Break cycle between Resource and Attributes

Attributes hold a reference to Resource via the attribute resolution
method. Let's add a weakref here to break the cycle.

Change-Id: I1cd22476c7d2d9ca0a090947b8f28d8afc413fc7
(cherry picked from commit c3021de31c23d70fb1d766f4ac3c521ad7ede7d5)

3 files changed, 22 insertions, 5 deletions

diff --git a/heat/engine/resource.py b/heat/engine/resource.py
index 2946c60d7..872beb4fb 100644
--- a/heat/engine/resource.py
+++ b/heat/engine/resource.py
@@ -209,7 +209,7 @@ class Resource(object):
         """
         return attributes.Attributes(self.name,
                                      self.attributes_schema,
-                                     self._resolve_all_attributes)
+                                     self._make_resolver(weakref.ref(self)))
 
     def __init__(self, name, definition, stack):
 
@@ -2269,3 +2269,17 @@ class Resource(object):
         except Exception:
             return False
         return True
+
+    @staticmethod
+    def _make_resolver(ref):
+        """Return an attribute resolution method.
+
+        This builds a resolver without a strong reference to this resource, to
+        break a possible cycle.
+        """
+        def resolve(attr):
+            res = ref()
+            if res is None:
+                raise RuntimeError("Resource collected")
+            return res._resolve_all_attributes(attr)
+        return resolve
diff --git a/heat/engine/resources/stack_resource.py b/heat/engine/resources/stack_resource.py
index 2978879a9..e0278e8fe 100644
--- a/heat/engine/resources/stack_resource.py
+++ b/heat/engine/resources/stack_resource.py
@@ -13,6 +13,7 @@
 
 import json
 import warnings
+import weakref
 
 from oslo_config import cfg
 from oslo_log import log as logging
@@ -87,7 +88,7 @@ class StackResource(resource.Resource):
             #       with all available outputs
             self.attributes = attributes.Attributes(
                 self.name, self.attributes_schema,
-                self._resolve_all_attributes)
+                self._make_resolver(weakref.ref(self)))
 
     def _needs_update(self, after, before, after_props, before_props,
                       prev_resource, check_init_complete=True):
diff --git a/heat/engine/resources/template_resource.py b/heat/engine/resources/template_resource.py
index 116b5731b..02c8306c5 100644
--- a/heat/engine/resources/template_resource.py
+++ b/heat/engine/resources/template_resource.py
@@ -11,6 +11,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import weakref
+
 from oslo_serialization import jsonutils
 from requests import exceptions
 import six
@@ -118,9 +120,9 @@ class TemplateResource(stack_resource.StackResource):
             tmpl, self.stack.env.param_defaults)
 
         self.attributes_schema.update(self.base_attributes_schema)
-        self.attributes = attributes.Attributes(self.name,
-                                                self.attributes_schema,
-                                                self._resolve_all_attributes)
+        self.attributes = attributes.Attributes(
+            self.name, self.attributes_schema,
+            self._make_resolver(weakref.ref(self)))
 
     def child_params(self):
         """Override method of child_params for the resource.