lnworker: handle multiple transports with same remote node id

If a remote node tries to establish a transport with us but we already
have an open transport with such a node id, there are two sane ways to go, either:
- keep existing transport open, reject new transport
- close existing transport, establish new transport

We could do either; I chose to do the second option here, as that is what
lnd and eclair seem to be doing.
Previously we would get into an inconsistent state: both transports open,
but only one of them stored in lnworker.peers.

electrum/lnworker.py

@@ -43,7 +43,7 @@ from .util import ignore_exceptions, make_aiohttp_session, SilentTaskGroup
 from .util import timestamp_to_datetime, random_shuffled_copy
 from .util import MyEncoder, is_private_netaddress
 from .logging import Logger
-from .lntransport import LNTransport, LNResponderTransport
+from .lntransport import LNTransport, LNResponderTransport, LNTransportBase
 from .lnpeer import Peer, LN_P2P_NETWORK_TIMEOUT
 from .lnaddr import lnencode, LnAddr, lndecode
 from .ecc import der_sig_from_sig_string
@@ -254,10 +254,7 @@ class LNWorker(Logger, NetworkRetryManager[LNPeerAddr]):
                 except Exception as e:
                     self.logger.info(f'handshake failure from incoming connection: {e!r}')
                     return
-                peer = Peer(self, node_id, transport)
+                await self._add_peer_from_transport(node_id=node_id, transport=transport)
-                with self.lock:
-                    self._peers[node_id] = peer
-                await self.taskgroup.spawn(peer.main_loop())
             try:
                 self.listen_server = await asyncio.start_server(cb, addr, netaddr.port)
             except OSError as e:
@@ -303,10 +300,18 @@ class LNWorker(Logger, NetworkRetryManager[LNPeerAddr]):
             raise ErrorAddingPeer("cannot connect to self")
         transport = LNTransport(self.node_keypair.privkey, peer_addr,
                                 proxy=self.network.proxy)
+        peer = await self._add_peer_from_transport(node_id=node_id, transport=transport)
+        return peer
+
+    async def _add_peer_from_transport(self, *, node_id: bytes, transport: LNTransportBase) -> Peer:
         peer = Peer(self, node_id, transport)
-        await self.taskgroup.spawn(peer.main_loop())
         with self.lock:
+            existing_peer = self._peers.get(node_id)
+            if existing_peer:
+                existing_peer.close_and_cleanup()
+            assert node_id not in self._peers
             self._peers[node_id] = peer
+        await self.taskgroup.spawn(peer.main_loop())
         return peer
 
     def peer_closed(self, peer: Peer) -> None:
@@ -2165,6 +2170,7 @@ class LNWallet(LNWorker):
             except Exception as e:
                 self.logger.info(f'failed to connect {host} {e}')
                 continue
+            # TODO close/cleanup the transport
         else:
             raise Exception('failed to connect')