Allow opening a portal through an existing channel

tractor/_portal.py

@@ -314,7 +314,8 @@ class LocalPortal:
 @asynccontextmanager
 async def open_portal(
     channel: Channel,
-    nursery: Optional[trio.Nursery] = None
+    nursery: Optional[trio.Nursery] = None,
+    start_msg_loop: bool = True,
 ) -> typing.AsyncGenerator[Portal, None]:
     """Open a ``Portal`` through the provided ``channel``.
 
@@ -332,6 +333,8 @@ async def open_portal(
         if channel.uid is None:
             await actor._do_handshake(channel)
 
+        msg_loop_cs = None
+        if start_msg_loop:
             msg_loop_cs: trio.CancelScope = await nursery.start(
                 partial(
                     actor._process_messages,
@@ -352,6 +355,7 @@ async def open_portal(
                 await channel.send(None)
 
             # cancel background msg loop task
+            if msg_loop_cs:
                 msg_loop_cs.cancel()
 
             nursery.cancel_scope.cancel()