Hide implementation-detail nursery of start when using run(..., stric…

…t_exception_groups=True)

newsfragments/2611.bugfix.rst

@@ -1 +1,4 @@
-With ``strict_exception_groups=True``, when you ran a function in a nursery which raised an exception before calling ``task_status.started()``, it previously got wrapped twice over in ``ExceptionGroup`` in some cases. It no longer does that, and also won't wrap any ``ExceptionGroup`` raised by the function itself.
+When a starting function raises before calling :func:`trio.TaskStatus.started`,
+:func:`trio.Nursery.start` will no longer wrap the exception in an undocumented
+``ExceptionGroup``. Previously, an ``ExceptionGroup`` was added when using
+``trio.run(..., strict_exception_groups=True)``.

trio/_core/_run.py

@@ -1103,13 +1103,6 @@ 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(
@@ -1218,7 +1211,11 @@ async def async_fn(arg1, arg2, *, task_status=trio.TASK_STATUS_IGNORED):
             raise RuntimeError("Nursery is closed to new arrivals")
         try:
             self._pending_starts += 1
-            async with open_nursery() as old_nursery:
+            # `strict_exception_groups=False` prevents the implementation-detail
+            # nursery from inheriting `strict_exception_groups=True` from the
+            # `run` option, which would cause it to wrap a pre-started()
+            # exception in an extra ExceptionGroup. See #2611.
+            async with open_nursery(strict_exception_groups=False) as old_nursery:
                 task_status: _TaskStatus[StatusT] = _TaskStatus(old_nursery, self)
                 thunk = functools.partial(async_fn, task_status=task_status)
                 task = GLOBAL_RUN_CONTEXT.runner.spawn_impl(