Lazily cache parsed value of list/json parameters

Parsing the value of JSON parameters at the time we create them (or
update the default value) results in exceptions occurring while creating
the Stack object, that are then not caught and transformed. The result
is an HTTP 500 Internal Error reported by the API, with an unhelpful
error message.

By not parsing the JSON until it is needed, we ensure that any errors
will occur during validation, where errors are appropriately transformed
(to StackValidationFailed) and annotated with the necessary information
to help the user determine the source.

Depends-On: https://review.opendev.org/#/c/748359/

Change-Id: I70e341c344d6254173ad4519276626230087263a
Story: 2007957
Task: 40443

heat/engine/parameters.py

@@ -369,32 +369,32 @@ class StringParam(Parameter):
 class ParsedParameter(Parameter):
     """A template parameter with cached parsed value."""
 
-    __slots__ = ('parsed',)
+    __slots__ = ('_parsed',)
 
     def __init__(self, name, schema, value=None):
         super(ParsedParameter, self).__init__(name, schema, value)
-        self._update_parsed()
+        self._parsed = None
 
-    def set_default(self, value):
-        super(ParsedParameter, self).set_default(value)
-        self._update_parsed()
-
-    def _update_parsed(self):
-        if self.has_value():
-            if self.user_value is not None:
-                self.parsed = self.parse(self.user_value)
+    @property
+    def parsed(self):
+        if self._parsed is None:
+            if self.has_value():
+                if self.user_value is not None:
+                    self._parsed = self.parse(self.user_value)
+                else:
+                    self._parsed = self.parse(self.default())
             else:
-                self.parsed = self.parse(self.default())
+                self._parsed = self.default_parsed()
+        return self._parsed
 
 
 class CommaDelimitedListParam(ParsedParameter, collections.Sequence):
     """A template parameter of type "CommaDelimitedList"."""
 
-    __slots__ = ('parsed',)
+    __slots__ = tuple()
 
-    def __init__(self, name, schema, value=None):
-        self.parsed = []
-        super(CommaDelimitedListParam, self).__init__(name, schema, value)
+    def default_parsed(self):
+        return []
 
     def parse(self, value):
         # only parse when value is not already a list
@@ -436,11 +436,10 @@ class CommaDelimitedListParam(ParsedParameter, collections.Sequence):
 class JsonParam(ParsedParameter):
     """A template parameter who's value is map or list."""
 
-    __slots__ = ('parsed',)
+    __slots__ = tuple()
 
-    def __init__(self, name, schema, value=None):
-        self.parsed = {}
-        super(JsonParam, self).__init__(name, schema, value)
+    def default_parsed(self):
+        return {}
 
     def parse(self, value):
         try:

heat/tests/test_parameters.py

@@ -379,7 +379,7 @@ class ParameterTestSpecific(common.HeatTestCase):
         schema = {'Type': 'Json',
                   'ConstraintDescription': 'wibble'}
         val = {"foo": "bar", "not_json": len}
-        err = self.assertRaises(ValueError,
+        err = self.assertRaises(exception.StackValidationFailed,
                                 new_parameter, 'p', schema, val)
         self.assertIn('Value must be valid JSON', str(err))
 
@@ -397,7 +397,7 @@ class ParameterTestSpecific(common.HeatTestCase):
         schema = {'Type': 'Json',
                   'ConstraintDescription': 'wibble'}
         val = "I am not a map"
-        err = self.assertRaises(ValueError,
+        err = self.assertRaises(exception.StackValidationFailed,
                                 new_parameter, 'p', schema, val)
         self.assertIn('Value must be valid JSON', str(err))