A batch run fails a node with no value on a triggering input instead of calling it, naming the input and its producer, and skips what waits on it

backend/tests/api/routes/test_runs.py

@@ -780,6 +780,54 @@ def test_a_failed_stage_skips_what_waits_on_it_and_fails_the_run(tmp_path):
             session.commit()
 
 
+def test_a_stage_with_nothing_on_an_input_fails_instead_of_being_called(tmp_path):
+    """The producer succeeded but published nothing on the port it waits on."""
+    model, score, report = (
+        MessageSpec(name=n, dtype=DType.FLOAT) for n in ("model", "score", "report")
+    )
+    flow = FlowDef(
+        name="split",
+        mode="batch",
+        nodes=[
+            NodeDef(id="fit", provides=[model, score]),
+            NodeDef(id="readout", requires=[score], provides=[report]),
+            NodeDef(id="evaluate", requires=[report]),
+        ],
+    )
+    store = FlowStore(tmp_path / "flows")
+    store.write_flow(flow)
+    store.write_node_source(
+        "split", "fit", "def process():\n    return {'model': 1.0}\n"
+    )
+    store.write_node_source(
+        "split", "readout", "def process(score):\n    return {'report': score}\n"
+    )
+    store.write_node_source("split", "evaluate", "def process(report):\n    pass\n")
+
+    service = RunService(controller=FlowController(store), queue=_Collect())
+    run_id = service.submit("split", {}).id
+    try:
+        service._drive(run_id)
+        with Session(db_engine) as session:
+            assert session.get(Run, run_id).status == "error"
+            rows = {
+                row.node: row
+                for row in session.exec(select(RunNode).where(RunNode.run_id == run_id))
+            }
+            assert {node: row.status for node, row in rows.items()} == {
+                "split.fit": "ok",
+                "split.readout": "error",
+                "split.evaluate": "skipped",
+            }
+            assert rows["split.readout"].error == "No value on input 'score' from 'fit'"
+    finally:
+        with Session(db_engine) as session:
+            for node in session.exec(select(RunNode).where(RunNode.run_id == run_id)):
+                session.delete(node)
+            session.delete(session.get(Run, run_id))
+            session.commit()
+
+
 def test_a_retry_is_a_new_run_that_names_the_one_it_repeats():
     """The way back from a run an engine restart interrupted.