Add sessions and cookies examples (#65)

Uses [`async-session`](https://crates.io/crates/async-session).

examples/README.md

@@ -13,3 +13,5 @@
 - [`error_handling_and_dependency_injection`](../examples/error_handling_and_dependency_injection.rs) - How to handle errors and dependency injection using trait objects.
 - [`tokio_postgres`](../examples/tokio_postgres.rs) - How to use a tokio-postgres and bb8 to query a database.
 - [`unix_domain_socket`](../examples/unix_domain_socket.rs) - How to run an Axum server over unix domain sockets.
+- [`sessions`](../examples/sessions.rs) - Sessions and cookies using [`async-session`](https://crates.io/crates/async-session).
+- [`tls_rustls`](../examples/tls_rustls.rs) - TLS with [`tokio-rustls`](https://crates.io/crates/tokio-rustls).

examples/sessions.rs

@@ -0,0 +1,109 @@
+use async_session::{MemoryStore, Session, SessionStore as _};
+use axum::{
+    async_trait,
+    extract::{FromRequest, RequestParts},
+    prelude::*,
+    response::IntoResponse,
+    AddExtensionLayer,
+};
+use headers::{HeaderMap, HeaderValue};
+use http::StatusCode;
+use serde::{Deserialize, Serialize};
+use std::net::SocketAddr;
+use uuid::Uuid;
+
+#[tokio::main]
+async fn main() {
+    // `MemoryStore` just used as an example. Don't use this in production.
+    let store = MemoryStore::new();
+
+    let app = route("/", get(handler)).layer(AddExtensionLayer::new(store));
+
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    hyper::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+async fn handler(user_id: UserIdFromSession) -> impl IntoResponse {
+    let (headers, user_id) = match user_id {
+        UserIdFromSession::FoundUserId(user_id) => (HeaderMap::new(), user_id),
+        UserIdFromSession::CreatedFreshUserId { user_id, cookie } => {
+            let mut headers = HeaderMap::new();
+            headers.insert(http::header::SET_COOKIE, cookie);
+            (headers, user_id)
+        }
+    };
+
+    dbg!(user_id);
+
+    headers
+}
+
+enum UserIdFromSession {
+    FoundUserId(UserId),
+    CreatedFreshUserId {
+        user_id: UserId,
+        cookie: HeaderValue,
+    },
+}
+
+#[async_trait]
+impl<B> FromRequest<B> for UserIdFromSession
+where
+    B: Send,
+{
+    type Rejection = (StatusCode, &'static str);
+
+    async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
+        let extract::Extension(store) = extract::Extension::<MemoryStore>::from_request(req)
+            .await
+            .expect("`MemoryStore` extension missing");
+
+        let headers = req.headers().expect("other extractor taken headers");
+
+        let cookie = if let Some(cookie) = headers
+            .get(http::header::COOKIE)
+            .and_then(|value| value.to_str().ok())
+            .map(|value| value.to_string())
+        {
+            cookie
+        } else {
+            let user_id = UserId::new();
+            let mut session = Session::new();
+            session.insert("user_id", user_id).unwrap();
+            let cookie = store.store_session(session).await.unwrap().unwrap();
+
+            return Ok(Self::CreatedFreshUserId {
+                user_id,
+                cookie: cookie.parse().unwrap(),
+            });
+        };
+
+        let user_id = if let Some(session) = store.load_session(cookie).await.unwrap() {
+            if let Some(user_id) = session.get::<UserId>("user_id") {
+                user_id
+            } else {
+                return Err((
+                    StatusCode::INTERNAL_SERVER_ERROR,
+                    "No `user_id` found in session",
+                ));
+            }
+        } else {
+            return Err((StatusCode::BAD_REQUEST, "No session found for cookie"));
+        };
+
+        Ok(Self::FoundUserId(user_id))
+    }
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, Copy)]
+struct UserId(Uuid);
+
+impl UserId {
+    fn new() -> Self {
+        Self(Uuid::new_v4())
+    }
+}