Fixed to_thread.run_sync() hanging on asyncio on StopIteration

Fixes #475.

docs/versionhistory.rst

@@ -33,6 +33,8 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
   scope
 - Fixed ``open_signal_receiver()`` inconsistently yielding integers instead of
   ``signal.Signals`` instances on the ``trio`` backend
+- Fixed ``to_thread.run_sync()`` hanging on asyncio if the target callable raises
+  ``StopIteration``
 
 **3.6.1**
 

src/anyio/_backends/_asyncio.py

@@ -674,6 +674,11 @@ def _report_result(
 
         if not future.cancelled():
             if exc is not None:
+                if isinstance(exc, StopIteration):
+                    new_exc = RuntimeError("coroutine raised StopIteration")
+                    new_exc.__cause__ = exc
+                    exc = new_exc
+
                 future.set_exception(exc)
             else:
                 future.set_result(result)

tests/test_to_thread.py

@@ -265,3 +265,17 @@ async def taskfunc2() -> None:
     task1 = asyncio_event_loop.create_task(taskfunc1())
     task2 = asyncio_event_loop.create_task(taskfunc2())
     asyncio_event_loop.run_until_complete(asyncio.gather(task1, task2))
+
+
+async def test_stopiteration() -> None:
+    """
+    Test that raising StopIteration in a worker thread raises a RuntimeError on the
+    caller.
+
+    """
+
+    def raise_stopiteration() -> NoReturn:
+        raise StopIteration
+
+    with pytest.raises(RuntimeError, match="coroutine raised StopIteration"):
+        await to_thread.run_sync(raise_stopiteration)