HOT intrinisic function list_join like Fn::Join

We have an increasing number of attributes which are a list of strings.

In HOT there is no way to join a list into a string as with the CFN
function Fn::Join.

This change implements HOT intrinsic function list_join using an
identical implementation as Fn::Join.

Tripleo will benefit from this function since the lack of a join
function in HOT is one of the barriers to transitioning to the
HOT template format.

Change-Id: I29e0b744271213429fff6922397b925eb9ccc204

doc/source/template_guide/hot_spec.rst

@@ -726,6 +726,26 @@ The *resource ID* of the referenced resources as used in the current template is
 given as single parameter to the get_resource function.
 
 
+list_join
+--------
+The *list_join* function joins a list of strings with the given delimiter.
+The syntax of the list_join function is as follows:
+
+::
+
+  list_join:
+  - <delimiter>
+  - <list to join>
+
+A sample use of this function with a simple list is shown below.
+
+::
+
+  list_join: [', ', ['one', 'two', 'and three']]
+
+This would resolve to "one, two, and three".
+
+
 resource_facade
 ---------------
 The *resource_facade* function allows a provider template to retrieve data

heat/engine/hot/functions.py

@@ -202,6 +202,20 @@ class GetFile(function.Function):
         return f
 
 
+class Join(cfn_funcs.Join):
+    '''
+    A function for joining strings.
+
+    Takes the form::
+
+        { "list_join" : [ "<delim>", [ "<string_1>", "<string_2>", ... ] }
+
+    And resolves to::
+
+        "<string_1><delim><string_2><delim>..."
+    '''
+
+
 class ResourceFacade(cfn_funcs.ResourceFacade):
     '''
     A function for obtaining data from the facade resource from within the
@@ -235,6 +249,7 @@ def function_mapping(version_key, version):
             'get_attr': GetAtt,
             'Fn::Select': cfn_funcs.Select,
             'Fn::Join': cfn_funcs.Join,
+            'list_join': Join,
             'Fn::Split': cfn_funcs.Split,
             'str_replace': Replace,
             'Fn::Replace': cfn_funcs.Replace,

heat/tests/test_hot.py

@@ -17,10 +17,10 @@ import six
 from heat.common import exception
 from heat.common import identifier
 from heat.common import template_format
-from heat.engine.cfn import functions as cfn_funcs
 from heat.engine import constraints
 from heat.engine import environment
 from heat.engine import function
+from heat.engine.hot import functions as hot_functions
 from heat.engine.hot import parameters as hot_param
 from heat.engine.hot import template as hot_template
 from heat.engine import parameters
@@ -585,8 +585,8 @@ class HOTemplateTest(HeatTestCase):
         parent_resource.stack = parser.Stack(utils.dummy_context(),
                                              'toplevel_stack',
                                              parser.Template(hot_tpl_empty))
-        del_policy = cfn_funcs.Join(parent_resource.stack,
-                                    'Fn::Join', ['eta', ['R', 'in']])
+        del_policy = hot_functions.Join(parent_resource.stack,
+                                        'list_join', ['eta', ['R', 'in']])
         parent_resource.t = rsrc_defn.ResourceDefinition(
             'parent', 'SomeType',
             deletion_policy=del_policy)