Convert exception messages to strings

Exception messages are expected to be strings, but it's not always
the case. Some exceptions are wrapped and passed as new exception
messages, which affects some exception handlers.

Finding all the occurrences may be difficult. For now, it's easier
to just convert the exception messages to strings in the NovaException
class.

Closes-Bug: #1801713

Change-Id: Ic470791aa2b08ca9911bf70cb3cc68652d3647f2

nova/exception.py

@@ -23,6 +23,7 @@ SHOULD include dedicated exception logging.
 """
 
 from oslo_log import log as logging
+import six
 
 import webob.exc
 from webob import util as woutil
@@ -77,15 +78,16 @@ class NovaException(Exception):
             except AttributeError:
                 pass
 
-        if not message:
-            try:
+        try:
+            if not message:
                 message = self.msg_fmt % kwargs
-
-            except Exception:
-                # NOTE(melwitt): This is done in a separate method so it can be
-                # monkey-patched during testing to make it a hard failure.
-                self._log_exception()
-                message = self.msg_fmt
+            else:
+                message = six.text_type(message)
+        except Exception:
+            # NOTE(melwitt): This is done in a separate method so it can be
+            # monkey-patched during testing to make it a hard failure.
+            self._log_exception()
+            message = self.msg_fmt
 
         self.message = message
         super(NovaException, self).__init__(message)

nova/tests/unit/test_exception.py

@@ -153,6 +153,8 @@ class NovaExceptionTestCase(test.NoDBTestCase):
     def test_error_msg(self):
         self.assertEqual('test',
                          six.text_type(exception.NovaException('test')))
+        self.assertEqual('test',
+                         exception.NovaException(Exception('test')).message)
 
     def test_default_error_msg_with_kwargs(self):
         class FakeNovaException(exception.NovaException):