backpressure on LISTEN?

• asyncpg version: 0.18.3
• PostgreSQL version: 9.5
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
• Python version: 3.6
• Platform:
• Do you use pgbouncer?:
• Did you install asyncpg with pip?:
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and
  uvloop?:

My application subscribes to postgres notifications and fans them out to subscribers using streaming http.

I see that notifications emerge in connection._process_notification, where they are dispatched via call_soon(), each time calling _call_listener(), which will synchronously call the callback that I specify in conn.add_listener(). In the callback, I insert a task into the queue for asynchronous processing...

I see a lot of tasks inserted this way, e.g. ~5000 before my application has a chance to process them. Therefore application memory has big unpredictable spikes, like +300k

Question: how do you apply back-pressure in this setup? is there a way to limit the number of postgres notifications that are being converted to tasks, to control overall number of in-flight tasks? Thanks!

先阅读 connection._process_notification、call_soon()、_call_listener() 和 conn.add_listener()，然后跟踪应用程序的队列如何创建任务。该 issue 未指定目标文件、测试或验收行为；请确定结果是否应当是 API 层面对进行中的通知设置限制，并为该行为添加覆盖测试。