Clarify dangerous use of exceptions in unit tests

Also, update unit tests to use more specific exception where possible
to make auditing easier in the future.

Change-Id: I4906e2f8e3ebacf0587f1471ca8dd46c73edc17a

nova/testing/README.rst

@@ -54,13 +54,33 @@ Writing Integration Tests
 
 TBD
 
-Tests and assertRaises
+Tests and Exceptions
-----------------------
+--------------------
-When asserting that a test should raise an exception, test against the
+A properly written test asserts that particular behavior occurs. This can
-most specific exception possible. An overly broad exception type (like
+be a success condition or a failure condition, including an exception.
-Exception) can mask errors in the unit test itself.
+When asserting that a particular exception is raised, the most specific
+exception possible should be used.
+
+In particular, testing for Exception being raised is almost always a
+mistake since it will match (almost) every exception, even those
+unrelated to the exception intended to be tested.
+
+This applies to catching exceptions manually with a try/except block,
+or using assertRaises().
 
 Example::
 
     self.assertRaises(exception.InstanceNotFound, db.instance_get_by_uuid,
                       elevated, instance_uuid)
+
+If a stubbed function/method needs a generic exception for testing
+purposes, test.TestingException is available.
+
+Example::
+
+    def stubbed_method(self):
+        raise test.TestingException()
+    self.stubs.Set(cls, 'inner_method', stubbed_method)
+
+    obj = cls()
+    self.assertRaises(test.TestingException, obj.outer_method)

nova/tests/test_db_api.py

@@ -46,11 +46,10 @@ class DbApiTestCase(test.TestCase):
         return db.instance_create(self.context, args)
 
     def test_ec2_ids_not_found_are_printable(self):
-
         def check_exc_format(method):
             try:
                 method(self.context, 'fake')
-            except Exception as exc:
+            except exception.NotFound as exc:
                 self.assertTrue('fake' in unicode(exc))
 
         check_exc_format(db.get_ec2_volume_id_by_uuid)