feat: idempotent handlers, deterministic activity ids

crates/adapters/image-converter/src/handler.rs

@@ -45,6 +45,17 @@ impl EventHandler for ImageConversionHandler {
             return Ok(());
         }
 
+        // `get` falls back to `{key}.webp` / `{key}.avif`, which would re-encode
+        // an already converted image on redelivery. Probe the exact key first:
+        // once the original is gone, an earlier delivery finished the conversion.
+        match self.storage.get_stream(&key).await {
+            Ok(_) => {}
+            Err(DomainError::NotFound(_)) => {
+                tracing::debug!("original {key} already converted, skipping");
+                return Ok(());
+            }
+            Err(e) => return Err(e),
+        }
         let bytes = self.storage.get(&key).await?;
         let format = self.format;
 

crates/adapters/image-converter/tests/conversion_handler_test.rs

@@ -162,3 +162,33 @@ async fn converts_jpeg_to_webp_and_swaps_key() {
     );
     assert!(storage.get("avatars/u1.webp").await.is_ok());
 }
+
+#[tokio::test]
+async fn image_conversion_is_idempotent_when_run_twice() {
+    for format in [Format::Webp, Format::Avif] {
+        let storage = in_memory_storage();
+        storage.store("posters/m1", &tiny_jpeg()).await.unwrap();
+        let image_ref = MockImageRef::new();
+        let handler = ImageConversionHandler::new(
+            Arc::clone(&storage) as Arc<dyn ObjectStorage>,
+            Arc::clone(&image_ref) as Arc<dyn ImageRefCommand>,
+            format,
+        );
+        let event = DomainEvent::ImageStored {
+            key: "posters/m1".into(),
+        };
+        let new_key = format!("posters/m1{}", format.extension());
+
+        handler.handle(&event).await.unwrap();
+        let converted = storage.get(&new_key).await.unwrap();
+        // Redelivery after the original was swapped and deleted.
+        handler.handle(&event).await.unwrap();
+
+        assert_eq!(image_ref.swaps().len(), 1, "one swap");
+        assert_eq!(
+            storage.get(&new_key).await.unwrap(),
+            converted,
+            "converted bytes are not re-encoded"
+        );
+    }
+}