Add bb8 connection pool example (#19)

I figure this will be quite common.

Cargo.toml

@@ -42,10 +42,13 @@ headers = { optional = true, version = "0.3" }
 
 [dev-dependencies]
 askama = "0.10.5"
+bb8 = "0.7.0"
+bb8-postgres = "0.7.0"
 hyper = { version = "0.14", features = ["full"] }
 reqwest = { version = "0.11", features = ["json", "stream"] }
 serde = { version = "1.0", features = ["derive"] }
 tokio = { version = "1.6.1", features = ["macros", "rt", "rt-multi-thread"] }
+tokio-postgres = "0.7.2"
 tracing = "0.1"
 tracing-subscriber = "0.2"
 uuid = "0.8"

examples/bb8_connection_pool.rs

@@ -0,0 +1,55 @@
+use awebframework::{extract::Extension, prelude::*, AddExtensionLayer};
+use bb8::Pool;
+use bb8_postgres::PostgresConnectionManager;
+use http::StatusCode;
+use std::net::SocketAddr;
+use tokio_postgres::NoTls;
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt::init();
+
+    // setup connection pool
+    let manager =
+        PostgresConnectionManager::new_from_stringlike("host=localhost user=postgres", NoTls)
+            .unwrap();
+    let pool = Pool::builder().build(manager).await.unwrap();
+
+    // build our application with some routes
+    let app = route("/", get(handler)).layer(AddExtensionLayer::new(pool));
+
+    // run it with hyper
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    app.serve(&addr).await.unwrap();
+}
+
+type ConnectionPool = Pool<PostgresConnectionManager<NoTls>>;
+
+async fn handler(
+    Extension(pool): Extension<ConnectionPool>,
+) -> Result<String, (StatusCode, String)> {
+    // We cannot get a connection directly via an extractor because
+    // `bb8::PooledConnection` contains a reference to the pool and
+    // `extract::FromRequest` cannot return types that contain references.
+    //
+    // So therefore we have to get a connection from the pool manually.
+    let conn = pool.get().await.map_err(internal_error)?;
+
+    let row = conn
+        .query_one("select 1 + 1", &[])
+        .await
+        .map_err(internal_error)?;
+    let two: i32 = row.try_get(0).map_err(internal_error)?;
+
+    Ok(two.to_string())
+}
+
+/// Utility function for mapping any error into a `500 Internal Server Error`
+/// response.
+fn internal_error<E>(err: E) -> (StatusCode, String)
+where
+    E: std::error::Error,
+{
+    (StatusCode::INTERNAL_SERVER_ERROR, err.to_string())
+}