Don't double-wrap exceptions in nurseries (#2826)

* add test

* Fix unnecessary wrapping of exception groups

* add test where child task raises an exception group

* refactor tests

* move tests into _core/_tests/test_run.py

* Update trio/_core/_tests/test_run.py

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

---------

Co-authored-by: CoolCat467 <52022020+CoolCat467@users.noreply.github.com>
Co-authored-by: Thomas Grainger <tagrain@gmail.com>

trio/_core/_run.py

@@ -1101,6 +1101,13 @@ def aborted(raise_cancel: _core.RaiseCancelT) -> Abort:
 
         popped = self._parent_task._child_nurseries.pop()
         assert popped is self
+
+        # don't unnecessarily wrap an exceptiongroup in another exceptiongroup
+        # see https://github.com/python-trio/trio/issues/2611
+        if len(self._pending_excs) == 1 and isinstance(
+            self._pending_excs[0], BaseExceptionGroup
+        ):
+            return self._pending_excs[0]
         if self._pending_excs:
             try:
                 return MultiError(

trio/_core/_tests/test_run.py

@@ -40,7 +40,7 @@
 )
 
 if sys.version_info < (3, 11):
-    from exceptiongroup import ExceptionGroup
+    from exceptiongroup import BaseExceptionGroup, ExceptionGroup
 
 
 T = TypeVar("T")
@@ -2538,3 +2538,62 @@ async def test_cancel_scope_no_cancellederror() -> None:
             raise ExceptionGroup("test", [RuntimeError(), RuntimeError()])
 
     assert not scope.cancelled_caught
+
+
+"""These tests are for fixing https://github.com/python-trio/trio/issues/2611
+where exceptions raised before `task_status.started()` got wrapped twice.
+"""
+
+
+async def raise_before(*, task_status: _core.TaskStatus[None]) -> None:
+    raise ValueError
+
+
+async def raise_after_started(*, task_status: _core.TaskStatus[None]) -> None:
+    task_status.started()
+    raise ValueError
+
+
+async def raise_custom_exception_group_before(
+    *, task_status: _core.TaskStatus[None]
+) -> None:
+    raise ExceptionGroup("my group", [ValueError()])
+
+
+def _check_exception(exc: pytest.ExceptionInfo[BaseException]) -> None:
+    assert isinstance(exc.value, BaseExceptionGroup)
+    assert len(exc.value.exceptions) == 1
+    assert isinstance(exc.value.exceptions[0], ValueError)
+
+
+async def _start_raiser(
+    raiser: Callable[[], Awaitable[None]], strict: bool | None = None
+) -> None:
+    async with _core.open_nursery(strict_exception_groups=strict) as nursery:
+        await nursery.start(raiser)
+
+
+@pytest.mark.parametrize("strict", [False, True])
+@pytest.mark.parametrize("raiser", [raise_before, raise_after_started])
+async def test_strict_before_started(
+    strict: bool, raiser: Callable[[], Awaitable[None]]
+) -> None:
+    with pytest.raises(BaseExceptionGroup if strict else ValueError) as exc:
+        await _start_raiser(raiser, strict)
+    if strict:
+        _check_exception(exc)
+
+
+# it was only when run from `trio.run` that the double wrapping happened
+@pytest.mark.parametrize("strict", [False, True])
+@pytest.mark.parametrize(
+    "raiser", [raise_before, raise_after_started, raise_custom_exception_group_before]
+)
+def test_trio_run_strict_before_started(
+    strict: bool, raiser: Callable[[], Awaitable[None]]
+) -> None:
+    expect_group = strict or raiser is raise_custom_exception_group_before
+    with pytest.raises(BaseExceptionGroup if expect_group else ValueError) as exc:
+        _core.run(_start_raiser, raiser, strict_exception_groups=strict)
+    if expect_group:
+        _check_exception(exc)