[mdns] unreference mDNS protocol when connection is closed

This allows cleanly shutting down the mDNS once all connections are
closed. Otherwise we hit a ResourceWarning when the event loop exits.

src/aioice/ice.py

@@ -30,16 +30,27 @@
 _mdns = threading.local()
 
 
-async def get_or_create_mdns_protocol() -> mdns.MDnsProtocol:
+async def get_or_create_mdns_protocol(subscriber: object) -> mdns.MDnsProtocol:
     if not hasattr(_mdns, "lock"):
         _mdns.lock = asyncio.Lock()
         _mdns.protocol = None
+        _mdns.subscribers = set()
     async with _mdns.lock:
         if _mdns.protocol is None:
             _mdns.protocol = await mdns.create_mdns_protocol()
+        _mdns.subscribers.add(subscriber)
     return _mdns.protocol
 
 
+async def unref_mdns_protocol(subscriber: object) -> None:
+    if hasattr(_mdns, "lock"):
+        async with _mdns.lock:
+            _mdns.subscribers.discard(subscriber)
+            if _mdns.protocol and not _mdns.subscribers:
+                await _mdns.protocol.close()
+                _mdns.protocol = None
+
+
 def candidate_pair_priority(
     local: Candidate, remote: Candidate, ice_controlling: bool
 ) -> int:
@@ -364,7 +375,7 @@ async def add_remote_candidate(self, remote_candidate: Candidate) -> None:
 
         # resolve mDNS candidate
         if mdns.is_mdns_hostname(remote_candidate.host):
-            mdns_protocol = await get_or_create_mdns_protocol()
+            mdns_protocol = await get_or_create_mdns_protocol(self)
             remote_addr = await mdns_protocol.resolve(remote_candidate.host)
             if remote_addr is None:
                 self.__log_info(
@@ -498,6 +509,9 @@ async def close(self) -> None:
         if self._check_list and not self._check_list_done:
             await self._check_list_state.put(ICE_FAILED)
 
+        # unreference mDNS
+        await unref_mdns_protocol(self)
+
         self._nominated.clear()
         for protocol in self._protocols:
             await protocol.close()

tests/test_ice.py

@@ -1092,6 +1092,9 @@ async def test_add_remote_candidate_mdns_bad(self):
         self.assertEqual(len(conn_a.remote_candidates), 0)
         self.assertEqual(conn_a._remote_candidates_end, False)
 
+        # close
+        await conn_a.close()
+
     @asynctest
     async def test_add_remote_candidate_mdns_good(self):
         """
@@ -1118,6 +1121,8 @@ async def test_add_remote_candidate_mdns_good(self):
         self.assertEqual(conn_a.remote_candidates[0].host, "1.2.3.4")
         self.assertEqual(conn_a._remote_candidates_end, False)
 
+        # close
+        await conn_a.close()
         await publisher.close()
 
     @asynctest