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!

先閱讀 connection._process_notification、call_soon()、_call_listener() 和 conn.add_listener()，然後追蹤應用程式的佇列如何建立工作。該 issue 未指定目標檔案、測試或驗收行為；請確定結果是否應該是 API 層級對處理中的通知設定限制，並為該行為新增涵蓋測試。