Implement notifications_iter for async connections

[LucaCappelletti94]

At this time, the notifications_iter method is solely available for the sync connections. Would it be possible to implement it also for the async case?

[weiznich]

It is certainly possible to implement an async equivalent of that method. I personally do not have the time to do that in a timely manner, but I might accept a PR for that.

[LucaCappelletti94]

I will look into it in the coming days, do you have any pointers in particular that I should take into special consideration?

[weiznich]

You need to use the Connection::poll_message function from tokio_postgres to fetch a potential notification message. That message will get you back a Notification or other message types. The other ones can be just ignored.

This needs to be done in that function:
https://github.com/weiznich/diesel_async/blob/main/src/pg/mod.rs#L892

We want to transform the notification type there to the PgNotification struct from diesel. We would then use a tokio channel to send that notification to the actual foreground task holding the AsyncPgConnectiion via a channel. That channel should be exposed as impl Stream<Item = QueryResult<PgNotification>> via a fn notification_stream() from AsyncPgConnection.

Now one thing that could happen is that the connection was constructed via AsyncPgConnection::try_from where we don't have a tokio_postgres::Connection at all. We either need to deprecated this function or document that using this function will disable the notification stream. For the later case we need to return an stream that always waiting on the next element.