Allow for session cookies to be lazily created (#39)

actix-session/src/cookie.rs

@@ -55,6 +55,7 @@ struct CookieSessionInner {
     name: String,
     path: String,
     domain: Option<String>,
+    lazy: bool,
     secure: bool,
     http_only: bool,
     max_age: Option<Duration>,
@@ -70,6 +71,7 @@ impl CookieSessionInner {
             name: "actix-session".to_owned(),
             path: "/".to_owned(),
             domain: None,
+            lazy: false,
             secure: true,
             http_only: true,
             max_age: None,
@@ -84,6 +86,11 @@ impl CookieSessionInner {
         state: impl Iterator<Item = (String, String)>,
     ) -> Result<(), Error> {
         let state: HashMap<String, String> = state.collect();
+
+        if self.lazy && state.is_empty() {
+            return Ok(());
+        }
+
         let value =
             serde_json::to_string(&state).map_err(CookieSessionError::Serialize)?;
         if value.len() > 4064 {
@@ -247,6 +254,15 @@ impl CookieSession {
         self
     }
 
+    /// When true, prevents adding session cookies to responses until
+    /// the session contains data. Default is `false`.
+    /// 
+    /// Useful when trying to comply with laws that require consent for setting cookies.
+    pub fn lazy(mut self, value: bool) -> CookieSession {
+        Rc::get_mut(&mut self.0).unwrap().lazy = value;
+        self
+    }
+
     /// Sets the `secure` field in the session cookie being built.
     ///
     /// If the `secure` field is set, a cookie will only be transmitted when the
@@ -426,6 +442,37 @@ mod tests {
             .any(|c| c.name() == "actix-session"));
     }
 
+    #[actix_rt::test]
+    async fn lazy_cookie() {
+        let mut app = test::init_service(
+            App::new()
+                .wrap(CookieSession::signed(&[0; 32]).secure(false).lazy(true))
+                .service(web::resource("/count").to(|ses: Session| async move {
+                    let _ = ses.set("counter", 100);
+                    "counting"
+                }))
+                .service(web::resource("/").to(|_ses: Session| async move {
+                    "test"
+                })),
+        )
+        .await;
+
+        let request = test::TestRequest::get().to_request();
+        let response = app.call(request).await.unwrap();
+        assert!(response
+            .response()
+            .cookies()
+            .count() == 0);
+
+        let request = test::TestRequest::with_uri("/count").to_request();
+        let response = app.call(request).await.unwrap();
+
+        assert!(response
+            .response()
+            .cookies()
+            .any(|c| c.name() == "actix-session"));
+    }
+
     #[actix_rt::test]
     async fn cookie_session_extractor() {
         let mut app = test::init_service(