add min of 1 to rpc_conn_pool_size

This change add a min value of 1 to
[oslo_messaging_rabbit]/rpc_conn_pool_size
such that there is always at least 1 connection avaiable.

This change add a runtime check to ensure that
[oslo_messaging_rabbit]/rpc_conn_pool_size is greater than
or equal too [oslo_messaging_rabbit]/conn_pool_min_size

Modified:
  oslo_messaging/tests/functional/test_rabbitmq.py

NOTE(stephenfin): We must set the 'conn_pool_min_size' config option in
the 'RabbitMQFailoverTests.test_failover_scenario' test to avoid raising
an error because the minimum is greater than the configured value. This
wasn't necessary on stable/train due to the presence of change
I0f86416623a0b718516147f0660b4df2b74cf867 ("Setup backend scenarios for
functional tests") which changed how our functional tests ran.

Change-Id: I2ad4b9f1d012c9f0586a932ac27d96da1bcc4e4c
Closes-Bug: #1899533
(cherry picked from commit 00d15eaeab)
(cherry picked from commit 8f5847dcbe)
(cherry picked from commit 7c9b070f5b)

oslo_messaging/_drivers/base.py

@@ -25,7 +25,8 @@ from oslo_messaging import exceptions
 base_opts = [
     cfg.IntOpt('rpc_conn_pool_size', default=30,
                deprecated_group='DEFAULT',
-               help='Size of RPC connection pool.'),
+               help='Size of RPC connection pool.',
+               min=1),
     cfg.IntOpt('conn_pool_min_size', default=2,
                help='The pool size limit for connections expiration policy'),
     cfg.IntOpt('conn_pool_ttl', default=1200,

oslo_messaging/_drivers/impl_rabbit.py

@@ -1370,6 +1370,11 @@ class RabbitDriver(amqpdriver.AMQPDriverBase):
         # the pool configuration properties
         max_size = conf.oslo_messaging_rabbit.rpc_conn_pool_size
         min_size = conf.oslo_messaging_rabbit.conn_pool_min_size
+        if max_size < min_size:
+            text = 'rpc_conn_pool_size: {max_size} must be greater than' \
+                   ' or equal to conn_pool_min_size:' \
+                   ' {min_size}'.format(max_size=max_size, min_size=min_size)
+            raise RuntimeError(text)
         ttl = conf.oslo_messaging_rabbit.conn_pool_ttl
 
         connection_pool = pool.ConnectionPool(

oslo_messaging/tests/drivers/test_impl_rabbit.py

@@ -31,6 +31,7 @@ from oslo_messaging._drivers import common as driver_common
 from oslo_messaging._drivers import impl_rabbit as rabbit_driver
 from oslo_messaging.exceptions import MessageDeliveryFailure
 from oslo_messaging.tests import utils as test_utils
+from oslo_messaging.transport import DriverLoadFailure
 from six.moves import mock
 
 load_tests = testscenarios.load_tests_apply_scenarios
@@ -141,6 +142,20 @@ class TestRabbitDriverLoad(test_utils.BaseTestCase):
 
         self.assertIsInstance(driver, rabbit_driver.RabbitDriver)
 
+    @mock.patch('oslo_messaging._drivers.impl_rabbit.Connection'
+                '.ensure_connection')
+    @mock.patch('oslo_messaging._drivers.impl_rabbit.Connection.reset')
+    def test_driver_load_max_less_than_min(self, fake_ensure, fake_reset):
+        self.config(
+            rpc_conn_pool_size=1, conn_pool_min_size=2,
+            group='oslo_messaging_rabbit')
+        self.messaging_conf.transport_url = self.transport_url
+        error = self.assertRaises(
+            DriverLoadFailure, oslo_messaging.get_transport, self.conf)
+        self.assertIn(
+            "rpc_conn_pool_size: 1 must be greater than or equal "
+            "to conn_pool_min_size: 2", str(error))
+
 
 class TestRabbitDriverLoadSSL(test_utils.BaseTestCase):
     scenarios = [

oslo_messaging/tests/functional/test_rabbitmq.py

@@ -50,6 +50,7 @@ class RabbitMQFailoverTests(test_utils.BaseTestCase):
         # correctly
         self.config(heartbeat_timeout_threshold=1,
                     rpc_conn_pool_size=1,
+                    conn_pool_min_size=1,
                     kombu_reconnect_delay=0,
                     rabbit_retry_interval=0,
                     rabbit_retry_backoff=0,