# Buggy code
def _change_state_for_tasks_failed_to_execute(self, session: Session = None):
    """
    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,
            # The TI.try_number will return raw try_number+1 since the
            # ti is not running. And we need to -1 to match the DB record.
            TI._try_number == try_number - 1,  # pylint: disable=protected-access
            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()
    if not tis_to_set_to_scheduled:
        return

    # set TIs to queued state
    filter_for_tis = TI.filter_for_tis(tis_to_set_to_scheduled)
    session.query(TI).filter(filter_for_tis).update(
        {TI.state: State.SCHEDULED, TI.queued_dttm: None}, synchronize_session=False
    )

    for task_instance in tis_to_set_to_scheduled:
        self.executor.queued_tasks.pop(task_instance.key)

    task_instance_str = "\n\t".join(repr(x) for x in tis_to_set_to_scheduled)
    self.log.info(
        "Set the following tasks to scheduled state:\n\t%s", task_instance_str
    )


# Traceback (_mysql_exceptions.OperationalError)
[2020-10-26 09:03:54,608] {{settings.py:49}} INFO - Configured default timezone Timezone('UTC')
[2020-10-26 09:04:05,467] {{scheduler_job.py:1327}} ERROR - Exception when executing SchedulerJob._run_scheduler_loop
Traceback (most recent call last):
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1277, in _execute_context
self.dialect.do_execute(
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/default.py", line 593, in do_execute
cursor.execute(statement, parameters)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 255, in execute
self.errorhandler(self, exc, value)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/connections.py", line 50, in defaulterrorhandler
raise errorvalue
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 252, in execute
res = self._query(query)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 379, in _query
self._do_get_result(db)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 182, in _do_get_result
self._result = result = self._get_result()
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 411, in _get_result
return self._get_db().store_result()
_mysql_exceptions.OperationalError: (1213, 'Deadlock found when trying to get lock; try restarting transaction')

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1308, in _execute
self._run_scheduler_loop()
File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1379, in _run_scheduler_loop
num_queued_tis = self._do_scheduling(session)
File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1451, in _do_scheduling
self._create_dag_runs(query.all(), session)
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3341, in all
return list(self)
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3503, in __iter__
return self._execute_and_instances(context)
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3528, in _execute_and_instances
result = conn.execute(querycontext.statement, self._params)
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1014, in execute
return meth(self, multiparams, params)
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/sql/elements.py", line 298, in _execute_on_connection
return connection._execute_clauseelement(self, multiparams, params)
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1127, in _execute_clauseelement
ret = self._execute_context(
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1317, in _execute_context
self._handle_dbapi_exception(
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1511, in _handle_dbapi_exception
util.raise_(
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/util/compat.py", line 178, in raise_
raise exception
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1277, in _execute_context
self.dialect.do_execute(
File "/home/airflow/.local/lib/python3.8/site-packages/sqlalchemy/engine/default.py", line 593, in do_execute
cursor.execute(statement, parameters)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 255, in execute
self.errorhandler(self, exc, value)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/connections.py", line 50, in defaulterrorhandler
raise errorvalue
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 252, in execute
res = self._query(query)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 379, in _query
self._do_get_result(db)
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 182, in _do_get_result
self._result = result = self._get_result()
File "/home/airflow/.local/lib/python3.8/site-packages/MySQLdb/cursors.py", line 411, in _get_result
return self._get_db().store_result()
sqlalchemy.exc.OperationalError: (_mysql_exceptions.OperationalError) (1213, 'Deadlock found when trying to get lock; try restarting transaction')
[SQL: SELECT dag.dag_id AS dag_dag_id, dag.root_dag_id AS dag_root_dag_id, dag.is_paused AS dag_is_paused, dag.is_subdag AS dag_is_subdag, dag.is_active AS dag_is_active, dag.last_scheduler_run AS dag_last_scheduler_run, dag.last_pickled AS dag_last_pickled, dag.last_expired AS dag_last_expired, dag.scheduler_lock AS dag_scheduler_lock, dag.pickle_id AS dag_pickle_id, dag.fileloc AS dag_fileloc, dag.owners AS dag_owners, dag.description AS dag_description, dag.default_view AS dag_default_view, dag.schedule_interval AS dag_schedule_interval, dag.concurrency AS dag_concurrency, dag.has_task_concurrency_limits AS dag_has_task_concurrency_limits, dag.next_dagrun AS dag_next_dagrun, dag.next_dagrun_create_after AS dag_next_dagrun_create_after
FROM dag
WHERE dag.is_paused IS false AND dag.is_active IS true AND dag.next_dagrun_create_after <= now() ORDER BY dag.next_dagrun_create_after
LIMIT %s FOR UPDATE]
[parameters: (10,)]
(Background on this error at: http://sqlalche.me/e/13/e3q8)
[2020-10-26 09:04:06,512] {{process_utils.py:102}} INFO - Sending Signals.SIGTERM to GPID 7437
[2020-10-26 09:04:07,029] {{process_utils.py:68}} INFO - Process psutil.Process(pid=7762, status='terminated', started='09:04:05') (7762) terminated with exit code None
[2020-10-26 09:04:07,122] {{process_utils.py:219}} INFO - Waiting up to 5 seconds for processes to exit...
[2020-10-26 09:04:07,126] {{process_utils.py:68}} INFO - Process psutil.Process(pid=7774, status='terminated', started='09:04:05') (7774) terminated with exit code None
[2020-10-26 09:04:07,128] {{process_utils.py:68}} INFO - Process psutil.Process(pid=7437, status='terminated', exitcode=0, started='09:03:54') (7437) terminated with exit code 0
[2020-10-26 09:04:07,128] {{process_utils.py:68}} INFO - Process psutil.Process(pid=7775, status='terminated', started='09:04:05') (7775) terminated with exit code None
[2020-10-26 09:04:07,129] {{process_utils.py:68}} INFO - Process psutil.Process(pid=7773, status='terminated', started='09:04:05') (7773) terminated with exit code None
[2020-10-26 09:04:07,129] {{process_utils.py:68}} INFO - Process psutil.Process(pid=7782, status='terminated', started='09:04:05') (7782) terminated with exit code None

# Fixed code
def _change_state_for_tasks_failed_to_execute(self, session: Session = None):
    """
    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,
            # The TI.try_number will return raw try_number+1 since the
            # ti is not running. And we need to -1 to match the DB record.
            TI._try_number == try_number - 1,  # pylint: disable=protected-access
            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, session=session).all()
    if not tis_to_set_to_scheduled:
        return

    # set TIs to queued state
    filter_for_tis = TI.filter_for_tis(tis_to_set_to_scheduled)
    session.query(TI).filter(filter_for_tis).update(
        {TI.state: State.SCHEDULED, TI.queued_dttm: None}, synchronize_session=False
    )

    for task_instance in tis_to_set_to_scheduled:
        self.executor.queued_tasks.pop(task_instance.key)

    task_instance_str = "\n\t".join(repr(x) for x in tis_to_set_to_scheduled)
    self.log.info(
        "Set the following tasks to scheduled state:\n\t%s", task_instance_str
    )