Fix "unexpected cancel" when parent is cancelled but needs to continue (#35)

resolves #34.

* When the async-with block (i.e., the parent task) is cancelled
  due to an unhandled exception from child tasks, `__aexit__()`
  sees the cancellation error instead of the source exception
  that triggered it. This should be wrapped as ExceptionGroup
  and the parent cancellation should be undone.
* The prior implementation from EdgeDB has been catching
  this parent cancellation as a full task cancellation, rather
  than a transient signal to exit the async-with block only.
* Fix this issue by taking the implementation of `_is_base_error()`
  from Python 3.11's `asyncio.TaskGroup` instead of changing
  `__aexit__()` method.
* Use more specific exception class in the test case to prevent
  unintended catch-all situation in terms of regression check.

Co-authored-by: Joongi Kim <joongi@lablup.com>

Author: bdowning

changes/35.feature.md

@@ -0,0 +1 @@
+Fix "unexpected cancel" bug in `TaskGroup`.

src/aiotools/taskgroup.py

@@ -211,7 +211,7 @@ def create_task(self, coro):
 
     def _is_base_error(self, exc):
         assert isinstance(exc, BaseException)
-        return not isinstance(exc, Exception)
+        return isinstance(exc, (SystemExit, KeyboardInterrupt))
 
     def _patch_task(self, task):
         # In Python 3.8 we'll need proper API on asyncio.Task to

tests/test_taskgroup.py

@@ -186,6 +186,32 @@ async def parent():
         assert results == []
 
 
+@pytest.mark.asyncio
+async def test_taskgroup_distinguish_inner_error_and_outer_cancel():
+
+    async def do_error():
+        await asyncio.sleep(0.5)
+        raise ValueError("bad stuff")
+
+    with VirtualClock().patch_loop():
+
+        with pytest.raises(TaskGroupError) as eg:
+            async with TaskGroup() as tg:
+                t1 = tg.create_task(do_error())
+                # The following sleep is cancelled due to do_error(),
+                # raising CancelledError!
+                await asyncio.sleep(1)
+                # We need to preserve the source exception instead of the outer
+                # cancellation.  __aexit__() should not treat CancelledError as a
+                # base error to handle such cases.
+
+        assert t1.done()
+        assert not t1.cancelled()
+        assert isinstance(t1.exception(), ValueError)
+        assert len(eg.value.__errors__) == 1
+        assert isinstance(eg.value.__errors__[0], ValueError)
+
+
 @pytest.mark.asyncio
 async def test_taskgroup_error():
     with VirtualClock().patch_loop():