fix(tests): fail loudly if cost model already updated

cardano_node_tests/tests/data/cost_models_list_185_297_v2_v3.json

@@ -409,7 +409,7 @@
         7305,
         -900,
         1716,
-        549,
+        960,
         57,
         85848,
         0,
@@ -419,9 +419,9 @@
         42921,
         4,
         2,
-        24548,
-        29498,
-        38,
+        30623,
+        28755,
+        75,
         1,
         898148,
         27279,
@@ -474,7 +474,7 @@
         7305,
         -900,
         1716,
-        549,
+        960,
         57,
         85848,
         0,
@@ -490,7 +490,7 @@
         7305,
         -900,
         1716,
-        549,
+        960,
         57,
         85848,
         0,
@@ -501,7 +501,7 @@
         7305,
         -900,
         1716,
-        549,
+        960,
         57,
         85848,
         0,
@@ -651,6 +651,59 @@
         1,
         1964219,
         24520,
-        3
+        3,
+        607153,
+        231697,
+        53144,
+        0,
+        1,
+        116711,
+        1957,
+        4,
+        231883,
+        10,
+        1000,
+        24838,
+        7,
+        1,
+        232010,
+        32,
+        321837444,
+        25087669,
+        18,
+        617887431,
+        67302824,
+        36,
+        356924,
+        18413,
+        45,
+        21,
+        219951,
+        9444,
+        1,
+        1000,
+        172116,
+        183150,
+        6,
+        24,
+        21,
+        213283,
+        618401,
+        1998,
+        28258,
+        1,
+        1000,
+        38159,
+        2,
+        22,
+        1000,
+        95933,
+        1,
+        1,
+        11,
+        1000,
+        277577,
+        12,
+        21
     ]
 }

cardano_node_tests/tests/test_node_upgrade.py

@@ -132,6 +132,7 @@ def test_update_cost_models(
         Test updating Plutus cost models after node upgrade. Runs only on step 2 of upgrade
         testing sequence.
 
+        * Check that the cost model update is actually needed at this point
         * Load cost model proposal from JSON file (PlutusV2 and PlutusV3 models)
         * Get default governance data (DReps, committee members, pools)
         * Submit cost model update governance action
@@ -142,6 +143,19 @@ def test_update_cost_models(
         cluster = cluster_singleton
         temp_template = common.get_test_id(cluster)
         cost_proposal_file = DATA_DIR / "cost_models_list_185_297_v2_v3.json"
+        cost_models_to_check = ("PlutusV2", "PlutusV3")
+
+        # This step is expected to run against a cost model that is still short. If it's
+        # already at the proposed length, the "cost model not yet updated" scenario was
+        # missed, e.g. the cluster started with it already updated.
+        assert conway_common.is_cost_model_update_needed(
+            cluster_obj=cluster,
+            cost_proposal_file=cost_proposal_file,
+            cost_models_to_check=cost_models_to_check,
+        ), (
+            "Cost model(s) already at or above the proposed length before the update. "
+            "The 'cost model not yet updated' test scenario would be missed."
+        )
 
         governance_data = governance_setup.get_default_governance(
             cluster_manager=cluster_manager, cluster_obj=cluster

cardano_node_tests/tests/tests_conway/conway_common.py

@@ -510,6 +510,31 @@ def propose_pparams_update(
     )
 
 
+def is_cost_model_update_needed(
+    cluster_obj: clusterlib.ClusterLib,
+    cost_proposal_file: pl.Path,
+    cost_models_to_check: tp.Sequence[str],
+) -> bool:
+    """Check whether any of `cost_models_to_check` is still below its proposed length.
+
+    `cost_proposal_file` can bundle cost models that aren't the actual subject of the
+    calling test, e.g. carried along unchanged because the CLI needs a full file. Only
+    `cost_models_to_check` is compared, other cost models in the file are ignored.
+
+    Returns:
+        True if at least one of `cost_models_to_check` is below its proposed length, i.e.
+        an update is still needed. False if all of them are already at or above it.
+    """
+    with open(cost_proposal_file, encoding="utf-8") as fp:
+        cost_models_in = json.load(fp)
+
+    live_cost_models = cluster_obj.g_query.get_protocol_params()["costModels"]
+    return any(
+        len(live_cost_models.get(name, [])) < len(cost_models_in[name])
+        for name in cost_models_to_check
+    )
+
+
 def update_cost_model(
     cluster_obj: clusterlib.ClusterLib,
     name_template: str,