feat: support redis xstream (#32586)

api/services/app_generate_service.py

@@ -38,6 +38,13 @@ if TYPE_CHECKING:
 class AppGenerateService:
     @staticmethod
     def _build_streaming_task_on_subscribe(start_task: Callable[[], None]) -> Callable[[], None]:
+        """
+        Build a subscription callback that coordinates when the background task starts.
+
+        - streams transport: start immediately (events are durable; late subscribers can replay).
+        - pubsub/sharded transport: start on first subscribe, with a short fallback timer so the task
+          still runs if the client never connects.
+        """
         started = False
         lock = threading.Lock()
 
@@ -54,10 +61,18 @@ class AppGenerateService:
                 started = True
                 return True
 
-        # XXX(QuantumGhost): dirty hacks to avoid a race between publisher and SSE subscriber.
-        # The Celery task may publish the first event before the API side actually subscribes,
-        # causing an "at most once" drop with Redis Pub/Sub. We start the task on subscribe,
-        # but also use a short fallback timer so the task still runs if the client never consumes.
+        channel_type = dify_config.PUBSUB_REDIS_CHANNEL_TYPE
+        if channel_type == "streams":
+            # With Redis Streams, we can safely start right away; consumers can read past events.
+            _try_start()
+
+            # Keep return type Callable[[], None] consistent while allowing an extra (no-op) call.
+            def _on_subscribe_streams() -> None:
+                _try_start()
+
+            return _on_subscribe_streams
+
+        # Pub/Sub modes (at-most-once): subscribe-gated start with a tiny fallback.
         timer = threading.Timer(SSE_TASK_START_FALLBACK_MS / 1000.0, _try_start)
         timer.daemon = True
         timer.start()