Fix exception cause/context lost before task_status.started()

newsfragments/3261.bugfix.rst

@@ -0,0 +1,2 @@
+``Nursery.start()`` now preserves the ``__cause__`` and ``__context__`` of
+exceptions raised before ``task_status.started()`` is called.

src/trio/_core/_run.py

@@ -33,7 +33,7 @@
 from .. import _core
 from .._abc import Clock, Instrument
 from .._deprecate import warn_deprecated
-from .._util import NoPublicConstructor, coroutine_or_error, final
+from .._util import NoPublicConstructor, coroutine_or_error, final, raise_saving_context
 from ._asyncgens import AsyncGenerators
 from ._concat_tb import concat_tb
 from ._entry_queue import EntryQueue, TrioToken
@@ -1466,7 +1466,7 @@ async def async_fn(arg1, arg2, *, task_status=trio.TASK_STATUS_IGNORED):
                     # cancel this nursery:
             except BaseExceptionGroup as exc:
                 if len(exc.exceptions) == 1:
-                    raise exc.exceptions[0] from None
+                    raise_saving_context(exc.exceptions[0])
                 raise TrioInternalError(
                     "Internal nursery should not have multiple tasks. This can be "
                     'caused by the user managing to access the "old" nursery in '

src/trio/_core/_tests/test_run.py

@@ -2920,6 +2920,27 @@ async def start_raiser() -> None:
         assert should_be_raiser_exc.exceptions == raiser_exc.exceptions
 
 
+async def test_start_exception_preserves_cause_and_context() -> None:
+    """Regression test for #3261: exceptions raised before task_status.started()
+    should preserve __cause__ and __context__."""
+
+    async def task(*, task_status: _core.TaskStatus[None]) -> None:
+        e = ValueError("foo")
+        e.__cause__ = SyntaxError("bar")
+        e.__context__ = TypeError("baz")
+        raise e
+
+    with pytest.RaisesGroup(
+        pytest.RaisesExc(
+            ValueError,
+            check=lambda exc: isinstance(exc.__cause__, SyntaxError)
+            and isinstance(exc.__context__, TypeError),
+        ),
+    ):
+        async with _core.open_nursery() as nursery:
+            await nursery.start(task)
+
+
 async def test_internal_error_old_nursery_multiple_tasks() -> None:
     async def error_func() -> None:
         raise ValueError