backpressure on LISTEN?

まだ誰も着手していません。

* **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](https://github.com/magicstack/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!

まず connection._process_notification、call_soon()、_call_listener()、conn.add_listener() を読み、次にアプリケーションのキューがどのようにタスクを作成するかを追跡してください。Issue では対象ファイル、テスト、受け入れ時の動作が指定されていないため、結果を処理中の通知に対する API レベルの上限とするかどうかを定義し、その動作のカバレッジを追加してください。

索引モデルが issue の本文から書いたものです。