The test fails occasionally with the below stacktrace, so I am marking this as Quarantined.

```
  _______________ TestSchedulerJob.test_scheduler_verify_pool_full _______________
  
  self = <tests.jobs.test_scheduler_job.TestSchedulerJob object at 0x7fbaaaba0f40>
  
      def test_scheduler_verify_pool_full(self):
          """
          Test task instances not queued when pool is full
          """
          dag = DAG(dag_id='test_scheduler_verify_pool_full', start_date=DEFAULT_DATE)
      
          BashOperator(
              task_id='dummy',
              dag=dag,
              owner='airflow',
              pool='test_scheduler_verify_pool_full',
              bash_command='echo hi',
          )
      
          dagbag = DagBag(
              dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"),
              include_examples=False,
              read_dags_from_db=True,
          )
          dagbag.bag_dag(dag=dag, root_dag=dag)
          dagbag.sync_to_db()
      
          session = settings.Session()
          pool = Pool(pool='test_scheduler_verify_pool_full', slots=1)
          session.add(pool)
          session.flush()
      
          dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
          SerializedDagModel.write_dag(dag)
      
          self.scheduler_job = SchedulerJob(executor=self.null_exec)
          self.scheduler_job.processor_agent = mock.MagicMock()
      
          # Create 2 dagruns, which will create 2 task instances.
          dr = dag.create_dagrun(
              run_type=DagRunType.SCHEDULED,
              execution_date=DEFAULT_DATE,
              state=State.RUNNING,
          )
  >       self.scheduler_job._schedule_dag_run(dr, session)
  
  tests/jobs/test_scheduler_job.py:2108: 
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  airflow/jobs/scheduler_job.py:1020: in _schedule_dag_run
      dag = dag_run.dag = self.dagbag.get_dag(dag_run.dag_id, session=session)
  airflow/utils/session.py:67: in wrapper
      return func(*args, **kwargs)
  airflow/models/dagbag.py:186: in get_dag
      self._add_dag_from_db(dag_id=dag_id, session=session)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  self = <airflow.models.dagbag.DagBag object at 0x7fbaaab95100>
  dag_id = 'test_scheduler_verify_pool_full'
  session = <sqlalchemy.orm.session.Session object at 0x7fbaaad9d0a0>
  
      def _add_dag_from_db(self, dag_id: str, session: Session):
          """Add DAG to DagBag from DB"""
          from airflow.models.serialized_dag import SerializedDagModel
      
          row = SerializedDagModel.get(dag_id, session)
          if not row:
  >           raise SerializedDagNotFound(f"DAG '{dag_id}' not found in serialized_dag table")
  E           airflow.exceptions.SerializedDagNotFound: DAG 'test_scheduler_verify_pool_full' not found in serialized_dag table
```
tests/jobs/test_scheduler_job.py
)
        
        self.scheduler_job.processor_agent.send_callback_to_execute.assert_called_once_with(expected_callback)
        
        
        
        real_dag = self.scheduler_job.dagbag.get_dag(dag.dag_id)
        self.scheduler_job._send_sla_callbacks_to_processor.assert_called_once_with(real_dag)
        session.rollback()
        session.close()
    def test_dagrun_callbacks_commited_before_sent(self, dag_maker):
        """
        Tests that before any callbacks are sent to the processor, the session is committed. This ensures
        that the dagrun details are up to date when the callbacks are run.
        """
        with dag_maker(dag_id='test_dagrun_callbacks_commited_before_sent'):
            DummyOperator(task_id='dummy')
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        self.scheduler_job.processor_agent = mock.Mock()
        self.scheduler_job._send_dag_callbacks_to_processor = mock.Mock()
        self.scheduler_job._schedule_dag_run = mock.Mock()
        dr = dag_maker.create_dagrun()
        session = settings.Session()
        ti = dr.get_task_instance('dummy')
        ti.set_state(State.SUCCESS, session)
        with mock.patch.object(settings, "USE_JOB_SCHEDULE", False), mock.patch(
            "airflow.jobs.scheduler_job.prohibit_commit"
        ) as mock_gaurd:
            mock_gaurd.return_value.__enter__.return_value.commit.side_effect = session.commit
            def mock_schedule_dag_run(*args, **kwargs):