Fix redisBackend _pubsub_listener just listen once (#134)

* fix: solve the redisBackend `_pubsub_listener` just listen once.

* chore: change the comment

Author: Fly-Playgroud

broadcaster/_backends/redis.py

@@ -43,14 +43,20 @@ async def next_published(self) -> Event:
     async def _pubsub_listener(self) -> None:
         # redis-py does not listen to the pubsub connection if there are no channels subscribed
         # so we need to wait until the first channel is subscribed to start listening
-        await self._ready.wait()
-        async for message in self._pubsub.listen():
-            if message["type"] == "message":
-                event = Event(
-                    channel=message["channel"].decode(),
-                    message=message["data"].decode(),
-                )
-                await self._queue.put(event)
+        while True:
+            await self._ready.wait()
+            async for message in self._pubsub.listen():
+                if message["type"] == "message":
+                    event = Event(
+                        channel=message["channel"].decode(),
+                        message=message["data"].decode(),
+                    )
+                    await self._queue.put(event)
+
+            # when no channel subscribed, clear the event.
+            # And then in next loop, event will blocked again until
+            # the new channel subscribed.Now asyncio.Task will not exit again.
+            self._ready.clear()
 
 
 StreamMessageType = typing.Tuple[bytes, typing.Tuple[typing.Tuple[bytes, typing.Dict[bytes, bytes]]]]