log: lazy evaluate without lock

log/handler.go

@@ -158,12 +158,15 @@ func RotatingFileHandler(path string, limit uint, formatter Format) (Handler, er
 	if counter == nil {
 		counter = new(countingWriter)
 	}
-	h := LazyHandler(FuncHandler(func(r *Record) error {
+	h := FuncHandler(func(r *Record) error {
 		_, err := counter.Write(formatter.Format(r))
 		return err
-	}))
+	})
 
 	return FuncHandler(func(r *Record) error {
+		// lazy evaluate, this needs no lock.
+		lazyEvaluateRecord(r)
+
 		counter.Lock()
 		defer counter.Unlock()
 		if counter.count > limit {
@@ -389,6 +392,12 @@ func BufferedHandler(bufSize int, h Handler) Handler {
 // it if you write your own Handler.
 func LazyHandler(h Handler) Handler {
 	return FuncHandler(func(r *Record) error {
+		lazyEvaluateRecord(r)
+		return h.Log(r)
+	})
+}
+
+func lazyEvaluateRecord(r *Record) {
 	// go through the values (odd indices) and reassign
 	// the values of any lazy fn to the result of its execution
 	hadErr := false
@@ -411,9 +420,6 @@ func LazyHandler(h Handler) Handler {
 	if hadErr {
 		r.Ctx = append(r.Ctx, errorKey, "bad lazy")
 	}
-
-		return h.Log(r)
-	})
 }
 
 func evaluateLazy(lz Lazy) (interface{}, error) {