lnpeer: on_ping: drain out-socket buffer

might help against some memory exhaustion attacks

f321x said:
> maybe a peer could just fill up our sockets outgoing buffer by ~234mb/h
> if they stop reading from their socket and keep sending pings every second.

Co-authored-by: f321x <f@f321x.com>

Author: SomberNight

electrum/lnpeer.py

@@ -385,7 +385,8 @@ async def on_ping(self, payload):
             await asyncio.sleep(min_delay - elapsed_since_last)
         self._last_ping_recv_time = time.monotonic()
         l = payload['num_pong_bytes']
-        self.send_message('pong', byteslen=l)
+        raw_msg = encode_msg('pong', byteslen=l)
+        await self.transport.send_bytes_and_drain(raw_msg)
 
     def on_pong(self, payload):
         self.pong_event.set()

tests/lnhelpers.py

@@ -214,6 +214,10 @@ def __init__(self, keypair, name):
     def send_bytes(self, data):
         self.other_mock_transport.queue.put_nowait(data)
 
+    async def send_bytes_and_drain(self, data):
+        self.send_bytes(data)
+
+
 def transport_pair(k1, k2, name1, name2):
     t1 = PutIntoOthersQueueTransport(k1, name1)
     t2 = PutIntoOthersQueueTransport(k2, name2)