unwrap exceptiongroup, add test

Author: jakkdl

src/trio/_channel.py

@@ -18,7 +18,15 @@
 
 from ._abc import ReceiveChannel, ReceiveType, SendChannel, SendType, T
 from ._core import Abort, RaiseCancelT, Task, enable_ki_protection
-from ._util import NoPublicConstructor, final, generic_function
+from ._util import (
+    NoPublicConstructor,
+    final,
+    generic_function,
+    raise_single_exception_from_group,
+)
+
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup
 
 if TYPE_CHECKING:
     from types import TracebackType
@@ -534,19 +542,29 @@ async def context_manager(
         *args: P.args, **kwargs: P.kwargs
     ) -> AsyncGenerator[trio._channel.RecvChanWrapper[T], None]:
         send_chan, recv_chan = trio.open_memory_channel[T](0)
-        async with trio.open_nursery(strict_exception_groups=True) as nursery:
-            agen = fn(*args, **kwargs)
-            send_semaphore = trio.Semaphore(0)
-            # `nursery.start` to make sure that we will clean up send_chan & agen
-            # If this errors we don't close `recv_chan`, but the caller
-            # never gets access to it, so that's not a problem.
-            await nursery.start(_move_elems_to_channel, agen, send_chan, send_semaphore)
-            # `async with recv_chan` could eat exceptions, so use sync cm
-            with RecvChanWrapper(recv_chan, send_semaphore) as wrapped_recv_chan:
-                yield wrapped_recv_chan
-            # User has exited context manager, cancel to immediately close the
-            # abandoned generator if it's still alive.
-            nursery.cancel_scope.cancel()
+        try:
+            async with trio.open_nursery(strict_exception_groups=True) as nursery:
+                agen = fn(*args, **kwargs)
+                send_semaphore = trio.Semaphore(0)
+                # `nursery.start` to make sure that we will clean up send_chan & agen
+                # If this errors we don't close `recv_chan`, but the caller
+                # never gets access to it, so that's not a problem.
+                await nursery.start(
+                    _move_elems_to_channel, agen, send_chan, send_semaphore
+                )
+                # `async with recv_chan` could eat exceptions, so use sync cm
+                with RecvChanWrapper(recv_chan, send_semaphore) as wrapped_recv_chan:
+                    yield wrapped_recv_chan
+                # User has exited context manager, cancel to immediately close the
+                # abandoned generator if it's still alive.
+                nursery.cancel_scope.cancel()
+        except BaseExceptionGroup as eg:
+            try:
+                raise_single_exception_from_group(eg)
+            except AssertionError:
+                raise RuntimeError(
+                    "Encountered exception during cleanup of generator object, as well as exception in the contextmanager body"
+                ) from eg
 
     async def _move_elems_to_channel(
         agen: AsyncGenerator[T, None],

src/trio/_tests/test_channel.py

@@ -416,18 +416,6 @@ async def do_send(s: trio.MemorySendChannel[int], v: int) -> None:
         r.receive_nowait()
 
 
-async def test_background_with_channel() -> None:
-    @background_with_channel
-    async def agen() -> AsyncGenerator[int]:
-        yield 1
-        await trio.sleep_forever()  # simulate deadlock
-
-    async with agen() as recv_chan:
-        async for x in recv_chan:
-            assert x == 1
-            break  # exit, cleanup should be quick
-
-
 async def test_background_with_channel_exhaust() -> None:
     @background_with_channel
     async def agen() -> AsyncGenerator[int]:
@@ -496,7 +484,7 @@ async def agen() -> AsyncGenerator[int]:
         yield 1
         raise ValueError("oae")
 
-    with RaisesGroup(ValueError):
+    with pytest.raises(ValueError, match=r"^oae$"):
         async with agen() as recv_chan:
             async for x in recv_chan:
                 assert x == 1
@@ -544,16 +532,20 @@ async def agen(stuff: list[str]) -> AsyncGenerator[int]:
             stuff.append("finally")
             raise ValueError("agen")
 
-    with RaisesGroup(
-        Matcher(ValueError, match="^agen$"),
-        Matcher(TypeError, match="^iterator$"),
-    ):
+    with pytest.raises(
+        RuntimeError,
+        match=r"^Encountered exception during cleanup of generator object, as well as exception in the contextmanager body.$",
+    ) as excinfo:
         async with agen(events) as recv_chan:
             async for i in recv_chan:  # pragma: no branch
                 assert i == 1
                 raise TypeError("iterator")
 
     assert events == ["GeneratorExit()", "finally"]
+    RaisesGroup(
+        Matcher(ValueError, match="^agen$"),
+        Matcher(TypeError, match="^iterator$"),
+    ).matches(excinfo.value.__cause__)
 
 
 async def test_background_with_channel_nested_loop() -> None:
@@ -571,3 +563,17 @@ async def agen() -> AsyncGenerator[int]:
                     assert (i, j) == (ii, jj)
                     jj += 1
             ii += 1
+
+
+async def test_doesnt_leak_cancellation() -> None:
+    @background_with_channel
+    async def agenfn() -> AsyncGenerator[None]:
+        with trio.CancelScope() as cscope:
+            cscope.cancel()
+            yield
+
+    with pytest.raises(AssertionError):
+        async with agenfn() as recv_chan:
+            async for _ in recv_chan:
+                pass
+        raise AssertionError("should be reachable")