Ensures DAG params order regardless of backend (#40156)

* Ensures DAG params order regardless of backend

Fixes #40154

This change adds an extra attribute to the serialized DAG param objects which helps us decide
the order of the deserialized params dictionary later even if the backend messes with us.

I decided not to limit this just to MySQL since the operation is inexpensive and may turn
out to be helpful.

I made sure the new test fails with the old implementation + MySQL. I assume this test will be
executed with MySQL somewhere in the build actions?

* Removes GitHub reference

Co-authored-by: Jed Cunningham <66968678+jedcunningham@users.noreply.github.com>

* Serialize DAG params as array of tuples to ensure ordering

Alternative to previous approach: We serialize the DAG params dict as a list of tuples which _should_ keep their ordering regardless of backend.

Backwards compatibility is ensured because if `encoded_params` is a `dict` (not the expected `list`) then `dict(encoded_params)` still works.

* Make backwards compatibility more explicit

Based on suggestions by @uranusjr with an additional fix to make mypy happy.

---------

Co-authored-by: Jed Cunningham <66968678+jedcunningham@users.noreply.github.com>
(cherry picked from commit 2149b4d)

Author: Usiel

airflow/serialization/schema.json

@@ -136,7 +136,7 @@
     "dag": {
       "type": "object",
       "properties": {
-        "params": { "$ref": "#/definitions/params_dict" },
+        "params": { "$ref": "#/definitions/params" },
         "_dag_id": { "type": "string" },
         "tasks": {  "$ref": "#/definitions/tasks" },
         "timezone": { "$ref": "#/definitions/timezone" },
@@ -206,9 +206,13 @@
       "type": "array",
       "additionalProperties": { "$ref": "#/definitions/operator" }
     },
-    "params_dict": {
-      "type": "object",
-      "additionalProperties": {"$ref": "#/definitions/param" }
+    "params": {
+      "type": "array",
+      "prefixItems": [
+          { "type": "string" },
+          { "$ref": "#/definitions/param" }
+      ],
+      "unevaluatedItems": false
     },
     "param": {
       "$comment": "A param for a dag / operator",
@@ -258,7 +262,7 @@
         "retry_delay": { "$ref": "#/definitions/timedelta" },
         "retry_exponential_backoff": { "type": "boolean" },
         "max_retry_delay": { "$ref": "#/definitions/timedelta" },
-        "params": { "$ref": "#/definitions/params_dict" },
+        "params": { "$ref": "#/definitions/params" },
         "priority_weight": { "type": "number" },
         "weight_rule": { "type": "string" },
         "executor_config": { "$ref": "#/definitions/dict" },

airflow/serialization/serialized_objects.py

@@ -777,17 +777,17 @@ def is_serialized(val):
         return class_(**kwargs)
 
     @classmethod
-    def _serialize_params_dict(cls, params: ParamsDict | dict):
-        """Serialize Params dict for a DAG or task."""
-        serialized_params = {}
+    def _serialize_params_dict(cls, params: ParamsDict | dict) -> list[tuple[str, dict]]:
+        """Serialize Params dict for a DAG or task as a list of tuples to ensure ordering."""
+        serialized_params = []
         for k, v in params.items():
             # TODO: As of now, we would allow serialization of params which are of type Param only.
             try:
                 class_identity = f"{v.__module__}.{v.__class__.__name__}"
             except AttributeError:
                 class_identity = ""
             if class_identity == "airflow.models.param.Param":
-                serialized_params[k] = cls._serialize_param(v)
+                serialized_params.append((k, cls._serialize_param(v)))
             else:
                 raise ValueError(
                     f"Params to a DAG or a Task can be only of type airflow.models.param.Param, "
@@ -796,10 +796,16 @@ def _serialize_params_dict(cls, params: ParamsDict | dict):
         return serialized_params
 
