Provide resource to allow clients to expand custom expressions

Clients may now format a string as if it were to be an expression for an attribute,
and have the server evaluate it using the same engine without passing through the
attribute engine.  This makes it easier, for example, to do nodeexec n1-n4 ipmitool -H {hardwaremanagement.manager}

confluent_server/confluent/config/configmanager.py

@@ -761,6 +761,14 @@ class ConfigManager(object):
                                                    decrypt=self.decrypt)
         return nodeobj
 
+    def expand_attrib_expression(self, nodelist, expression):
+        if type(nodelist) in (unicode, str):
+            nodelist = (nodelist,)
+        for node in nodelist:
+            cfgobj = self._cfgstore['nodes'][node]
+            fmt = _ExpressionFormat(cfgobj, node)
+            yield (node, fmt.format(expression))
+
     def get_node_attributes(self, nodelist, attributes=(), decrypt=None):
         if decrypt is None:
             decrypt = self.decrypt

confluent_server/confluent/core.py

@@ -122,6 +122,7 @@ def _init_core():
         'attributes': {
             'all': PluginRoute({'handler': 'attributes'}),
             'current': PluginRoute({'handler': 'attributes'}),
+            'expression': PluginRoute({'handler': 'attributes'}),
         },
         'boot': {
             'nextdevice': PluginRoute({

confluent_server/confluent/plugins/configuration/attributes.py

@@ -152,6 +152,20 @@ def update_nodegroup(group, element, configmanager, inputdata):
     return retrieve_nodegroup(group, element, configmanager, inputdata)
 
 
+def _expand_expression(nodes, configmanager, inputdata):
+    expression = inputdata.get_attributes(list(nodes)[0])
+    if type(expression) is dict:
+        expression = expression['expression']
+    if type(expression) is dict:
+        expression = expression['expression']
+    for expanded in configmanager.expand_attrib_expression(nodes, expression):
+        yield msg.KeyValueData({'value': expanded[1]}, expanded[0])
+
+
+def create(nodes, element, configmanager, inputdata):
+    if nodes is not None and element[-1] == 'expression':
+        return _expand_expression(nodes, configmanager, inputdata)
+
 def update_nodes(nodes, element, configmanager, inputdata):
     updatedict = {}
     for node in nodes: