How to I receive `NOTIFY` from other processes?

[letharion]

I have this simple test program:

let conn = Connection::connect("postgres://test:test@localhost", &SslMode::None).unwrap();
conn.execute("LISTEN chan", &[]);
let notifications = conn.notifications();

conn.execute("NOTIFY chan, 'rust'", &[]);
let mut it = notifications.iter();
for i in 0..99999999 {
    let a = it.next();
    match a {
       Some(b) => {
           println!("{:?}", b);
       }
       _ => {}
    }
}

The notification that I send with 'rust' as payload gets printed, and everything looks fine. However, if I then go into psql, and send NOTIFY chan, 'psql'; while the loop is running, that notification never seems to show up.

I also did a long sleep while I sent notifications from psql, instead of long loop, but that didn't have any other effect. (Not that I really know why it would)

The other way works fine however. If I LISTEN chan; from psql, then I receive the 'rust' payload.

I'm probably doing something wrong that has an easy fix, but I can't work out what it is, any help would be appreciated. :)

$ rustc --version
rustc 1.5.0-beta.2 (b0f440163 2015-10-28)
$ psql --version
psql (PostgreSQL) 9.4.5

[letharion]

Just found HandleNotice http://sfackler.github.io/rust-postgres/doc/v0.10.2/postgres/trait.HandleNotice.html which looks relevant. :)

[sfackler]

The iterator returned by notifications.iter() will only return notifications that the driver's already received and processed. Any notification that has been sent after the last operation on the connection that touched the network (e.g. query) will not be found. notifications.blocking_iter() and notifications.timeout_iter() will return iterators that will check the network for incoming notifications.

With notifications.iter(), you can "touch the network" to get it to pick up pending notifications:

let mut notifications = conn.notifications();
let mut it = notifications.iter();
for i in 0..99999999 {
    let a = it.next();
    match a {
       Some(b) => {
           println!("{:?}", b);
       }
       _ => conn.query("", &[]), // execute a noop query to pick up new messages
    }
}

Or just use blocking_iter().

HandleNotice is actually separate from the LISTEN and NOTIFY system. It handles messages the server sends about things like the server shutting down.

[letharion]

Thank you very much, blocking_iter looks like just what I need.
Want me to submit something back as a documentation update around this? :)

[sfackler]

Please do! The documentation on iter right now is not great about explaining how this stuff works.