Modify 'if' Macro to allow optional properties

Change-Id: I931d88e79fc077d12fc9bd39009061ffe87f1262
Story: 2007388
Task: 38973

doc/source/template_guide/hot_spec.rst

@@ -414,7 +414,15 @@ The complete list of supported condition functions is::
 --------------------
 The key with value ``2021-04-16`` or ``wallaby`` indicates that the YAML
 document is a HOT template and it may contain features added and/or removed
-up until the Wallaby release. The complete list of supported functions is::
+up until the Wallaby release.
+
+This version adds a 2-argument variant of the ``if`` function. When the
+condition is false and no third argument is supplied, the entire enclosing item
+(which may be e.g. a list item, a key-value pair in a dict, or a property
+value) will be elided. This allows for e.g. conditional definition of
+properties while keeping the default value when the condition is false.
+
+The complete list of supported functions is::
 
   digest
   filter
@@ -1875,6 +1883,27 @@ template except for ``if`` conditions. You can use the ``if`` condition
 in the property values in the ``resources`` section and ``outputs`` sections
 of a template.
 
+Beginning with the ``wallaby`` template version, the third argument is
+optional. If only two arguments are passed, the entire enclosing item is
+removed when the condition is false.
+
+For example:
+
+.. code-block:: yaml
+
+    conditions:
+      override_name: {not: {equals: [{get_param: server_name}, ""]}}
+
+    resources:
+      test_server:
+        type: OS::Nova::Server
+        properties:
+          name: {if: [override_name, {get_param: server_name}]}
+
+In this example, the default name for the server (which is generated by Heat
+when the property value is not specified) would be used when the
+``server_name`` parameter value is an empty string.
+
 not
 ---
 The ``not`` function acts as a NOT operator.

heat/engine/hot/functions.py

@@ -1275,13 +1275,16 @@ class If(function.Macro):
     evaluates to false.
     """
 
+    def _read_args(self):
+        return self.args
+
     def parse_args(self, parse_func):
         try:
             if (not self.args or
                     not isinstance(self.args, collections.Sequence) or
                     isinstance(self.args, str)):
                 raise ValueError()
-            condition, value_if_true, value_if_false = self.args
+            condition, value_if_true, value_if_false = self._read_args()
         except ValueError:
             msg = _('Arguments to "%s" must be of the form: '
                     '[condition_name, value_if_true, value_if_false]')
@@ -1299,6 +1302,40 @@ class If(function.Macro):
         return self.template.conditions(self.stack).is_enabled(cond)
 
 
+class IfNullable(If):
+    """A function to return corresponding value based on condition evaluation.
+
+    Takes the form::
+
+        if:
+          - <condition_name>
+          - <value_if_true>
+          - <value_if_false>
+
+    The value_if_true to be returned if the specified condition evaluates
+    to true, the value_if_false to be returned if the specified condition
+    evaluates to false.
+
+    If the value_if_false is omitted and the condition is false, the enclosing
+    item (list item, dictionary key/value pair, property definition) will be
+    treated as if it were not mentioned in the template::
+
+        if:
+          - <condition_name>
+          - <value_if_true>
+    """
+
+    def _read_args(self):
+        if not (2 <= len(self.args) <= 3):
+            raise ValueError()
+
+        if len(self.args) == 2:
+            condition, value_if_true = self.args
+            return condition, value_if_true, Ellipsis
+
+        return self.args
+
+
 class ConditionBoolean(function.Function):
     """Abstract parent class of boolean condition functions."""
 

heat/engine/hot/template.py

@@ -783,7 +783,8 @@ class HOTemplate20210416(HOTemplate20180831):
         # functions added in 2016-10-14
         'yaql': hot_funcs.Yaql,
         'map_replace': hot_funcs.MapReplace,
-        'if': hot_funcs.If,
+        # Modified in 2021-04-16
+        'if': hot_funcs.IfNullable,
 
         # functions added in 2017-02-24
         'filter': hot_funcs.Filter,

heat/tests/test_hot.py

@@ -71,6 +71,10 @@ hot_pike_tpl_empty = template_format.parse('''
 heat_template_version: 2017-09-01
 ''')
 
+hot_wallaby_tpl_empty = template_format.parse('''
+heat_template_version: 2021-04-16
+''')
+
 hot_tpl_empty_sections = template_format.parse('''
 heat_template_version: 2013-05-23
 parameters:
@@ -1506,14 +1510,43 @@ resources:
         self.assertEqual('', self.stack['AResource'].properties['Foo'])
 
     def test_if_invalid_args(self):
-        snippet = {'if': ['create_prod', 'one_value']}
+        snippets = [
+            {'if': ['create_prod', 'one_value']},
+            {'if': ['create_prod', 'one_value', 'two_values', 'three_values']},
+        ]
         tmpl = template.Template(hot_newton_tpl_empty)
+        for snippet in snippets:
             exc = self.assertRaises(exception.StackValidationFailed,
                                     self.resolve, snippet, tmpl)
             self.assertIn('Arguments to "if" must be of the form: '
                           '[condition_name, value_if_true, value_if_false]',
                           str(exc))
 
+    def test_if_nullable_invalid_args(self):
+        snippets = [
+            {'if': ['create_prod']},
+            {'if': ['create_prod', 'one_value', 'two_values', 'three_values']},
+        ]
+        tmpl = template.Template(hot_wallaby_tpl_empty)
+        for snippet in snippets:
+            exc = self.assertRaises(exception.StackValidationFailed,
+                                    self.resolve, snippet, tmpl)
+            self.assertIn('Arguments to "if" must be of the form: '
+                          '[condition_name, value_if_true, value_if_false]',
+                          str(exc))
+
+    def test_if_nullable(self):
+        snippet = {
+            'single': {'if': [False, 'value_if_true']},
+            'nested_true': {'if': [True, {'if': [False, 'foo']}, 'bar']},
+            'nested_false': {'if': [False, 'baz', {'if': [False, 'quux']}]},
+            'control': {'if': [False, True, None]},
+        }
+
+        tmpl = template.Template(hot_wallaby_tpl_empty)
+        resolved = self.resolve(snippet, tmpl, None)
+        self.assertEqual({'control': None}, resolved)
+
     def test_if_condition_name_non_existing(self):
         snippet = {'if': ['cd_not_existing', 'value_true', 'value_false']}
         tmpl = template.Template(hot_newton_tpl_empty)

releasenotes/notes/if-macro-optional-properties-40647f036903731b.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    The ``wallaby`` template version introduces a new 2-argument form of the
+    ``if`` function. This allows users to specify optional property values, so
+    that when the condition is false Heat treats it the same as if no value
+    were specified for the property at all. The behaviour of existing templates
+    is unchanged, even after updating the template version to ``wallaby``.