Fixed cyclic references in to_thread.run_sync() on asyncio (#887)

Co-authored-by: Alex Grönholm <[email protected]>

Author: Wankupi

docs/versionhistory.rst

@@ -33,6 +33,9 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 - Fixed ``anyio.Path.iterdir()`` making a blocking call in Python 3.13
   (`#873 <https://github.com/agronholm/anyio/issues/873>`_; PR by @cbornet and
   @agronholm)
+- Fixed ``anyio.to_thread.run_sync()`` needlessly holding on to references of the
+  context, function, arguments and others until the next work item on asyncio
+  (PR by @Wankupi)
 
 **4.8.0**
 

src/anyio/_backends/_asyncio.py

@@ -3,6 +3,7 @@
 import array
 import asyncio
 import concurrent.futures
+import contextvars
 import math
 import os
 import socket
@@ -974,7 +975,10 @@ def run(self) -> None:
                             self._report_result, future, result, exception
                         )
 
+                    del result, exception
+
                 self.queue.task_done()
+                del item, context, func, args, future, cancel_scope
 
     def stop(self, f: asyncio.Task | None = None) -> None:
         self.stopping = True
@@ -2433,7 +2437,9 @@ async def run_sync_in_worker_thread(  # type: ignore[return]
                     worker = WorkerThread(root_task, workers, idle_workers)
                     worker.start()
                     workers.add(worker)
-                    root_task.add_done_callback(worker.stop)
+                    root_task.add_done_callback(
+                        worker.stop, context=contextvars.Context()
+                    )
                 else:
                     worker = idle_workers.pop()
 

tests/test_to_thread.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
 import asyncio
+import gc
+import sys
 import threading
 import time
 from concurrent.futures import Future, ThreadPoolExecutor
@@ -23,7 +25,7 @@
 )
 from anyio.from_thread import BlockingPortalProvider
 
-from .conftest import asyncio_params
+from .conftest import asyncio_params, no_other_refs
 
 pytestmark = pytest.mark.anyio
 
@@ -360,3 +362,30 @@ def dummy() -> None:
                 portal.call(event.set)
 
         assert len(threads) == 1
+
+
+@pytest.mark.skipif(
+    sys.implementation.name == "pypy",
+    reason=(
+        "gc.get_referrers is broken on PyPy (see "
+        "https://github.com/pypy/pypy/issues/5075)"
+    ),
+)
+async def test_run_sync_worker_cyclic_references() -> None:
+    class Foo:
+        pass
+
+    def foo(_: Foo) -> None:
+        pass
+
+    cvar = ContextVar[Foo]("cvar")
+    contextval = Foo()
+    arg = Foo()
+    cvar.set(contextval)
+    await to_thread.run_sync(foo, arg)
+    cvar.set(Foo())
+    gc.collect()
+
+    assert gc.get_referrers(contextval) == no_other_refs()
+    assert gc.get_referrers(foo) == no_other_refs()
+    assert gc.get_referrers(arg) == no_other_refs()