Fix early resource property value validation

Currently resource property validation fails if resource it
depends on is not created.

This change skips a property constraint validation if its intrinsic
function resolution does not find the dependant resource.

Change-Id: I2056c89437e8df2a59726b60c3c12f19f713d84d
Partial-Bug: #1317636

heat/engine/properties.py

@@ -278,7 +278,9 @@ class Property(object):
         elif t == Schema.BOOLEAN:
             _value = self._get_bool(value)
 
-        if validate:
+        # property value resolves to None if resource it depends on is not
+        # created. So, if value is None skip constraint validation.
+        if value is not None and validate:
             self.schema.validate_constraints(_value, self.context)
 
         return _value

heat/tests/test_properties.py

@@ -1031,6 +1031,20 @@ class PropertiesTest(testtools.TestCase):
         err = self.assertRaises(ValueError, props.get, 'foo')
         self.assertEqual('foo resolution failed!', str(err))
 
+    def test_resolve_returns_none(self):
+        schema = {'foo': {'Type': 'String', "MinLength": "5"}}
+
+        def test_resolver(prop):
+            return None
+
+        props = properties.Properties(schema,
+                                      {'foo': 'get_attr: [db, value]'},
+                                      test_resolver)
+        try:
+            self.assertIsNone(props.validate())
+        except exception.StackValidationFailed:
+            self.fail("Constraints should not have been evaluated.")
+
     def test_schema_from_params(self):
         params_snippet = {
             "DBUsername": {