fix(backend/postgres): allow concurrent pubs

This fix adds an asyncio.Lock to avoid
`asyncpg.exceptions._base.InterfaceError: cannot perform operation: another operation is in progress`

fixes encode#22

broadcaster/_backends/postgres.py

@@ -13,19 +13,23 @@ def __init__(self, url: str):
 
     async def connect(self) -> None:
         self._conn = await asyncpg.connect(self._url)
+        self._lock = asyncio.Lock()
         self._listen_queue: asyncio.Queue = asyncio.Queue()
 
     async def disconnect(self) -> None:
         await self._conn.close()
 
     async def subscribe(self, channel: str) -> None:
-        await self._conn.add_listener(channel, self._listener)
+        async with self._lock:
+            await self._conn.add_listener(channel, self._listener)
 
     async def unsubscribe(self, channel: str) -> None:
-        await self._conn.remove_listener(channel, self._listener)
+        async with self._lock:
+            await self._conn.remove_listener(channel, self._listener)
 
     async def publish(self, channel: str, message: str) -> None:
-        await self._conn.execute("SELECT pg_notify($1, $2);", channel, message)
+        async with self._lock:
+            await self._conn.execute("SELECT pg_notify($1, $2);", channel, message)
 
     def _listener(self, *args: Any) -> None:
         connection, pid, channel, payload = args

tests/test_concurrent.py

@@ -28,3 +28,35 @@ async def test_broadcast(setup_broadcast):
             event = await subscriber.get()
             assert event.channel == channel
             assert event.message == msg
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(["setup_broadcast"], URLS, indirect=True)
+async def test_sub(setup_broadcast):
+    uid = uuid4()
+    channel1 = f"chatroom-{uid}1"
+    channel2 = f"chatroom-{uid}2"
+
+    to_sub = [
+        setup_broadcast._backend.subscribe(channel1),
+        setup_broadcast._backend.subscribe(channel2),
+    ]
+    await asyncio.gather(*to_sub)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(["setup_broadcast"], URLS, indirect=True)
+async def test_unsub(setup_broadcast):
+    uid = uuid4()
+    channel1 = f"chatroom-{uid}1"
+    channel2 = f"chatroom-{uid}2"
+
+    await setup_broadcast._backend.subscribe(channel1)
+    await setup_broadcast._backend.subscribe(channel2)
+
+    to_unsub = [
+        setup_broadcast._backend.unsubscribe(channel1),
+        setup_broadcast._backend.unsubscribe(channel2),
+    ]
+
+    await asyncio.gather(*to_unsub)