Add the config option for Oslo Messaging executor type

* With Pymysql driver and "eventlet" Oslo Messaging executor type
  Mistral seems to work fine. Just in case there's a regression in
  comparison with using "blocking" executor this patch makes a
  a change in a form of a config option that defines an RPC
  executor rather making it hardcoded.

Change-Id: Id73364eee29f2113fc983718b9891a496ca32ee4

mistral/config.py

@@ -111,6 +111,14 @@ rpc_response_timeout_opt = cfg.IntOpt(
     help=_('Seconds to wait for a response from a call.')
 )
 
+oslo_rpc_executor = cfg.StrOpt(
+    'oslo_rpc_executor',
+    default='eventlet',
+    choices=['eventlet', 'blocking', 'threading'],
+    help=_('Executor type used by Oslo Messaging framework. Defines how '
+           'Oslo Messaging based RPC subsystem processes incoming calls.')
+)
+
 expiration_token_duration = cfg.IntOpt(
     'expiration_token_duration',
     default=30,
@@ -559,6 +567,7 @@ CONF.register_opt(auth_type_opt)
 CONF.register_opt(js_impl_opt)
 CONF.register_opt(rpc_impl_opt)
 CONF.register_opt(rpc_response_timeout_opt)
+CONF.register_opt(oslo_rpc_executor)
 CONF.register_opt(expiration_token_duration)
 
 CONF.register_opts(api_opts, group=API_GROUP)
@@ -596,6 +605,7 @@ default_group_opts = itertools.chain(
         js_impl_opt,
         rpc_impl_opt,
         rpc_response_timeout_opt,
+        oslo_rpc_executor,
         expiration_token_duration
     ]
 )

mistral/engine/engine_server.py

@@ -61,10 +61,7 @@ class EngineServer(service_base.MistralService):
         self._rpc_server = rpc.get_rpc_server_driver()(cfg.CONF.engine)
         self._rpc_server.register_endpoint(self)
 
-        # Note(ddeja): Engine needs to be run in default (blocking) mode
-        # since using another mode may lead to a deadlock.
-        # See https://review.openstack.org/#/c/356343 for more info.
-        self._rpc_server.run(executor='blocking')
+        self._rpc_server.run(executor=cfg.CONF.oslo_rpc_executor)
 
         self._notify_started('Engine server started.')
 

mistral/engine/workflow_handler.py

@@ -112,7 +112,7 @@ def _check_and_complete(wf_ex_id):
                     else 4
                 )
 
-                # Rescheduling this check may not happen if erros are
+                # Rescheduling this check may not happen if errors are
                 # raised in the business logic. If the error is DB related
                 # and not considered fatal (e.g. disconnect, deadlock), the
                 # retry annotation around the method will ensure that the

mistral/rpc/base.py

@@ -168,7 +168,7 @@ class RPCServer(object):
         raise NotImplementedError
 
     @abc.abstractmethod
-    def run(self, executor='blocking'):
+    def run(self, executor='eventlet'):
         """Runs the RPC server.
 
         :param executor: Executor used to process incoming requests. Different

mistral/rpc/kombu/kombu_server.py

@@ -83,7 +83,7 @@ class KombuRPCServer(rpc_base.RPCServer, kombu_base.Base):
         """Return whether server is running."""
         return self._running.is_set()
 
-    def run(self, executor='blocking'):
+    def run(self, executor='eventlet'):
         if self._thread is None:
             self._thread = threading.Thread(target=self._run, args=(executor,))
             self._thread.daemon = True

mistral/rpc/oslo/oslo_server.py

@@ -35,7 +35,7 @@ class OsloRPCServer(rpc.RPCServer):
     def register_endpoint(self, endpoint):
         self.endpoints.append(endpoint)
 
-    def run(self, executor='blocking'):
+    def run(self, executor='eventlet'):
         target = messaging.Target(
             topic=self.topic,
             server=self.server_id