONTEXT
 
     @atexit.register

From 18569b82ecce25d8305be9d701a233442c5499d5 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Wed, 25 Dec 2024 11:55:41 -0500
Subject: [PATCH 4/7] documentation

---
 docs/source/examples/single_use_workers.py |  4 ++--
 docs/source/reference.rst                  | 17 ++++++++++++++---
 trio_parallel/__init__.py                  |  1 +
 trio_parallel/_impl.py                     |  2 ++
 4 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/docs/source/examples/single_use_workers.py b/docs/source/examples/single_use_workers.py
index 6b8d53c6..5a379073 100644
--- a/docs/source/examples/single_use_workers.py
+++ b/docs/source/examples/single_use_workers.py
@@ -31,10 +31,10 @@ async def amain():
                 nursery.start_soon(ctx.run_sync, worker, i)
 
     print("dual use worker behavior:")
-    async with trio_parallel.open_worker_context(retire=after_dual_use) as ctx:
+    async with trio_parallel.cache_scope(retire=after_dual_use):
         async with trio.open_nursery() as nursery:
             for i in range(10):
-                nursery.start_soon(ctx.run_sync, worker, i)
+                nursery.start_soon(trio_parallel.run_sync, worker, i)
 
     print("default behavior:")
     async with trio.open_nursery() as nursery:
diff --git a/docs/source/reference.rst b/docs/source/reference.rst
index 11daedb5..e9f4e900 100644
--- a/docs/source/reference.rst
+++ b/docs/source/reference.rst
@@ -138,6 +138,15 @@ lifetime is required in a subset of your application.
 .. autoclass:: WorkerContext()
    :members:
 
+Alternatively, you can implicitly override the default context of :func:`run_sync`
+in any subset of the task tree using `cache_scope()`. This async context manager
+sets an internal TreeVar_ so that the current task and all nested subtasks operate
+using an internal, isolated `WorkerContext`, without having to manually pass a
+context object around.
+
+.. autofunction:: cache_scope
+   :async-with: ctx
+
 One typical use case for configuring workers is to set a policy for taking a worker
 out of service. For this, use the ``retire`` argument. This example shows how to
 build (trivial) stateless and stateful worker retirement policies.
@@ -145,11 +154,11 @@ build (trivial) stateless and stateful worker retirement policies.
 .. literalinclude:: examples/single_use_workers.py
 
 A more realistic use-case might examine the worker process's memory usage (e.g. with
-`psutil <https://psutil.readthedocs.io/en/latest/>`_) and retire if usage is too high.
+psutil_) and retire if usage is too high.
 
 If you are retiring workers frequently, like in the single-use case, a large amount
-of process startup overhead will be incurred with the default worker type. If your
-platform supports it, an alternate `WorkerType` might cut that overhead down.
+of process startup overhead will be incurred with the default "spawn" worker type.
+If your platform supports it, an alternate `WorkerType` might cut that overhead down.
 
 .. autoclass:: WorkerType()
 
@@ -161,4 +170,6 @@ You probably won't use these... but create an issue if you do and need help!
 .. autofunction:: default_context_statistics
 
 .. _cloudpickle: https://github.com/cloudpipe/cloudpickle
+.. _psutil: https://psutil.readthedocs.io/en/latest/
 .. _service: https://github.com/richardsheridan/trio-parallel/issues/348
+.. _TreeVar: https://tricycle.readthedocs.io/en/latest/reference.html#tricycle.TreeVar
diff --git a/trio_parallel/__init__.py b/trio_parallel/__init__.py
index 9b437753..a4fd4403 100644
--- a/trio_parallel/__init__.py
+++ b/trio_parallel/__init__.py
@@ -3,6 +3,7 @@
 from ._impl import (
     run_sync,
     open_worker_context,
+    cache_scope,
     WorkerContext,
     WorkerType,
     current_default_worker_limiter,
diff --git a/trio_parallel/_impl.py b/trio_parallel/_impl.py
index 59187f61..6cb349b8 100644
--- a/trio_parallel/_impl.py
+++ b/trio_parallel/_impl.py
@@ -390,6 +390,8 @@ async def cache_scope(
     worker_type=WorkerType.SPAWN,
 ):
     """
+    Override the configuration of `trio_parallel.run_sync()` in this and all subtasks.
+
     The context will automatically wait for any running workers to become idle when
     exiting the scope. Since this wait cannot be cancelled, it is more convenient to
     only pass the context object to tasks that cannot outlive the scope, for example,

From fbc5ab3eb6c58c016bad3815dc17923fdfa97403 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Wed, 25 Dec 2024 11:59:15 -0500
Subject: [PATCH 5/7] nit

---
 trio_parallel/_impl.py | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/trio_parallel/_impl.py b/trio_parallel/_impl.py
index 6cb349b8..59ce1a9f 100644
--- a/trio_parallel/_impl.py
+++ b/trio_parallel/_impl.py
@@ -271,16 +271,10 @@ def configure_default_context(
         DEFAULT_CONTEXT = ctx
 
