I have just faced the same issue on OpenStack Ocata:

2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters [req-5670b1fa-afd0-4f30-a7c5-f3973fce2442 bcbadd6e37124fcabffeebd8cc186406 fb7bc775e86241f6b6807a06fd767c30 - default default] DB exception wrapped.
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters Traceback (most recent call last):
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     context)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/default.py", line 450, in do_execute
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     cursor.execute(statement, parameters)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/cursors.py", line 166, in execute
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     result = self._query(query)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/cursors.py", line 322, in _query
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     conn.query(q)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 835, in query
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     self._affected_rows = self._read_query_result(unbuffered=unbuffered)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 1019, in _read_query_result
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     result.read()
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 1302, in read
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     first_packet = self.connection._read_packet()
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 971, in _read_packet
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     recv_data = self._read_bytes(bytes_to_read)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 985, in _read_bytes
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters     self._sock.settimeout(self._read_timeout)
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters AttributeError: 'NoneType' object has no attribute 'settimeout'
2019-07-24 06:53:38.146 2329 ERROR oslo_db.sqlalchemy.exc_filters 
2019-07-24 06:53:38.147 2329 INFO nova.api.openstack.wsgi [req-4759ecc7-60fa-4ac0-8eff-3048981b854b 08e8ac57fb2447fe9c01e44d4baa423a 227a774fb9404585b854db2e6694ba63 - default default] HTTP exception thrown: Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
2019-07-24 06:53:38.148 2329 INFO nova.osapi_compute.wsgi.server [req-4759ecc7-60fa-4ac0-8eff-3048981b854b 08e8ac57fb2447fe9c01e44d4baa423a 227a774fb9404585b854db2e6694ba63 - default default] 100.113.0.213,127.0.0.1 "GET /v2/227a774fb9404585b854db2e6694ba63/servers/detail?name=%5Ecaas-piaas-hcvart2-piaas-hcvart2-lb-01%24 HTTP/1.1" status: 500 len: 430 time: 0.2303760
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions [req-5670b1fa-afd0-4f30-a7c5-f3973fce2442 bcbadd6e37124fcabffeebd8cc186406 fb7bc775e86241f6b6807a06fd767c30 - default default] Unexpected exception in API method
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions Traceback (most recent call last):
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/api/openstack/extensions.py", line 338, in wrapped
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return f(*args, **kwargs)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/api/openstack/compute/flavors.py", line 46, in detail
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     limited_flavors = self._get_flavors(req)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/api/openstack/compute/flavors.py", line 111, in _get_flavors
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     limit=limit, marker=marker)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/oslo_versionedobjects/base.py", line 184, in wrapper
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     result = fn(cls, context, *args, **kwargs)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/objects/flavor.py", line 707, in get_all
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     marker=marker)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/db/api.py", line 1557, in flavor_get_all
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     sort_dir=sort_dir, limit=limit, marker=marker)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/db/sqlalchemy/api.py", line 180, in wrapper
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return f(*args, **kwargs)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/db/sqlalchemy/api.py", line 266, in wrapped
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return f(context, *args, **kwargs)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/nova/db/sqlalchemy/api.py", line 5115, in flavor_get_all
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     inst_types = query.all()
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2588, in all
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return list(self)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2736, in __iter__
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return self._execute_and_instances(context)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2749, in _execute_and_instances
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     close_with_result=True)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2740, in _connection_from_session
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     **kw)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 905, in connection
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     execution_options=execution_options)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 910, in _connection_for_bind
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     engine, execution_options)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 334, in _connection_for_bind
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     conn = bind.contextual_connect()
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2041, in contextual_connect
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     **kwargs)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 92, in __init__
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     self.dispatch.engine_connect(self, self.__branch)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/event/attr.py", line 256, in __call__
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     fn(*args, **kw)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/engines.py", line 72, in _connect_ping_listener
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     connection.scalar(select([1]))
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 844, in scalar
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return self.execute(object, *multiparams, **params).scalar()
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 914, in execute
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return meth(self, multiparams, params)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     return connection._execute_clauseelement(self, multiparams, params)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     compiled_sql, distilled_params
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     context)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1334, in _handle_dbapi_exception
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     self._autorollback()
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 791, in _autorollback
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     self._root._rollback_impl()
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 670, in _rollback_impl
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     self._handle_dbapi_exception(e, None, None, None, None)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1266, in _handle_dbapi_exception
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     exc_info
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/util/compat.py", line 200, in raise_from_cause
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     reraise(type(exception), exception, tb=exc_tb)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 668, in _rollback_impl
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     self.engine.dialect.do_rollback(self.connection)
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/sqlalchemy/dialects/mysql/base.py", line 2526, in do_rollback
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     dbapi_connection.rollback()
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 771, in rollback
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     self._execute_command(COMMAND.COM_QUERY, "ROLLBACK")
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 1033, in _execute_command
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions     raise err.InterfaceError("(0, '')")
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions InterfaceError: (pymysql.err.InterfaceError) (0, '')
2019-07-24 06:53:38.149 2329 ERROR nova.api.openstack.extensions 
2019-07-24 06:53:38.150 2329 INFO nova.api.openstack.wsgi [req-5670b1fa-afd0-4f30-a7c5-f3973fce2442 bcbadd6e37124fcabffeebd8cc186406 fb7bc775e86241f6b6807a06fd767c30 - default default] HTTP exception thrown: Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.

It occurred when running a Rally test. Attached is a full log from all nova cloud controller units.