fluksio.arrived(): a node can tell which inputs the wave behind it wrote, so a value written again unchanged is seen

worker/fluksio_worker/worker_main.py

@@ -83,6 +83,8 @@ CHUNK = 1024 * 1024
 _RPC: Any = None
 #: The call being served, so an event can say which one it belongs to.
 _CALL_ID = ""
+#: The input ports the wave behind this call wrote, or None if it cannot say.
+_ARRIVED: list[str] | None = None
 
 
 def _emit(event: dict[str, Any]) -> None:
@@ -116,6 +118,16 @@ class _Reporter(ModuleType):
         """
         _emit({"event": "emit", "outputs": dict(ports)})
 
+    def arrived(self) -> set[str] | None:
+        """The input ports the message that started this run wrote.
+
+        The other inputs still hold what they held before. A value written
+        again unchanged is here too, which is how a node tells somebody
+        repeating a command from a value that only sits in state. None when
+        the engine cannot say, such as when the flow starts.
+        """
+        return None if _ARRIVED is None else set(_ARRIVED)
+
     def save_artifact(
         self,
         source: Any,
@@ -545,7 +557,7 @@ def _drain(generator: Any) -> Any:
 
 
 def main() -> None:
-    global _RPC, _CALL_ID
+    global _RPC, _CALL_ID, _ARRIVED
 
     _RPC = os.fdopen(os.dup(1), "w")
     # Everything the node writes to the real stdout now goes to the server log.
@@ -558,6 +570,7 @@ def main() -> None:
             continue
         request = json.loads(line)
         _CALL_ID = str(request.get("call_id") or "")
+        _ARRIVED = request.get("arrived")
         captured = _Capped()
         # ``id`` back untouched: the engine matches it against what it sent, so
         # a pipe that has slipped a call is caught rather than handing one node