control-connection fallback: treat empty session keyspace as unset

[dkropachev]

Problem

Control-connection fallback treats an empty-string Session.keyspace as a real keyspace, while the rest of session setup treats it as unset.

SkipPoolCreation binds the shared control connection to the raw value:

python-driver/cassandra/cluster.py

Lines 2894 to 2898 in 5651509

	if fallback_mode is ControlConnectionQueryFallback.SkipPoolCreation:
	control_connection = self.cluster.control_connection
	conflict = control_connection._attach_application_session(self.keyspace, self)
	if conflict is not None:
	raise InvalidRequest(conflict)

The on-demand fallback path does the same, then attempts to select any keyspace that is not None:

python-driver/cassandra/cluster.py

Lines 5735 to 5741 in 5651509

	keyspace = self.session.keyspace
	new_application_session = \
	self.session not in control_connection._application_sessions
	conflict = control_connection._attach_application_session(keyspace, self.session)
	if conflict is not None:
	self._set_final_exception(InvalidRequest(conflict))
	return _NOT_SET

python-driver/cassandra/cluster.py

Lines 5766 to 5770 in 5651509

	if keyspace is not None and connection.keyspace != keyspace:
	return self._set_control_connection_keyspace(
	connection, host, keyspace, message=message, cb=cb,
	request_id=request_id,
	provisional_session=provisional_session)

A session created with keyspace='' can therefore conflict with an otherwise equivalent no-keyspace session and can issue USE "" through fallback.

Expected behavior

Normalize a falsey session keyspace to None before control-connection binding and dispatch. Sessions using '' and None should share the no-keyspace binding, and fallback should send the original query without an empty USE request.

Acceptance criteria

• Fallback treats keyspace='' as no keyspace.
• SkipPoolCreation treats keyspace='' as no keyspace.
• Empty-string and None sessions do not conflict solely because of their representation.
• No USE "" request is sent.

Related

• control-connection fallback: preserve session keyspace #1013 introduced keyspace isolation for control-connection fallback.