Add preview for NestedStack

Override ``child_template`` and ``child_params`` to allow previewing of
NestedStack as a group of resources.

Implements: blueprint preview-stack (partial)
Change-Id: I3acde4ccc31731aee1209c747fdbcc4446be63bc

heat/engine/resources/stack.py

@@ -61,13 +61,7 @@ class NestedStack(stack_resource.StackResource):
 
     update_allowed_keys = ('Properties',)
 
-    def handle_adopt(self, resource_data=None):
-        return self._create_with_template(resource_adopt_data=resource_data)
-
-    def handle_create(self):
-        return self._create_with_template()
-
-    def _create_with_template(self, resource_adopt_data=None):
+    def child_template(self):
         try:
             template_data = urlfetch.get(self.properties[self.TEMPLATE_URL])
         except (exceptions.RequestException, IOError) as r_exc:
@@ -76,10 +70,21 @@ class NestedStack(stack_resource.StackResource):
                              {'url': self.properties[self.TEMPLATE_URL],
                               'exc': str(r_exc)})
 
-        template = template_format.parse(template_data)
+        return template_format.parse(template_data)
 
+    def child_params(self):
+        return self.properties[self.PARAMETERS]
+
+    def handle_adopt(self, resource_data=None):
+        return self._create_with_template(resource_adopt_data=resource_data)
+
+    def handle_create(self):
+        return self._create_with_template()
+
+    def _create_with_template(self, resource_adopt_data=None):
+        template = self.child_template()
         return self.create_with_template(template,
-                                         self.properties[self.PARAMETERS],
+                                         self.child_params(),
                                          self.properties[self.TIMEOUT_IN_MINS],
                                          adopt_data=resource_adopt_data)
 

heat/tests/test_nested_stack.py

@@ -15,6 +15,8 @@
 
 import copy
 import json
+import mock
+from requests import exceptions
 
 from oslo.config import cfg
 
@@ -598,6 +600,42 @@ Resources:
 
         self.m.VerifyAll()
 
+    def test_child_params(self):
+        t = template_format.parse(self.test_template)
+        stack = self.parse_stack(t)
+        nested_stack = stack['the_nested']
+        nested_stack.properties.data[nested_stack.PARAMETERS] = {'foo': 'bar'}
+
+        self.assertEqual({'foo': 'bar'}, nested_stack.child_params())
+
+    @mock.patch.object(urlfetch, 'get')
+    def test_child_template_when_file_is_fetched(self, mock_get):
+        mock_get.return_value = 'template_file'
+        t = template_format.parse(self.test_template)
+        stack = self.parse_stack(t)
+        nested_stack = stack['the_nested']
+
+        with mock.patch('heat.common.template_format.parse') as mock_parse:
+            mock_parse.return_value = 'child_template'
+            self.assertEqual('child_template', nested_stack.child_template())
+            mock_parse.assert_called_once_with('template_file')
+
+    @mock.patch.object(urlfetch, 'get')
+    def test_child_template_when_fetching_file_fails(self, mock_get):
+        mock_get.side_effect = exceptions.RequestException()
+        t = template_format.parse(self.test_template)
+        stack = self.parse_stack(t)
+        nested_stack = stack['the_nested']
+        self.assertRaises(ValueError, nested_stack.child_template)
+
+    @mock.patch.object(urlfetch, 'get')
+    def test_child_template_when_io_error(self, mock_get):
+        mock_get.side_effect = IOError()
+        t = template_format.parse(self.test_template)
+        stack = self.parse_stack(t)
+        nested_stack = stack['the_nested']
+        self.assertRaises(ValueError, nested_stack.child_template)
+
 
 class ResDataResource(generic_rsrc.GenericResource):
     def handle_create(self):