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
#
#
#
#
import datetime
import itertools
import logging
import multiprocessing
import os
import signal
import sys
import threading
import time
from collections import defaultdict
from contextlib import ExitStack, redirect_stderr, redirect_stdout, suppress
from datetime import timedelta
from multiprocessing.connection import Connection as MultiprocessingConnection
from typing import Any, DefaultDict, Dict, Iterable, List, Optional, Set, Tuple
import tenacity
from setproctitle import setproctitle
from sqlalchemy import and_, func, not_, or_
from sqlalchemy.exc import OperationalError