copy config file to backup if unmarshal error occurs

backend/util/util.go

@@ -1,7 +1,9 @@
 package util
 
 import (
+	"io"
 	"math/rand"
+	"os"
 	"time"
 )
 
@@ -18,3 +20,20 @@ func ShuffleSlice(a []int) {
 	rand.Seed(time.Now().UnixNano())
 	rand.Shuffle(len(a), func(i, j int) { a[i], a[j] = a[j], a[i] })
 }
+
+func CopyFile(srcPath, dstPath string) error {
+	fin, err := os.Open(srcPath)
+	if err != nil {
+		return err
+	}
+	defer fin.Close()
+
+	fout, err := os.Create(dstPath)
+	if err != nil {
+		return err
+	}
+	defer fout.Close()
+
+	_, err = io.Copy(fout, fin)
+	return err
+}