Added demo program to show use of LISTEN/NOTIFY.

Cargo.toml

@@ -3,6 +3,7 @@ members = [
     ".",
     "sqlx-core",
     "sqlx-macros",
+    "examples/postgres-listen",
     "examples/realworld-postgres"
 ]
 

examples/postgres-listen/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "postgres-listen"
+version = "0.1.0"
+edition = "2018"
+workspace = "../.."
+
+[dependencies]
+async-std = { version = "1.4.0", features = [ "attributes", "unstable" ] }
+sqlx = { path = "../..", features = [ "postgres", "tls" ] }
+futures = "0.3.1"

examples/postgres-listen/README.md

@@ -0,0 +1,18 @@
+Postgres LISTEN/NOTIFY
+======================
+
+## Usage
+
+Declare the database URL. This example does not include any reading or writing of data.
+
+```
+export DATABASE_URL="postgres://postgres@localhost/postgres"
+```
+
+Run.
+
+```
+cargo run
+```
+
+The example program should connect to the database, and create a LISTEN loop on a predefined set of channels. A NOTIFY task will be spawned which will connect to the same database and will emit notifications on a 5 second interval.

examples/postgres-listen/src/main.rs

@@ -0,0 +1,57 @@
+use std::time::Duration;
+
+use async_std::stream;
+use futures::stream::StreamExt;
+use sqlx::postgres::PgPoolExt;
+
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    println!("Building PG pool.");
+    let conn_str =
+        std::env::var("DATABASE_URL").expect("Env var DATABASE_URL is required for this example.");
+    let pool = sqlx::PgPool::new(&conn_str).await?;
+
+    let notify_pool = pool.clone();
+    let _t = async_std::task::spawn(async move {
+        stream::interval(Duration::from_secs(5))
+            .for_each(move |_| notify(notify_pool.clone()))
+            .await
+    });
+
+    println!("Starting LISTEN loop.");
+    let mut listener = pool.listen(&["chan0", "chan1", "chan2"]);
+    let mut counter = 0usize;
+    loop {
+        let res = listener.recv().await;
+        println!("[from recv]: {:?}", res);
+        counter += 1;
+        if counter >= 3 {
+            break;
+        }
+    }
+
+    let stream = listener.into_stream();
+    futures::pin_mut!(stream);
+    while let Some(res) = stream.next().await {
+        println!("[from stream]: {:?}", res);
+    }
+
+    Ok(())
+}
+
+async fn notify(pool: sqlx::PgPool) {
+    let mut conn = match pool.acquire().await {
+        Ok(conn) => conn,
+        Err(err) => return println!("[from notify]: {:?}", err),
+    };
+    let res = sqlx::Executor::send(
+        &mut conn,
+        r#"
+        NOTIFY "chan0", '{"payload": 0}';
+        NOTIFY "chan1", '{"payload": 1}';
+        NOTIFY "chan2", '{"payload": 2}';
+    "#,
+    )
+    .await;
+    println!("[from notify]: {:?}", res);
+}