Fixed expired worker thread being sometimes assigned a task

Fixes #323.
agronholm · Jun 23, 2021 · 49f666e · 49f666e
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diff --git a/docs/versionhistory.rst b/docs/versionhistory.rst
@@ -3,6 +3,11 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Fixed idle thread pruning on asyncio sometimes causing an expired worker thread to be assigned a
+  task
+
 **3.2.0**
 
 - Added Python 3.10 compatibility

diff --git a/src/anyio/_backends/_asyncio.py b/src/anyio/_backends/_asyncio.py
@@ -792,9 +792,9 @@ async def run_sync_in_worker_thread(
                     if now - idle_workers[0].idle_since < WorkerThread.MAX_IDLE_TIME:
                         break
 
-                    worker = idle_workers.popleft()
-                    worker.root_task.remove_done_callback(worker.stop)
-                    worker.stop()
+                    expired_worker = idle_workers.popleft()
+                    expired_worker.root_task.remove_done_callback(expired_worker.stop)
+                    expired_worker.stop()
 
             worker.queue.put_nowait((func, args, future))
             return await future