Fix spew when event loop exception handler is invoked after loop closure

newsfragments/134.bugfix.rst

@@ -0,0 +1,4 @@
+Fix an issue where a call to ``TrioEventLoop.call_exception_handler()`` after
+the loop was closed would attempt to call a method on ``None``. This pattern
+can be encountered if an ``aiohttp`` session is garbage-collected without being
+properly closed, for example.

tests/test_trio_asyncio.py

@@ -45,6 +45,15 @@ async def test_get_running_loop():
         assert asyncio.get_running_loop() == loop
 
 
+@pytest.mark.trio
+async def test_exception_after_closed(caplog):
+    async with trio_asyncio.open_loop() as loop:
+        pass
+    loop.call_exception_handler({"message": "Test exception after loop closed"})
+    assert len(caplog.records) == 1
+    assert caplog.records[0].message == "Test exception after loop closed"
+
+
 @pytest.mark.trio
 async def test_tasks_get_cancelled():
     record = []

trio_asyncio/_async.py

@@ -36,6 +36,13 @@ def default_exception_handler(self, context):
         # Call the original default handler so we get the full info in the log
         super().default_exception_handler(context)
 
+        if self._nursery is None:
+            # Event loop is already closed; don't do anything further.
+            # Some asyncio libraries call the asyncio exception handler
+            # from their __del__ methods, e.g., aiohttp for "Unclosed
+            # client session".
+            return
+
         # Also raise an exception so it can't go unnoticed
         exception = context.get("exception")
         if exception is None: