backpressure on LISTEN?

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* **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!

Leggi prima connection._process_notification, call_soon(), _call_listener() e conn.add_listener(), poi traccia il modo in cui la coda dell’applicazione crea le attività. L’issue non indica un file di destinazione, un test o un comportamento di accettazione; definisci se il risultato debba essere un limite a livello API per le notifiche in corso e aggiungi la coverage per questo comportamento.

Scritto dal modello di indicizzazione a partire dal testo della issue.