propagate exceptions from cancel scope to nursery __aexit__ manually

To further reduce extraneous exception frames, CancelScope gets
create() constructor and close() methods, which nursery __aexit__
uses directly rather than going through a scope context manager.

trio/_core/_run.py

@@ -123,6 +123,16 @@ class CancelScope:
     cancel_called = attr.ib(default=False)
     cancelled_caught = attr.ib(default=False)
 
+    @staticmethod
+    def create(deadline, shield):
+        task = _core.current_task()
+        scope = CancelScope()
+        scope._scope_task = task
+        scope._add_task(task)
+        scope.deadline = deadline
+        scope.shield = shield
+        return scope
+
     def __repr__(self):
         return "<cancel scope object at {:#x}>".format(id(self))
 
@@ -211,6 +221,12 @@ def _exc_filter(self, exc):
             return None
         return exc
 
+    def _close(self, exc):
+        self._remove_task(self._scope_task)
+        if exc is not None:
+            filtered_exc = MultiError.filter(self._exc_filter, exc)
+            return filtered_exc
+
     # Note we explicitly avoid @contextmanager since it adds extraneous stack
     # frames to exceptions.
     @enable_ki_protection
@@ -219,30 +235,21 @@ def __enter__(self):
 
     @enable_ki_protection
     def __exit__(self, etype, exc, tb):
-        try:
-            if exc is not None:
-                filtered_exc = MultiError.filter(self._exc_filter, exc)
-                if filtered_exc is None:
-                    return True
-                elif filtered_exc == exc:
-                    return False
-                else:
-                    raise filtered_exc
-        finally:
-            self._remove_task(self._scope_task)
+        filtered_exc = self._close(exc)
+        if filtered_exc is None:
+            return True
+        elif filtered_exc == exc:
+            return False
+        else:
+            raise filtered_exc
 
 
 def open_cancel_scope(*, deadline=inf, shield=False):
     """Returns a context manager which creates a new cancellation scope.
 
     """
 
-    scope = CancelScope()
-    scope._scope_task = _core.current_task()
-    scope._add_task(scope._scope_task)
-    scope.deadline = deadline
-    scope.shield = shield
-    return scope
+    return CancelScope.create(deadline, shield)
 
 
 ################################################################
@@ -348,27 +355,22 @@ class NurseryManager:
 
     @enable_ki_protection
     async def __aenter__(self):
-        self._scope_manager = open_cancel_scope()
-        scope = self._scope_manager.__enter__()
-        self._nursery = Nursery(current_task(), scope)
+        self._scope = CancelScope.create(deadline=inf, shield=False)
+        self._nursery = Nursery(current_task(), self._scope)
         return self._nursery
 
     @enable_ki_protection
     async def __aexit__(self, etype, exc, tb):
         new_exc = await self._nursery._nested_child_finished(exc)
         if new_exc:
-            try:
-                if self._scope_manager.__exit__(
-                    type(new_exc), new_exc, new_exc.__traceback__
-                ):
-                    return True
-            except BaseException as scope_manager_exc:
-                if scope_manager_exc == exc:
-                    return False
-                raise  # scope_manager_exc
-            raise new_exc
+            scope_exc = self._scope._close(new_exc)
+            if scope_exc is None:
+                return True
+            elif scope_exc == exc:
+                return False
+            else:
+                raise scope_exc
         else:
-            self._scope_manager.__exit__(None, None, None)
             return True
 
     def __enter__(self):