Avoid chaining current exception when exiting task-groups on asyncio (#…

…864)

Fixes #863.

docs/versionhistory.rst

@@ -3,6 +3,13 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+ **UNRELEASED**
+
+- Fixed traceback formatting growing quadratically with level of ``TaskGroup``
+  nesting on asyncio due to exception chaining when raising ``ExceptionGroups``
+  in ``TaskGroup.__aexit__``
+  (`#863 <https://github.com/agronholm/anyio/issues/863>`_; PR by @tapetersen)
+
 **4.8.0**
 
 - Added **experimental** support for running functions in subinterpreters on Python

src/anyio/_backends/_asyncio.py

@@ -764,9 +764,13 @@ async def __aexit__(
 
                 self._active = False
                 if self._exceptions:
+                    # The exception that got us here should already have been
+                    # added to self._exceptions so it's ok to break exception
+                    # chaining and avoid adding a "During handling of above..."
+                    # for each nesting level.
                     raise BaseExceptionGroup(
                         "unhandled errors in a TaskGroup", self._exceptions
-                    )
+                    ) from None
                 elif exc_val:
                     raise exc_val
             except BaseException as exc:

tests/test_taskgroups.py

@@ -1837,3 +1837,13 @@ async def test_patched_asyncio_task(monkeypatch: MonkeyPatch) -> None:
     )
     async with create_task_group() as tg:
         tg.start_soon(sleep, 0)
+
+
+async def test_exception_groups_suppresses_exc_context() -> None:
+    with pytest.raises(
+        cast(type[ExceptionGroup[Exception]], ExceptionGroup)
+    ) as exc_info:
+        async with create_task_group():
+            raise Exception("Error")
+
+    assert exc_info.value.__suppress_context__