A local call waiting for a slot is reached by a stop, a delete or a cancel and ends without running; what a call emits after its flow was stopped is dropped as its result is; a node run by hand whose flow is stopped or deleted tells its caller but publishes no node_error; cancelling a node also ends its calls on workers

backend/tests/flow/test_workers.py

@@ -176,6 +176,40 @@ def test_a_running_node_can_be_cancelled(pool):
         stopper.join()
 
 
+def test_a_call_waiting_for_a_slot_is_cancelled_before_it_runs(pool):
+    """A cancel missed a call queued for a slot, so it took the slot that
+    cancelling the node ahead of it freed, and ran."""
+    outcome: dict[str, object] = {}
+
+    def call(node: str, code: str) -> None:
+        try:
+            outcome[node] = pool.run("demo", node, code, {}, f"demo.{node}", 30)
+        except NodeCancelled as exc:
+            outcome[node] = exc
+
+    sleep = "import time\n\n\ndef process():\n    time.sleep(30)\n"
+    ahead = threading.Thread(target=call, args=("ahead", sleep), daemon=True)
+    ahead.start()
+    deadline = time.monotonic() + 10
+    while ("", "demo.ahead") not in pool._running:
+        assert time.monotonic() < deadline, "the first call never started"
+        time.sleep(0.01)
+    queued = threading.Thread(
+        target=call, args=("queued", "def process():\n    return 1\n"), daemon=True
+    )
+    queued.start()
+    deadline = time.monotonic() + 5
+    while not pool.cancel("demo.queued"):
+        assert time.monotonic() < deadline, "the queued call was not found"
+        time.sleep(0.01)
+
+    assert pool.cancel("demo.ahead")
+    ahead.join(timeout=10)
+    queued.join(timeout=10)
+    assert isinstance(outcome["queued"], NodeCancelled)
+    assert "waited" in str(outcome["queued"])
+
+
 def test_a_result_that_is_not_json_is_refused(pool):
     with pytest.raises(Exception, match="cannot be sent back as JSON"):
         run(pool, "def process():\n    return {'out': {1, 2}}\n")