From 83652f59db7639bcc477d45132eb393a06cb4de8 Mon Sep 17 00:00:00 2001
From: Oliver Steele <steele@osteele.com>
Date: Thu, 3 Aug 2017 15:11:34 +0200
Subject: [PATCH] Actually cache the drop resolution

---
 values/drop.go  | 35 ++++++++++++++---------------------
 values/value.go |  2 +-
 2 files changed, 15 insertions(+), 22 deletions(-)

diff --git a/values/drop.go b/values/drop.go
index c768961..6c1b372 100644
--- a/values/drop.go
+++ b/values/drop.go
@@ -1,6 +1,8 @@
 package values
 
-import "sync"
+import (
+	"sync"
+)
 
 type drop interface {
 	ToLiquid() interface{}
@@ -16,31 +18,22 @@ func ToLiquid(value interface{}) interface{} {
 	}
 }
 
-// embed this in a struct to give it default implementations of the Value interface
 type dropWrapper struct {
 	d drop
 	v Value
+	sync.Once
 }
 
-// Single mutex, until proven finer granularity helps.
-var dropResolverMu sync.Mutex
-
-func (w dropWrapper) Resolve() Value {
-	if w.v == nil {
-		dropResolverMu.Lock()
-		defer dropResolverMu.Unlock()
-		if w.v == nil {
-			w.v = ValueOf(w.d.ToLiquid())
-		}
-	}
+func (w *dropWrapper) Resolve() Value {
+	w.Do(func() { w.v = ValueOf(w.d.ToLiquid()) })
 	return w.v
 }
 
-func (w dropWrapper) Equal(o Value) bool          { return w.Resolve().Equal(o) }
-func (w dropWrapper) Less(o Value) bool           { return w.Resolve().Less(o) }
-func (w dropWrapper) IndexValue(i Value) Value    { return w.Resolve().IndexValue(i) }
-func (w dropWrapper) Contains(o Value) bool       { return w.Resolve().Contains(o) }
-func (w dropWrapper) Int() int                    { return w.Resolve().Int() }
-func (w dropWrapper) Interface() interface{}      { return w.Resolve().Interface() }
-func (w dropWrapper) PropertyValue(k Value) Value { return w.Resolve().PropertyValue(k) }
-func (w dropWrapper) Test() bool                  { return w.Resolve().Test() }
+func (w *dropWrapper) Equal(o Value) bool          { return w.Resolve().Equal(o) }
+func (w *dropWrapper) Less(o Value) bool           { return w.Resolve().Less(o) }
+func (w *dropWrapper) IndexValue(i Value) Value    { return w.Resolve().IndexValue(i) }
+func (w *dropWrapper) Contains(o Value) bool       { return w.Resolve().Contains(o) }
+func (w *dropWrapper) Int() int                    { return w.Resolve().Int() }
+func (w *dropWrapper) Interface() interface{}      { return w.Resolve().Interface() }
+func (w *dropWrapper) PropertyValue(k Value) Value { return w.Resolve().PropertyValue(k) }
+func (w *dropWrapper) Test() bool                  { return w.Resolve().Test() }
diff --git a/values/value.go b/values/value.go
index 1bfee83..1ce993a 100644
--- a/values/value.go
+++ b/values/value.go
@@ -41,7 +41,7 @@ func ValueOf(value interface{}) Value { // nolint: gocyclo
 	// interfaces
 	switch v := value.(type) {
 	case drop:
-		return dropWrapper{d: v}
+		return &dropWrapper{d: v}
 	case yaml.MapSlice:
 		return mapSliceValue{slice: v}
 	case Value: