# Buggy code
def sync_to_db(self, session: Optional[Session] = None):
    """Save attributes about list of DAG to the DB."""
    # To avoid circular import - airflow.models.dagbag -> airflow.models.dag -> airflow.models.dagbag
    from airflow.models.dag import DAG
    from airflow.models.serialized_dag import SerializedDagModel

    def _serialze_dag_capturing_errors(dag, session):
        """
        Try to serialize the dag to the DB, but make a note of any errors.

        We can't place them directly in import_errors, as this may be retried, and work the next time
        """
        if dag.is_subdag:
            return []
        try:
            # We cant use bulk_write_to_db as we want to capture each error individually
            SerializedDagModel.write_dag(
                dag,
                min_update_interval=settings.MIN_SERIALIZED_DAG_UPDATE_INTERVAL,
                session=session,
            )
            return []
        except OperationalError:
            raise
        except Exception:  # pylint: disable=broad-except
            return [
                (
                    dag.fileloc,
                    traceback.format_exc(
                        limit=-self.dagbag_import_error_traceback_depth
                    ),
                )
            ]

    # Retry 'DAG.bulk_write_to_db' & 'SerializedDagModel.bulk_sync_to_db' in case
    # of any Operational Errors
    # In case of failures, provide_session handles rollback
    for attempt in tenacity.Retrying(
        retry=tenacity.retry_if_exception_type(exception_types=OperationalError),
        wait=tenacity.wait_random_exponential(multiplier=0.5, max=5),
        stop=tenacity.stop_after_attempt(settings.MAX_DB_RETRIES),
        before_sleep=tenacity.before_sleep_log(self.log, logging.DEBUG),
        reraise=True,
    ):
        with attempt:
            serialize_errors = []
            self.log.debug(
                "Running dagbag.sync_to_db with retries. Try %d of %d",
                attempt.retry_state.attempt_number,
                settings.MAX_DB_RETRIES,
            )
            self.log.debug("Calling the DAG.bulk_sync_to_db method")
            try:
                # Write Serialized DAGs to DB, capturing errors
                for dag in self.dags.values():
                    serialize_errors.extend(
                        _serialze_dag_capturing_errors(dag, session)
                    )

                DAG.bulk_write_to_db(self.dags.values(), session=session)
            except OperationalError:
                session.rollback()
                raise
            # Only now we are "complete" do we update import_errors - don't want to record errors from
            # previous failed attempts
            self.import_errors.update(dict(serialize_errors))


# 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 sync_to_db(self, session: Optional[Session] = None):
    """Save attributes about list of DAG to the DB."""
    # To avoid circular import - airflow.models.dagbag -> airflow.models.dag -> airflow.models.dagbag
    from airflow.models.dag import DAG
    from airflow.models.serialized_dag import SerializedDagModel

    def _serialze_dag_capturing_errors(dag, session):
        """
        Try to serialize the dag to the DB, but make a note of any errors.

        We can't place them directly in import_errors, as this may be retried, and work the next time
        """
        if dag.is_subdag:
            return []
        try:
            # We cant use bulk_write_to_db as we want to capture each error individually
            SerializedDagModel.write_dag(
                dag,
                min_update_interval=settings.MIN_SERIALIZED_DAG_UPDATE_INTERVAL,
                session=session,
            )
            return []
        except OperationalError:
            raise
        except Exception:  # pylint: disable=broad-except
            return [
                (
                    dag.fileloc,
                    traceback.format_exc(
                        limit=-self.dagbag_import_error_traceback_depth
                    ),
                )
            ]

    # Retry 'DAG.bulk_write_to_db' & 'SerializedDagModel.bulk_sync_to_db' in case
    # of any Operational Errors
    # In case of failures, provide_session handles rollback
    for attempt in run_with_db_retries(logger=self.log):
        with attempt:
            serialize_errors = []
            self.log.debug(
                "Running dagbag.sync_to_db with retries. Try %d of %d",
                attempt.retry_state.attempt_number,
                settings.MAX_DB_RETRIES,
            )
            self.log.debug("Calling the DAG.bulk_sync_to_db method")
            try:
                # Write Serialized DAGs to DB, capturing errors
                for dag in self.dags.values():
                    serialize_errors.extend(
                        _serialze_dag_capturing_errors(dag, session)
                    )

                DAG.bulk_write_to_db(self.dags.values(), session=session)
            except OperationalError:
                session.rollback()
                raise
            # Only now we are "complete" do we update import_errors - don't want to record errors from
            # previous failed attempts
            self.import_errors.update(dict(serialize_errors))