How are Postgres server restarts handled?

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Hi guys,

We are using asyncpg in a web application.

Upon starting the web server, we do the following two things:

1. `db = asyncpg.create_pool` to create the connection pool for the application's DB connections.
1. `conn = await db.pool.acquire(); await conn.add_listener('foo', _callback)` to register long-lived listeners to catch and handle `NOTIFY` events from the DB.

Recently I restarted the Postgres server and noticed something interesting. First, connections to the DB using the `db` pool object saw no interruption and were still able to connect to the DB server despite it having a new PID (since the process had been restarted).

However, the `conn` the application relied on to receive async notifications from the server was dead/closed and so the application logic dependent upon receiving those notifications was not functioning. (This is how I reproduced this issue and discovered the cause to have been a PG server restart.)

* Why is the pool unaffected by restart? It makes sense the connection instance is dead since the socket is closed after restarting the server, but I'm a bit surprised the pool is not affected.
* I suppose the best strategy for checking for dead DB connections would be to simply check `conn.is_closed` periodically - or, is there some "on connection close" callback I can define to retry/reopen a connection (since obtaining new connections from the pool still works after restart)?

Inizia esaminando gli entry point asyncpg.create_pool, pool.acquire, add_listener e conn.is_closed descritti nel report, insieme al comportamento di PostgreSQL NOTIFY durante il riavvio di un server. Determina come dovrebbero comportarsi le connessioni del pool e i listener di lunga durata, quindi documenta il modo supportato per rilevare e recuperare le connessioni dei listener chiuse.

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