 
-async def close_at_run_end(ctx):
-    try:
-        await trio.sleep_forever()
-    finally:
-        await ctx._aclose()  # noqa: ASYNC102
-
-
 CACHE_SCOPE_TREEVAR = tricycle.TreeVar("tp_cache_scope")
+
 if sys.platform == "win32":
-    DEFAULT_CONTEXT_RUNVAR = trio.lowlevel.RunVar("win32_ctx")
+    DEFAULT_CONTEXT_RUNVAR = trio.lowlevel.RunVar("tp_win32_ctx")
     DEFAULT_CONTEXT_PARAMS = {}
 
     def get_default_context():
@@ -297,6 +291,12 @@ def get_default_context():
             DEFAULT_CONTEXT_RUNVAR.set(ctx)
         return ctx
 
+    async def close_at_run_end(ctx):
+        try:
+            await trio.sleep_forever()
+        finally:
+            await ctx._aclose()  # noqa: ASYNC102
+
 else:
 
     def get_default_context():
@@ -390,7 +390,8 @@ async def cache_scope(
     worker_type=WorkerType.SPAWN,
 ):
     """
-    Override the configuration of `trio_parallel.run_sync()` in this and all subtasks.
+    Override the configuration of `trio_parallel.run_sync()` in this task and all
+    subtasks.
 
     The context will automatically wait for any running workers to become idle when
     exiting the scope. Since this wait cannot be cancelled, it is more convenient to

From a812d847698795ce5771376083dacec9343ece54 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Wed, 25 Dec 2024 12:27:26 -0500
Subject: [PATCH 6/7] document interactions, test them too

---
 docs/source/reference.rst         |  2 +-
 trio_parallel/_impl.py            |  7 +++----
 trio_parallel/_tests/test_impl.py | 17 +++++++++++++++++
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/docs/source/reference.rst b/docs/source/reference.rst
index e9f4e900..5f3516b0 100644
--- a/docs/source/reference.rst
+++ b/docs/source/reference.rst
@@ -145,7 +145,7 @@ using an internal, isolated `WorkerContext`, without having to manually pass a
 context object around.
 
 .. autofunction:: cache_scope
-   :async-with: ctx
+   :async-with:
 
 One typical use case for configuring workers is to set a policy for taking a worker
 out of service. For this, use the ``retire`` argument. This example shows how to
diff --git a/trio_parallel/_impl.py b/trio_parallel/_impl.py
index 59ce1a9f..b4f46c0f 100644
--- a/trio_parallel/_impl.py
+++ b/trio_parallel/_impl.py
@@ -393,10 +393,9 @@ async def cache_scope(
     Override the configuration of `trio_parallel.run_sync()` in this task and all
     subtasks.
 
-    The context will automatically wait for any running workers to become idle when
-    exiting the scope. Since this wait cannot be cancelled, it is more convenient to
-    only pass the context object to tasks that cannot outlive the scope, for example,
-    by using a :class:`~trio.Nursery`.
+    The internal `WorkerContext` is passed implicitly down the task tree and can
+    be overridden by nested scopes. Explicit `WorkerContext` objects from
+    `open_worker_context` will not be overridden.
 
     Args:
       idle_timeout (float): The time in seconds an idle worker will
diff --git a/trio_parallel/_tests/test_impl.py b/trio_parallel/_tests/test_impl.py
index 74373b93..c0bf2b6b 100644
--- a/trio_parallel/_tests/test_impl.py
+++ b/trio_parallel/_tests/test_impl.py
@@ -226,6 +226,7 @@ async def make_a_cache_scope_around_nursery(task_status):
             await _assert_worker_pid(pid, False)
             task_status.started()
             await e.wait()
+        await _assert_worker_pid(pid, True)
 
     async def assert_elsewhere_in_task_tree():
         await _assert_worker_pid(pid, False)
@@ -237,3 +238,19 @@ async def assert_elsewhere_in_task_tree():
         await nursery.start(make_a_cache_scope_around_nursery)
         # this line tests the main difference from contextvars vs treevars
         shared_nursery.start_soon(assert_elsewhere_in_task_tree)
+
+
+async def test_cache_scope_overrides_nested():
+    pid1 = await run_sync(os.getpid)
+    async with _impl.cache_scope():
+        pid2 = await run_sync(os.getpid)
+        async with _impl.cache_scope():
+            await _assert_worker_pid(pid1, False)
+            await _assert_worker_pid(pid2, False)
+
+
+async def test_cache_scope_doesnt_override_explicit_context():
+    async with _impl.open_worker_context() as ctx:
+        pid = await ctx.run_sync(os.getpid)
+        async with _impl.cache_scope():
+            assert pid == await ctx.run_sync(os.getpid)

From 243f48e121631f500c354819c1759676b47510c5 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Wed, 25 Dec 2024 12:50:43 -0500
Subject: [PATCH 7/7] add newsfragment

---
 newsfragments/455.feature.rst | 3 +++
 1 file changed, 3 insertions(+)
 create mode 100644 newsfragments/455.feature.rst

diff --git a/newsfragments/455.feature.rst b/newsfragments/455.feature.rst
new file mode 100644
index 00000000..73c77742
--- /dev/null
+++ b/newsfragments/455.feature.rst
@@ -0,0 +1,3 @@
+Add `cache_scope()`, an async context manager that can override the behavior of
+`trio_parallel.run_sync()` in a subtree of your Trio tasks with an implicit
+`WorkerContext`.