     @classmethod
-    def _deserialize_params_dict(cls, encoded_params: dict) -> ParamsDict:
+    def _deserialize_params_dict(cls, encoded_params: list[tuple[str, dict]]) -> ParamsDict:
         """Deserialize a DAG's Params dict."""
+        if isinstance(encoded_params, collections.abc.Mapping):
+            # in 2.9.2 or earlier params were serialized as JSON objects
+            encoded_param_pairs: Iterable[tuple[str, dict]] = encoded_params.items()
+        else:
+            encoded_param_pairs = encoded_params
+
         op_params = {}
-        for k, v in encoded_params.items():
+        for k, v in encoded_param_pairs:
             if isinstance(v, dict) and "__class" in v:
                 op_params[k] = cls._deserialize_param(v)
             else:

tests/models/test_serialized_dag.py

@@ -206,6 +206,22 @@ def test_get_dag_dependencies_default_to_empty(self, dag_dependencies_fields):
         expected_dependencies = {dag_id: [] for dag_id in example_dags}
         assert SDM.get_dag_dependencies() == expected_dependencies
 
+    def test_order_of_dag_params_is_stable(self):
+        """
+        This asserts that we have logic in place which guarantees the order
+        of the params is maintained - even if the backend (e.g. MySQL) mutates
+        the serialized DAG JSON.
+        """
+        example_dags = make_example_dags(example_dags_module)
+        example_params_trigger_ui = example_dags.get("example_params_trigger_ui")
+        before = list(example_params_trigger_ui.params.keys())
+
+        SDM.write_dag(example_params_trigger_ui)
+        retrieved_dag = SDM.get_dag("example_params_trigger_ui")
+        after = list(retrieved_dag.params.keys())
+
+        assert before == after
+
     def test_order_of_deps_is_consistent(self):
         """
         Previously the 'dag_dependencies' node in serialized dag was converted to list from set.

tests/serialization/test_dag_serialization.py

@@ -232,7 +232,7 @@ def detect_task_dependencies(task: Operator) -> DagDependency | None:  # type: i
         },
         "edge_info": {},
         "dag_dependencies": [],
-        "params": {},
+        "params": [],
     },
 }
 
@@ -2034,6 +2034,25 @@ def test_params_upgrade(self):
         assert isinstance(dag.params.get_param("none"), Param)
         assert dag.params["str"] == "str"
 
+    def test_params_serialization_from_dict_upgrade(self):
+        """In <=2.9.2 params were serialized as a JSON object instead of a list of key-value pairs.
+        This test asserts that the params are still deserialized properly."""
+        serialized = {
+            "__version": 1,
+            "dag": {
+                "_dag_id": "simple_dag",
+                "fileloc": "/path/to/file.py",
+                "tasks": [],
+                "timezone": "UTC",
+                "params": {"my_param": {"__class": "airflow.models.param.Param", "default": "str"}},
+            },
+        }
+        dag = SerializedDAG.from_dict(serialized)
+
+        param = dag.params.get_param("my_param")
+        assert isinstance(param, Param)
+        assert param.value == "str"
+
     def test_params_serialize_default_2_2_0(self):
         """In 2.0.0, param ``default`` was assumed to be json-serializable objects and were not run though
         the standard serializer function.  In 2.2.2 we serialize param ``default``.  We keep this
@@ -2045,7 +2064,7 @@ def test_params_serialize_default_2_2_0(self):
                 "fileloc": "/path/to/file.py",
                 "tasks": [],
                 "timezone": "UTC",
-                "params": {"str": {"__class": "airflow.models.param.Param", "default": "str"}},
+                "params": [["str", {"__class": "airflow.models.param.Param", "default": "str"}]],
             },
         }
         SerializedDAG.validate_schema(serialized)
@@ -2062,14 +2081,17 @@ def test_params_serialize_default(self):
                 "fileloc": "/path/to/file.py",
                 "tasks": [],
                 "timezone": "UTC",
-                "params": {
-                    "my_param": {
-                        "default": "a string value",
-                        "description": "hello",
-                        "schema": {"__var": {"type": "string"}, "__type": "dict"},
-                        "__class": "airflow.models.param.Param",
-                    }
-                },
+                "params": [
+                    [
+                        "my_param",
+                        {
+                            "default": "a string value",
+                            "description": "hello",
+                            "schema": {"__var": {"type": "string"}, "__type": "dict"},
+                            "__class": "airflow.models.param.Param",
+                        },
+                    ]
+                ],
             },
         }
         SerializedDAG.validate_schema(serialized)