add a keys filter to make liquid iteration of maps predictable

(fixes random test failure)

site/site_test.go

@@ -197,7 +197,7 @@ tags: [software]
 	// add a page (no date)
 	content = `---
 ---
-{% for tag in site.tags %}<h1>{{tag[0]}}</h1>{% for post in tag[1] %}
+{% for tag in site.tags | keys | sort %}<h1>{{tag}}</h1>{% for post in site.tags[tag] %}
 {{post.title}}
 {% endfor %}
 {% endfor %}

templates/filters.go

@@ -30,6 +30,7 @@ func loadJekyllFilters(e *liquid.Engine, siteUrl string, includesDir string) {
 	e.RegisterFilter("group_by", groupByFilter)
 	e.RegisterFilter("group_by_exp", groupByExpFilter)
 	e.RegisterFilter("sort", sortFilter)
+	e.RegisterFilter("keys", keysFilter)
 	e.RegisterFilter("where", whereFilter)
 	e.RegisterFilter("where_exp", whereExpFilter)
 	e.RegisterFilter("xml_escape", xml.Marshal)
@@ -144,6 +145,16 @@ func groupByFilter(array []map[string]interface{}, property string) []map[string
 	return result
 }
 
+func keysFilter(m map[string]interface{}) []string {
+	keys := make([]string, len(m))
+	i := 0
+	for k := range m {
+		keys[i] = k
+		i++
+	}
+	return keys
+}
+
 func sortFilter(array []interface{}, key interface{}, nilFirst func(bool) bool) []interface{} {
 	nf := nilFirst(true)
 	result := make([]interface{}, len(array))