Add API for expected endpoint exceptions

Review I4e7b19dc730342091fd70a717065741d56da4555 gives a lot of the
background here, but the idea is that some exceptions raised by an RPC
endpoint method do not indicate any sort of failure and should not be
logged by the server as an error.

The classic example of this is conductor's instance_get() method raising
InstanceNotFound. This is perfectly normal and should not be considered
an error.

The new API is a decorator which you can use with RPC endpoints methods
to indicate which exceptions are expected:

    @messaging.expected_exceptions(InstanceNotFound)
    def instance_get(self, context, instance_id):
        ...

but we also need to expose the ExpectedException type itself so that
direct "local" users of the endpoint class know what type will be used
to wrap expected exceptions. For example, Nova has an ExceptionHelper
class which unwraps the original exception from an ExpectedException and
re-raises it.

I've changed from client_exceptions() and ClientException to make it
more clear it's intent. I felt that the "client" naming gave the
impression it was intended for use on the client side.

Change-Id: Ieec4600bd6b70cf31ac7925a98a517b84acada4d

doc/source/server.rst

@@ -12,3 +12,7 @@ Server
 
 .. autoclass:: MessageHandlingServer
    :members:
+
+.. autofunction:: expected_exceptions
+
+.. autoexception:: ExpectedException

oslo/messaging/_drivers/amqpdriver.py

@@ -37,7 +37,8 @@ class AMQPIncomingMessage(base.IncomingMessage):
         self.msg_id = msg_id
         self.reply_q = reply_q
 
-    def _send_reply(self, conn, reply=None, failure=None, ending=False):
+    def _send_reply(self, conn, reply=None, failure=None,
+                    ending=False, log_failure=True):
         if failure:
             failure = rpc_common.serialize_remote_exception(failure)
 
@@ -56,9 +57,9 @@ class AMQPIncomingMessage(base.IncomingMessage):
         else:
             conn.direct_send(self.msg_id, rpc_common.serialize_msg(msg))
 
-    def reply(self, reply=None, failure=None):
+    def reply(self, reply=None, failure=None, log_failure=True):
         with self.listener.driver._get_connection() as conn:
-            self._send_reply(conn, reply, failure)
+            self._send_reply(conn, reply, failure, log_failure=log_failure)
             self._send_reply(conn, ending=True)
 
 

oslo/messaging/_drivers/base.py

@@ -33,7 +33,7 @@ class IncomingMessage(object):
         self.message = message
 
     @abc.abstractmethod
-    def reply(self, reply=None, failure=None):
+    def reply(self, reply=None, failure=None, log_failure=True):
         "Send a reply or failure back to the client."
 
 

oslo/messaging/_drivers/impl_fake.py

@@ -31,7 +31,7 @@ class FakeIncomingMessage(base.IncomingMessage):
         super(FakeIncomingMessage, self).__init__(listener, ctxt, message)
         self._reply_q = reply_q
 
-    def reply(self, reply=None, failure=None):
+    def reply(self, reply=None, failure=None, log_failure=True):
         # FIXME: handle failure
         if self._reply_q:
             self._reply_q.put(reply)

oslo/messaging/_executors/base.py

@@ -16,6 +16,8 @@ import abc
 import logging
 import sys
 
+from oslo import messaging
+
 _LOG = logging.getLogger(__name__)
 
 
@@ -33,6 +35,10 @@ class ExecutorBase(object):
             reply = self.callback(incoming.ctxt, incoming.message)
             if reply:
                 incoming.reply(reply)
+        except messaging.ExpectedException as e:
+            _LOG.debug('Expected exception during message handling (%s)' %
+                       e.exc_info[1])
+            incoming.reply(failure=e.exc_info, log_failure=False)
         except Exception:
             # sys.exc_info() is deleted by LOG.exception().
             exc_info = sys.exc_info()

oslo/messaging/rpc/__init__.py

@@ -15,12 +15,14 @@
 
 __all__ = [
     'ClientSendError',
+    'ExpectedException',
     'NoSuchMethod',
     'RPCClient',
     'RPCDispatcher',
     'RPCDispatcherError',
     'RPCVersionCapError',
     'UnsupportedVersion',
+    'expected_exceptions',
     'get_rpc_server',
 ]
 

oslo/messaging/rpc/server.py

@@ -89,7 +89,13 @@ return values from the methods. By supplying a serializer object, a server can
 deserialize arguments from - serialize return values to - primitive types.
 """
 
-__all__ = ['get_rpc_server']
+__all__ = [
+    'get_rpc_server',
+    'ExpectedException',
+    'expected_exceptions',
+]
+
+import sys
 
 from oslo.messaging.rpc import dispatcher as rpc_dispatcher
 from oslo.messaging import server as msg_server
@@ -117,3 +123,38 @@ def get_rpc_server(transport, target, endpoints,
     dispatcher = rpc_dispatcher.RPCDispatcher(endpoints, serializer)
     return msg_server.MessageHandlingServer(transport, target,
                                             dispatcher, executor)
+
+
+class ExpectedException(Exception):
+    """Encapsulates an expected exception raised by an RPC endpoint
+
+    Merely instantiating this exception records the current exception
+    information, which  will be passed back to the RPC client without
+    exceptional logging.
+    """
+    def __init__(self):
+        self.exc_info = sys.exc_info()
+
+
+def expected_exceptions(*exceptions):
+    """Decorator for RPC endpoint methods that raise expected exceptions.
+
+    Marking an endpoint method with this decorator allows the declaration
+    of expected exceptions that the RPC server should not consider fatal,
+    and not log as if they were generated in a real error scenario.
+
+    Note that this will cause listed exceptions to be wrapped in an
+    ExpectedException, which is used internally by the RPC sever. The RPC
+    client will see the original exception type.
+    """
+    def outer(func):
+        def inner(*args, **kwargs):
+            try:
+                return func(*args, **kwargs)
+            except Exception as e:
+                if type(e) in exceptions:
+                    raise ExpectedException()
+                else:
+                    raise
+        return inner
+    return outer

tests/test_expected_exceptions.py

@@ -0,0 +1,54 @@
+
+# Copyright 2012 OpenStack Foundation
+# Copyright 2013 Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from oslo import messaging
+
+from tests import utils as test_utils
+
+
+class TestExpectedExceptions(test_utils.BaseTestCase):
+
+    def test_exception(self):
+        e = None
+        try:
+            try:
+                raise ValueError()
+            except Exception:
+                raise messaging.ExpectedException()
+        except messaging.ExpectedException as e:
+            self.assertIsInstance(e, messaging.ExpectedException)
+            self.assertTrue(hasattr(e, 'exc_info'))
+            self.assertIsInstance(e.exc_info[1], ValueError)
+
+    def test_decorator_expected(self):
+        class FooException(Exception):
+            pass
+
+        @messaging.expected_exceptions(FooException)
+        def naughty():
+            raise FooException()
+
+        self.assertRaises(messaging.ExpectedException, naughty)
+
+    def test_decorator_unexpected(self):
+        class FooException(Exception):
+            pass
+
+        @messaging.expected_exceptions(FooException)
+        def really_naughty():
+            raise ValueError()
+
+        self.assertRaises(ValueError, really_naughty)

tests/test_rabbit.py

@@ -60,7 +60,8 @@ class TestSendReceive(test_utils.BaseTestCase):
 
     _failure = [
         ('success', dict(failure=False)),
-        ('failure', dict(failure=True)),
+        ('failure', dict(failure=True, expected=False)),
+        ('expected_failure', dict(failure=True, expected=True)),
     ]
 
     _timeout = [
@@ -134,7 +135,8 @@ class TestSendReceive(test_utils.BaseTestCase):
                         raise ZeroDivisionError
                     except Exception:
                         failure = sys.exc_info()
-                    msgs[i].reply(failure=failure)
+                    msgs[i].reply(failure=failure,
+                                  log_failure=not self.expected)
                 else:
                     msgs[i].reply({'bar': msgs[i].message['foo']})
             senders[i].join()