Bugfix: Allow getting details of a DAG with null start_date (REST API) (#13959)

airflow/api_connexion/openapi/v1.yaml

@@ -1937,6 +1937,7 @@ components:
               type: string
               format: 'date-time'
               readOnly: true
+              nullable: true
             dag_run_timeout:
               nullable: true
               $ref: '#/components/schemas/TimeDelta'

tests/api_connexion/endpoints/test_dag_endpoint.py

@@ -42,6 +42,7 @@ class TestDagEndpoint(unittest.TestCase):
     dag_id = "test_dag"
     task_id = "op1"
     dag2_id = "test_dag2"
+    dag3_id = "test_dag3"
 
     @staticmethod
     def clean_db():
@@ -80,10 +81,16 @@ class TestDagEndpoint(unittest.TestCase):
         with DAG(cls.dag2_id, start_date=datetime(2020, 6, 15)) as dag2:  # no doc_md
             DummyOperator(task_id=cls.task_id)
 
+        with DAG(cls.dag3_id) as dag3:  # DAG start_date set to None
+            DummyOperator(task_id=cls.task_id, start_date=datetime(2019, 6, 12))
+
         cls.dag = dag  # type:ignore
         cls.dag2 = dag2  # type: ignore
+        cls.dag3 = dag3  # tupe: ignore
+
         dag_bag = DagBag(os.devnull, include_examples=False)
-        dag_bag.dags = {dag.dag_id: dag, dag2.dag_id: dag2}
+        dag_bag.dags = {dag.dag_id: dag, dag2.dag_id: dag2, dag3.dag_id: dag3}
+
         cls.app.dag_bag = dag_bag  # type:ignore
 
     @classmethod
@@ -251,6 +258,38 @@ class TestGetDagDetails(TestDagEndpoint):
         }
         assert response.json == expected
 
+    def test_should_response_200_for_null_start_date(self):
+        response = self.client.get(
+            f"/api/v1/dags/{self.dag3_id}/details", environ_overrides={'REMOTE_USER': "test"}
+        )
+        assert response.status_code == 200
+        expected = {
+            "catchup": True,
+            "concurrency": 16,
+            "dag_id": "test_dag3",
+            "dag_run_timeout": None,
+            "default_view": "tree",
+            "description": None,
+            "doc_md": None,
+            "fileloc": __file__,
+            "file_token": FILE_TOKEN,
+            "is_paused": None,
+            "is_subdag": False,
+            "orientation": "LR",
+            "owners": [],
+            "params": {},
+            "schedule_interval": {
+                "__type": "TimeDelta",
+                "days": 1,
+                "microseconds": 0,
+                "seconds": 0,
+            },
+            "start_date": None,
+            "tags": None,
+            "timezone": "Timezone('UTC')",
+        }
+        assert response.json == expected
+
     def test_should_respond_200_serialized(self):
         # Create empty app with empty dagbag to check if DAG is read from db
         with conf_vars({("api", "auth_backend"): "tests.test_utils.remote_user_api_auth_backend"}):