Completely disable rpc workers when rpc_workers=0

Currently at least 1 rpc worker is launched even when a user requests
zero workers by setting rpc_workers=0. The setting of rpc_workers=0 is
used when ml2-ovn plugin is used without any additional agent, and in
this deployment pattern the single rpc worker is not at all used.

This change ensures no rpc worker is launched when rpc_workers options
is explicitly set to 0. This may be classified as a breaking change,
but is consistent with the earlier change[1] for rpc_workers=0.

[1] 3e1e2d63b3

Closes-Bug: #2052484
Change-Id: I878e50c3192ecd3b145ded0ab8394845a089696e

doc/source/admin/config-wsgi.rst

@@ -150,3 +150,13 @@ events from the various neutron agents. Signs that there are too few
 can be agent heartbeats arriving late, nova vif bindings timing out
 on the hypervisors, or rpc message timeout exceptions in agent logs
 (for example, "broken pipe" errors).
+
+There is also the rpc_state_report_workers option, which determines
+the number fo RPC worker processes dedicated to process state reports
+from the various agents. This may be increased to resolve frequent delay
+in processing agents heartbeats.
+
+.. note::
+   If OVN ML2 plugin is used without any additional agents, neutron requires
+   no worker for RPC message processing. Set both rpc_workers and
+   rpc_state_report_workers to 0, to disable RPC workers.

neutron/conf/service.py

@@ -31,7 +31,8 @@ SERVICE_OPTS = [
     cfg.IntOpt('rpc_workers',
                help=_('Number of RPC worker processes for service. '
                       'If not specified, the default is equal to half the '
-                      'number of API workers.')),
+                      'number of API workers. If set to 0, no RPC worker '
+                      'is launched.')),
     cfg.IntOpt('rpc_state_report_workers',
                default=1,
                help=_('Number of RPC worker processes dedicated to the state '

neutron/service.py

@@ -189,9 +189,15 @@ def _get_rpc_workers(plugin=None):
                       workers)
         raise NotImplementedError()
 
+    rpc_workers = []
+
+    if workers > 0:
         # passing service plugins only, because core plugin is among them
-    rpc_workers = [RpcWorker(service_plugins,
-                             worker_process_count=workers)]
+        rpc_workers.append(
+            RpcWorker(service_plugins, worker_process_count=workers))
+    else:
+        LOG.warning('No rpc workers are launched. Make sure no agent is used '
+                    'in this deployment.')
 
     if (cfg.CONF.rpc_state_report_workers > 0 and
             plugin.rpc_state_report_workers_supported()):

neutron/tests/unit/test_service.py

@@ -54,12 +54,15 @@ class TestRunRpcWorkers(base.BaseTestCase):
             with mock.patch('neutron.service.RpcReportsWorker'):
                 service._get_rpc_workers(plugin=mock.Mock())
         init_call = mock_rpc_worker.call_args
+        if expected_passed_value > 0:
             expected_call = mock.call(
                 mock.ANY, worker_process_count=expected_passed_value)
             self.assertEqual(expected_call, init_call)
+        else:
+            mock_rpc_worker.assert_not_called()
 
     def test_rpc_workers_zero(self):
-        self._test_rpc_workers(0, 1)
+        self._test_rpc_workers(0, 0)
 
     def test_rpc_workers_default_api_workers_default(self):
         workers = max(int(self.worker_count / 2), 1)

releasenotes/notes/bug-2052484-9a80c1d035349879.yaml

@@ -0,0 +1,16 @@
+---
+upgrade:
+  - |
+    Now setting ``[DEFAULT] rpc_workers = 0`` completely disables rpc workers.
+    In a deployment with additional agents, like the dhcp-agent, this option
+    should be set to a positive value. Note that all notifications from
+    neutron-server to agents were disabled when ``[DEFAULT] rpc_workers = 0``
+    is set in 22.0.0 release, so this was the requiremenet actually added in
+    that release.
+
+fixes:
+  - |
+    [`bug 2052484 <https://bugs.launchpad.net/neutron/+bug/2002484>`_]
+    Now setting ``[DEFAULT] rpc_workers = 0`` completely disables rpc workers.
+    Previously one rpc worker was launched even though ``0`` is specifically
+    requested.