prefer simple async -> sync

* asgiref async_to_sync was causing a Redis connection _for each_ call
to emit_channel_notification i.e. every event that the callback receiver
processes. This is a "known" issue
https://github.com/django/channels_redis/pull/130#issuecomment-424274470
and the advise is to slow downn the rate at which you call
async_to_sync. That is not an option for us. Instead, we put the async
group_send call onto the event loop for the current thread and wait for
it to be processed immediately.

The known issue has to do with event loop + socket relationship. Each
connection to redis is achieved via a socket. That conection can only be
waiting on by the event loop that corresponds to the calling thread.
async_to_sync creates a _new thread_ for each invocation. Thus, a new
connection to redis is required. Thus, the excess redis connections that
can be observed via netstat | grep redis | wc -l.

awx/main/consumers.py

@@ -5,6 +5,7 @@ import logging
 import codecs
 import datetime
 import hmac
+import asyncio
 
 from django.utils.encoding import force_bytes
 from django.utils.encoding import smart_str
@@ -200,28 +201,66 @@ class EventConsumer(AsyncJsonWebsocketConsumer):
         await self.send(event['text'])
 
 
-def emit_channel_notification(group, payload):
+def run_sync(func):
+    event_loop = None
     try:
-        payload = json.dumps(payload, cls=DjangoJSONEncoder)
+        event_loop = asyncio.get_event_loop()
+    except RuntimeError:
+        event_loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(event_loop)
+    return event_loop.run_until_complete(func)
+
+
+def _dump_payload(payload):
+    try:
+        return json.dumps(payload, cls=DjangoJSONEncoder)
     except ValueError:
-        logger.error("Invalid payload emitting channel {} on topic: {}".format(group, payload))
+        logger.error("Invalid payload to emit")
+        return None
+
+
+async def emit_channel_notification_async(group, payload):
+    payload_dumped = _dump_payload(payload)
+    if payload_dumped is None:
+        return
+
+    channel_layer = get_channel_layer()
+    await channel_layer.group_send(
+        group,
+        {
+            "type": "internal.message",
+            "text": payload_dumped
+        },
+    )
+
+    await channel_layer.group_send(
+        BROADCAST_GROUP,
+        {
+            "type": "internal.message",
+            "text": wrap_broadcast_msg(group, payload_dumped),
+        },
+    )
+
+
+def emit_channel_notification(group, payload):
+    payload_dumped = _dump_payload(payload)
+    if payload_dumped is None:
         return
 
     channel_layer = get_channel_layer()
 
-    async_to_sync(channel_layer.group_send)(
+    run_sync(channel_layer.group_send(
         group,
         {
             "type": "internal.message",
-            "text": payload
+            "text": payload_dumped
         },
-    )
+    ))
 
-    async_to_sync(channel_layer.group_send)(
+    run_sync(channel_layer.group_send(
         BROADCAST_GROUP,
         {
             "type": "internal.message",
-            "text": wrap_broadcast_msg(group, payload),
+            "text": wrap_broadcast_msg(group, payload_dumped),
         },
-    )
-
+    ))