If you have a DAG with `max_active_runs=1` and it is running some tasks, and then you manually trigger another run of this DAG, both runs end up not scheduling anything and you see this message over and over in the logs:

```
[2020-10-16 14:00:34,118]  2437572 {{airflow.jobs.scheduler_job.SchedulerJob scheduler_job.py:1657}} INFO - DAG scenario1_case2_01_1 already has 1 active runs, not queuing any more tasks
```

A longer term fix for this would be to introduce a "queued" state for DagRuns, and then when manually creating dag runs set it to queued, and only have the scheduler set DagRuns to running.

That may or may not be less work than fixing the current mechanism.
airflow/jobs/scheduler_job.py
"""
        If there are tasks left over in the executor,
        we set them back to SCHEDULED to avoid creating hanging tasks.
        :param session: session for ORM operations
        """
        if not self.executor.queued_tasks:
            return
        filter_for_ti_state_change = (
            [and_(
                TI.dag_id == dag_id,
                TI.task_id == task_id,
                TI.execution_date == execution_date,
                
                
                TI._try_number == try_number - 1,  
                TI.state == State.QUEUED)
                for dag_id, task_id, execution_date, try_number
                in self.executor.queued_tasks.keys()])
        ti_query = session.query(TI).filter(or_(*filter_for_ti_state_change))
        tis_to_set_to_scheduled: List[TI] = with_row_locks(ti_query).all()