control-connection fallback: treat pools without live connections as unusable

[dkropachev]

Problem

Fallback considers every non-shutdown pool usable:

python-driver/cassandra/cluster.py

Lines 5029 to 5045 in b666e5c

	def _has_usable_node_pool(self):
	try:
	pools = tuple(self.session._pools.values())
	except (AttributeError, TypeError):
	return False
	return any(pool and not pool.is_shutdown for pool in pools)
	def _fallback_to_control_connection(self):
	fallback_mode = self.session.cluster.allow_control_connection_query_fallback
	if fallback_mode is ControlConnectionQueryFallback.Disabled:
	return False
	if self._host or self._control_connection_query_attempted:
	return False
	if fallback_mode is ControlConnectionQueryFallback.SkipPoolCreation:
	return True
	return not self._has_usable_node_pool()

A pool can remain non-shutdown while having no live connections. Its borrow_connection() then fails, but the pool suppresses control-connection fallback and the request ends with NoHostAvailable.

The current test encodes this behavior:

python-driver/tests/unit/test_response_future.py

Lines 615 to 630 in b666e5c

	def test_control_connection_fallback_not_used_when_pool_can_serve(self):
	session = self.make_basic_session()
	session.cluster.allow_control_connection_query_fallback = ControlConnectionQueryFallback.Fallback
	session.cluster._default_load_balancing_policy.make_query_plan.return_value = ['ip1']
	pool = Mock(is_shutdown=False)
	pool.borrow_connection.side_effect = NoConnectionsAvailable()
	session._pools = {'ip1': pool}
	connection = self.make_control_connection()
	session.cluster.control_connection._connection = connection
	rf = self.make_response_future(session)
	rf.send_request()
	connection.send_msg.assert_not_called()
	with pytest.raises(NoHostAvailable):
	rf.result()

Expected behavior

Treat pools with no open/live connections as unusable and allow fallback after the query plan is exhausted.