Use empty list in list_join for validation

Calling list_join on the list of ResourceGroup's attributes in a
SoftwareConfig fails during template validation with:

ERROR: Property error : my_config: config "list_join" must operate on a
list

Closes-Bug: 1358831
Change-Id: I39d2da7a28ba53fbe3a95bff0198fa5c2f87aa3b
(cherry picked from commit 61a9d167c0f36a2846e39fd33a77ce2755801fe6)

2 files changed, 23 insertions, 0 deletions

diff --git a/heat/engine/cfn/functions.py b/heat/engine/cfn/functions.py
index d7f4f042d..6d5fc789d 100644
--- a/heat/engine/cfn/functions.py
+++ b/heat/engine/cfn/functions.py
@@ -278,6 +278,8 @@ class Join(function.Function):
 
     def result(self):
         strings = function.resolve(self._strings)
+        if strings is None:
+            strings = []
         if (isinstance(strings, basestring) or
                 not isinstance(strings, collections.Sequence)):
             raise TypeError(_('"%s" must operate on a list') % self.fn_name)
diff --git a/heat/tests/test_hot.py b/heat/tests/test_hot.py
index 452c14dae..019af719e 100644
--- a/heat/tests/test_hot.py
+++ b/heat/tests/test_hot.py
@@ -301,6 +301,27 @@ class HOTemplateTest(HeatTestCase):
         err = self.assertRaises(KeyError, tmpl.__getitem__, tmpl.OUTPUTS)
         self.assertIn('Value', str(err))
 
+    def test_resource_group_list_join(self):
+        """Test list_join on a ResourceGroup's inner attributes
+
+        This should not fail during validation (i.e. before the ResourceGroup
+        can return the list of the runtime values.
+        """
+        hot_tpl = template_format.parse('''
+        heat_template_version: 2013-05-23
+        resources:
+          rg:
+            type: OS::Heat::ResourceGroup
+            properties:
+              count: 3
+              resource_def:
+                type: OS::Nova::Server
+        ''')
+        tmpl = parser.Template(hot_tpl)
+        stack = parser.Stack(utils.dummy_context(), 'test_stack', tmpl)
+        snippet = {'Fn::Join': ["\n", {'get_attr': ['rg', 'name']}]}
+        self.assertEqual(self.resolve(snippet, tmpl, stack), '')
+
     def test_str_replace(self):
         """Test str_replace function."""