Validate nova server's metadata

This prevents "foo" getting passed into the metadata.
This can happen when using get_attr() to return a dynamic value.

Change-Id: I17eab659e65a51963e510add4d71fbc90a3a3a53
Closes-bug: #1338811

heat/engine/clients/os/nova.py

@@ -11,6 +11,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import collections
 import email
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
@@ -375,6 +376,10 @@ echo -e '%s\tALL=(ALL)\tNOPASSWD: ALL' >> /etc/sudoers
         Serialize non-string metadata values before sending them to
         Nova.
         """
+        if not isinstance(metadata, collections.Mapping):
+            raise exception.StackValidationFailed(message=_(
+                "nova server metadata needs to be a Map."))
+
         return dict((key, (value if isinstance(value,
                                                six.string_types)
                            else json.dumps(value))

heat/tests/test_nova_client.py

@@ -15,6 +15,7 @@
 import mock
 from novaclient import exceptions as nova_exceptions
 from oslo.config import cfg
+import six
 import uuid
 
 from heat.common import exception
@@ -232,6 +233,14 @@ class NovaUtilsMetadataTests(NovaClientPluginTestCase):
         expected = {'test_key': 'null'}
         self.assertEqual(expected, self.nova_plugin.meta_serialize(original))
 
+    def test_serialize_no_value(self):
+        """This test is to prove that the user can only pass in a dict to nova
+        metadata.
+        """
+        excp = self.assertRaises(exception.StackValidationFailed,
+                                 self.nova_plugin.meta_serialize, "foo")
+        self.assertIn('metadata needs to be a Map', six.text_type(excp))
+
     def test_serialize_combined(self):
         original = {
             'test_key_1': 123,