Get rid of circular references in Resource and Function

Circular references cause practically every bit of data that Heat uses to
remain in memory until the completion of an operation, and even then to
only be freed once the loop is detected by the garbage collector. By
breaking all of the loops using weak references, we can ensure that things
will get freed when they are no longer referenced without the need to wait
for garbage collection (which should also take a lot less time). This
change removes the loops from Resource and Function objects back to the
Stack.

Change-Id: Ibf80e95e69a2f27ed29754a2e0f1125e8eed0775
Closes-Bug: #1454873

heat/engine/function.py

@@ -14,6 +14,7 @@
 import abc
 import collections
 import itertools
+import weakref
 
 import six
 
@@ -33,10 +34,20 @@ class Function(object):
             { <fn_name> : <args> }
         """
         super(Function, self).__init__()
-        self.stack = stack
+        self._stackref = weakref.ref(stack) if stack is not None else None
         self.fn_name = fn_name
         self.args = args
 
+    @property
+    def stack(self):
+        ref = self._stackref
+        if ref is None:
+            return None
+
+        stack = ref()
+        assert stack is not None, "Need a reference to the Stack object"
+        return stack
+
     def validate(self):
         """
         Validate arguments without resolving the function.

heat/engine/resource.py

@@ -15,6 +15,7 @@ import base64
 import contextlib
 import datetime as dt
 import warnings
+import weakref
 
 from oslo_config import cfg
 from oslo_log import log as logging
@@ -218,6 +219,16 @@ class Resource(object):
         self.replaced_by = resource.replaced_by
         self.current_template_id = resource.current_template_id
 
+    @property
+    def stack(self):
+        stack = self._stackref()
+        assert stack is not None, "Need a reference to the Stack object"
+        return stack
+
+    @stack.setter
+    def stack(self, stack):
+        self._stackref = weakref.ref(stack)
+
     def reparse(self):
         self.properties = self.t.properties(self.properties_schema,
                                             self.context)