Load Linux Bridge Trunk extension if service plugin configured

Load the Linux Driver Trunk agent extension driver only if the service
plugin is enabled. If the service plugin is not enabled, the agent
extension will not subscribe to the port events.

Change-Id: I883abb5ef122ac7f8fb029d0334fa2d68893f8aa
Closes-Bug: #1930443

neutron/services/trunk/drivers/linuxbridge/agent/driver.py

@@ -22,6 +22,7 @@ from neutron.api.rpc.callbacks import events
 from neutron.api.rpc.handlers import resources_rpc
 from neutron.services.trunk.drivers.linuxbridge.agent import trunk_plumber
 from neutron.services.trunk.rpc import agent as trunk_rpc
+from neutron.services.trunk import utils as trunk_utils
 
 
 LOG = logging.getLogger(__name__)
@@ -29,7 +30,8 @@ LOG = logging.getLogger(__name__)
 
 def init_handler(resource, event, trigger, payload=None):
     """Handler for agent init event."""
-    LinuxBridgeTrunkDriver()
+    if trunk_utils.is_trunk_service_loaded():
+        LinuxBridgeTrunkDriver()
 
 
 @registry.has_registry_receivers

neutron/services/trunk/utils.py

@@ -14,6 +14,8 @@
 
 from neutron_lib.api import extensions
 from neutron_lib.plugins import directory
+from neutron_lib.utils import runtime
+from oslo_config import cfg
 
 from neutron.common import utils as common_utils
 
@@ -53,3 +55,15 @@ def is_driver_compatible(context, driver, interface, host_agent_types):
 
     # For an agent-based driver, both interface and agent compat is required.
     return is_interface_compatible and driver.agent_type in host_agent_types
+
+
+def is_trunk_service_loaded():
+    for service_plugins in cfg.CONF.service_plugins:
+        try:
+            klass = runtime.load_class_by_alias_or_classname(
+                'neutron.service_plugins', service_plugins)
+            if klass.__name__ == 'TrunkPlugin':
+                return True
+        except ImportError:
+            continue
+    return False