Add a nursery in Swarm

To avoid using the one in `Service`

libp2p/network/swarm.py

@@ -1,8 +1,9 @@
 import logging
-from typing import Dict, List
+from typing import Dict, List, Optional
 
 from async_service import Service
 from multiaddr import Multiaddr
+import trio
 
 from libp2p.io.abc import ReadWriteCloser
 from libp2p.network.connection.net_connection_interface import INetConn
@@ -49,6 +50,8 @@ class Swarm(Service, INetworkService):
     connections: Dict[ID, INetConn]
     listeners: Dict[str, IListener]
     common_stream_handler: StreamHandlerFn
+    listener_nursery: Optional[trio.Nursery]
+    event_listener_nursery_created: trio.Event
 
     notifees: List[INotifee]
 
@@ -72,8 +75,21 @@ class Swarm(Service, INetworkService):
         # Ignore type here since mypy complains: https://github.com/python/mypy/issues/2427
         self.common_stream_handler = create_default_stream_handler(self)  # type: ignore
 
+        self.listener_nursery = None
+        self.event_listener_nursery_created = trio.Event()
+
     async def run(self) -> None:
+        async with trio.open_nursery() as nursery:
+            # Create a nursery for listener tasks.
+            self.listener_nursery = nursery
+            self.event_listener_nursery_created.set()
+            try:
                 await self.manager.wait_finished()
+            finally:
+                # The service ended. Cancel listener tasks.
+                nursery.cancel_scope.cancel()
+                # Indicate that the nursery has been cancelled.
+                self.listener_nursery = None
 
     def get_peer_id(self) -> ID:
         return self.self_id
@@ -207,6 +223,9 @@ class Swarm(Service, INetworkService):
               - Call listener listen with the multiaddr
               - Map multiaddr to listener
         """
+        # We need to wait until `self.listener_nursery` is created.
+        await self.event_listener_nursery_created.wait()
+
         for maddr in multiaddrs:
             if str(maddr) in self.listeners:
                 return True
@@ -250,7 +269,9 @@ class Swarm(Service, INetworkService):
                 self.listeners[str(maddr)] = listener
                 # TODO: `listener.listen` is not bounded with nursery. If we want to be
                 #   I/O agnostic, we should change the API.
-                await listener.listen(maddr, self.manager._task_nursery)  # type: ignore
+                if self.listener_nursery is None:
+                    raise SwarmException("swarm instance hasn't been run")
+                await listener.listen(maddr, self.listener_nursery)
 
                 # Call notifiers since event occurred
                 self.notify_listen(maddr)