Avoid circular references in Macro class

Just as we store a weakref to the stack in the Function class, we must
also do the same with the template in the Macro class. Otherwise we end
up with circular references that don't get cleaned up until the garbage
collector runs.

Change-Id: Ia615de961f52e9fc5ef90e0668720b42c864a605
Closes-Bug: #1623706

heat/engine/function.py

@@ -140,9 +140,19 @@ class Macro(Function):
     def __init__(self, stack, fn_name, raw_args, parse_func, template):
         """Initialise with the argument syntax tree and parser function."""
         super(Macro, self).__init__(stack, fn_name, raw_args)
-        self.template = template
+        self._tmplref = weakref.ref(template) if template is not None else None
         self.parsed = self.parse_args(parse_func)
 
+    @property
+    def template(self):
+        ref = self._tmplref
+        if ref is None:
+            return None
+
+        tmpl = ref()
+        assert tmpl is not None, "Need a reference to the Template object"
+        return tmpl
+
     @abc.abstractmethod
     def parse_args(self, parse_func):
         """Parse the macro using the supplied parsing function.