feat: idempotent handlers, deterministic activity ids

crates/adapters/activitypub/src/urls.rs

@@ -38,3 +38,34 @@ pub fn watchlist_entry_url(
     ))
     .expect("base_url is always a valid URL prefix")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// k-ap derives the Create activity id from the object id (UUIDv5), so
+    /// re-handling an event only re-sends the same activity id if our object
+    /// ids are pure functions of the entity. Update/Delete/Move ids are minted
+    /// randomly inside k-ap 0.5 with no override (see ADR-0015): those repeat
+    /// semantically idempotent activities under a fresh id.
+    #[test]
+    fn activitypub_object_ids_are_stable_across_event_redelivery() {
+        let instance = InstanceIdentity::new("https://diary.example");
+        let review = ReviewId::from_uuid(uuid::Uuid::new_v4());
+        let user = uuid::Uuid::new_v4();
+        let movie = uuid::Uuid::new_v4();
+
+        assert_eq!(
+            review_url(&instance, &review),
+            review_url(&instance, &review)
+        );
+        assert_eq!(
+            goal_url(&instance, user, 2026),
+            goal_url(&instance, user, 2026)
+        );
+        assert_eq!(
+            watchlist_entry_url(&instance, user, movie),
+            watchlist_entry_url(&instance, user, movie)
+        );
+    }
+}