Config option to disable the DHCP functions

This patch adds a new config option ``enable_traditional_dhcp``,
if set False, neutron-server will disable:
* DHCP provisioning block
* DHCP scheduler API extension
* Network scheduling mechanism
* DHCP RPC/notification

Partially-Implements: bp/distributed-dhcp-for-ml2-ovs
Related-Bug: #1900934

Change-Id: Icfbfc9691c5cf837406ff4291b3e3ed4970b26ee

neutron/api/rpc/agentnotifiers/dhcp_rpc_agent_api.py

@@ -220,7 +220,7 @@ class DhcpAgentNotifyAPI(object):
             enabled_agents = self._get_enabled_agents(
                 context, network, agents, method, payload)
 
-            if method == 'port_create_end':
+            if method == 'port_create_end' and enabled_agents:
                 high_agent = enabled_agents.pop(
                     random.randint(0, len(enabled_agents) - 1))
                 self._notify_high_priority_agent(

neutron/api/rpc/handlers/dhcp_rpc.py

@@ -23,6 +23,7 @@ from neutron_lib.callbacks import resources
 from neutron_lib import constants
 from neutron_lib.db import api as db_api
 from neutron_lib import exceptions
+from neutron_lib.exceptions import agent as agent_exc
 from neutron_lib.plugins import directory
 from neutron_lib.plugins import utils as p_utils
 from oslo_config import cfg
@@ -88,6 +89,20 @@ class DhcpRpcCallback(object):
             nets = plugin.list_active_networks_on_active_dhcp_agent(
                 context, host)
         else:
+            # If no active DHCP agent or agent admin state is DOWN,
+            # return empty network list for RPC to avoid unexpected
+            # resource creation on remote host when the DHCP agent
+            # scheduler extension is not supported.
+            try:
+                agent = plugin._get_agent_by_type_and_host(
+                    context, constants.AGENT_TYPE_DHCP, host)
+            except agent_exc.AgentNotFoundByTypeHost:
+                LOG.debug("DHCP Agent not found on host %s", host)
+                return []
+            if not agent.admin_state_up:
+                LOG.debug("DHCP Agent admin state is down on host %s", host)
+                return []
+
             filters = dict(admin_state_up=[True])
             nets = plugin.get_networks(context, filters=filters)
         return nets

neutron/conf/common.py

@@ -145,7 +145,14 @@ core_opts = [
                       "Setting to any positive integer means that on failure "
                       "the connection is retried that many times. "
                       "For example, setting to 3 means total attempts to "
-                      "connect will be 4."))
+                      "connect will be 4.")),
+    cfg.BoolOpt('enable_traditional_dhcp', default=True,
+                help=_('If False, neutron-server will disable the following '
+                       'DHCP-agent related functions:'
+                       '1. DHCP provisioning block '
+                       '2. DHCP scheduler API extension '
+                       '3. Network scheduling mechanism '
+                       '4. DHCP RPC/notification')),
 ]
 
 core_cli_opts = [

neutron/extensions/dhcpagentscheduler.py

@@ -22,12 +22,16 @@ from neutron_lib.api import extensions as api_extensions
 from neutron_lib.api import faults
 from neutron_lib.plugins import directory
 from neutron_lib import rpc as n_rpc
+from oslo_config import cfg
+from oslo_log import log as logging
 
 from neutron.api import extensions
 from neutron.api.v2 import resource
 from neutron import policy
 from neutron import wsgi
 
+LOG = logging.getLogger(__name__)
+
 
 class NetworkSchedulerController(wsgi.Controller):
     def index(self, request, **kwargs):
@@ -126,3 +130,10 @@ def notify(context, action, network_id, agent_id):
     info = {'id': agent_id, 'network_id': network_id}
     notifier = n_rpc.get_notifier('network')
     notifier.info(context, action, {'agent': info})
+
+
+def disable_extension_by_config(aliases):
+    if not cfg.CONF.enable_traditional_dhcp:
+        if 'dhcp_agent_scheduler' in aliases:
+            aliases.remove('dhcp_agent_scheduler')
+        LOG.info('Disabled dhcp_agent_scheduler extension.')

neutron/plugins/ml2/plugin.py

@@ -119,6 +119,7 @@ from neutron.db import securitygroups_rpc_base as sg_db_rpc
 from neutron.db import segments_db
 from neutron.db import subnet_service_type_mixin
 from neutron.db import vlantransparent_db
+from neutron.extensions import dhcpagentscheduler as dhcp_ext
 from neutron.extensions import filter_validation
 from neutron.extensions import vlantransparent
 from neutron.ipam import exceptions as ipam_exc
@@ -239,6 +240,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
             sg_rpc.disable_security_group_extension_by_config(aliases)
             vlantransparent._disable_extension_by_config(aliases)
             filter_validation._disable_extension_by_config(aliases)
+            dhcp_ext.disable_extension_by_config(aliases)
             self._aliases = self._filter_extensions_by_mech_driver(aliases)
         return self._aliases
 
@@ -288,6 +290,8 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
 
     def _setup_dhcp(self):
         """Initialize components to support DHCP."""
+        if not cfg.CONF.enable_traditional_dhcp:
+            return
         self.network_scheduler = importutils.import_object(
             cfg.CONF.network_scheduler_driver
         )
@@ -366,9 +370,10 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         """Initialize RPC notifiers for agents."""
         self.ovo_notifier = ovo_rpc.OVOServerRpcInterface()
         self.notifier = rpc.AgentNotifierApi(topics.AGENT)
-        self.agent_notifiers[const.AGENT_TYPE_DHCP] = (
+        if cfg.CONF.enable_traditional_dhcp:
-            dhcp_rpc_agent_api.DhcpAgentNotifyAPI()
+            self.agent_notifiers[const.AGENT_TYPE_DHCP] = (
-        )
+                dhcp_rpc_agent_api.DhcpAgentNotifyAPI()
+            )
 
     @log_helpers.log_method_call
     def start_rpc_listeners(self):
@@ -1375,6 +1380,8 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
             raise psec_exc.PortSecurityAndIPRequiredForSecurityGroups()
 
     def _setup_dhcp_agent_provisioning_component(self, context, port):
+        if not cfg.CONF.enable_traditional_dhcp:
+            return
         subnet_ids = [f['subnet_id'] for f in port['fixed_ips']]
         if (db.is_dhcp_active_on_any_subnet(context, subnet_ids) and
             len(self.get_dhcp_agents_hosting_networks(context,

releasenotes/notes/config_disable_traditional_dhcp-e553e2bd327c37c5.yaml

@@ -0,0 +1,10 @@
+---
+other:
+  - |
+    Added a new config option ``enable_traditional_dhcp`` for neutron server,
+    if it is set to False, neutron server will disable DHCP provisioning
+    block, DHCP scheduler API extension, network scheduling mechanism and
+    DHCP RPC/notification. This option can be used with the
+    ``dhcp`` extension of the OVS agent to enable distributed DHCP, or
+    for a deployment which needs
+    to disable the DHCP agent related functions permanently.