add exists template function

docs/templates.md

@@ -376,3 +376,38 @@ Render:
 ```
 QUEUE-NAME
 ```
+
+#### varExists
+
+Tests for the existence of a variable in the variables file and returns true
+if it exists, otherwise false. The first argument should be a map containing
+the variable in question, the second is a string containing the name of the
+variable to test for.
+
+Example:
+```
+[[ if varExists . "constraint" ]]
+constraint {
+  attribute = "${[[ .constraint.attribute ]]}"
+  value = "[[ .constraint.value ]]"
+}
+[[ end ]]
+```
+
+Given a vars yaml file containing:
+```
+---
+constraint:
+  attribute: meta.foo
+  value: bar
+```
+
+Render:
+```
+constraint {
+  attribute = "${meta.foo}"
+  value = "bar"
+}
+```
+
+If `constraint` is missing the constraint stanza will not be rendered.

template/funcs.go

@@ -35,9 +35,20 @@ func funcMap(consulClient *consul.Client) template.FuncMap {
 		"timeNowTimezone":    timeNowTimezoneFunc(),
 		"toLower":            toLower,
 		"toUpper":            toUpper,
+		"varExists":          varExists,
 	}
 }
 
+func varExists(i interface{}, name string) (ok bool, err error) {
+	switch kv := i.(type) {
+	case map[string]interface{}:
+		_, ok = kv[name]
+	default:
+		err = errors.New("varExists cannot be used on non-maps")
+	}
+	return
+}
+
 func consulKeyFunc(consulClient *consul.Client) func(string) (string, error) {
 	return func(s string) (string, error) {