When record.args is None, it should not give an exception.

Also included a unit test which will validate that scenario.
Error occured at oslo_privsep implementation, which is fixed upstream:
3c47348ced

Change-Id: I6dd34f80662d2733b37f8d8d1b9f81eb6161f346

oslo_log/formatters.py

@@ -270,7 +270,7 @@ class ContextFormatter(logging.Formatter):
 
         if six.PY2:
             should_use_unicode = True
-            for arg in record.args:
+            for arg in record.args or []:
                 try:
                     six.text_type(arg)
                 except UnicodeDecodeError:

oslo_log/tests/unit/test_custom_loghandler.py

@@ -0,0 +1,51 @@
+# Copyright (c) 2016 OpenStack Foundation
+#
+# 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.
+
+"""Unit Tests for oslo.log with custom log handler"""
+
+
+import logging
+
+from oslo_log import log
+
+from oslo_log.tests.unit.test_log import LogTestBase
+
+
+class CustomLogHandler(logging.StreamHandler):
+    # Custom loghandler to mimick the error which was later fixed by
+    # https://github.com/openstack/oslo.privsep/commit/3c47348ced0d3ace1113ba8de8dff015792b0b89
+
+    def emit(self, record):
+        # Make args None; this was the error, which broke oslo_log formatting
+        record.args = None  # This is intentionally wrong
+
+        super(CustomLogHandler, self).emit(record)
+
+
+class CustomLogHandlerTestCase(LogTestBase):
+    def setUp(self):
+        super(CustomLogHandlerTestCase, self).setUp()
+        self.config(logging_context_format_string="HAS CONTEXT "
+                                                  "[%(request_id)s]: "
+                                                  "%(message)s",
+                    logging_default_format_string="NOCTXT: %(message)s",
+                    logging_debug_format_suffix="--DBG")
+        self.log = log.getLogger('')  # obtain root logger instead of 'unknown'
+        self._add_handler_with_cleanup(self.log, handler=CustomLogHandler)
+        self._set_log_level_with_cleanup(self.log, logging.DEBUG)
+
+    def test_log(self):
+        message = 'foo'
+        self.log.info(message)
+        self.assertEqual("NOCTXT: %s\n" % message, self.stream.getvalue())