From 53afb72ae12955edaac20533d9fcfda12b630336 Mon Sep 17 00:00:00 2001
From: mpl <mathieu.lonjaret@gmail.com>
Date: Sat, 25 Jul 2020 01:44:34 +0200
Subject: [PATCH] Make a copy of .lastdone before updating it

Fixes #20
---
 main.go | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/main.go b/main.go
index a478ad3..5b457b2 100644
--- a/main.go
+++ b/main.go
@@ -445,8 +445,20 @@ func markDone(dldir, location string) error {
 	if *verboseFlag {
 		log.Printf("Marking %v as done", location)
 	}
-	// TODO(mpl): back up .lastdone before overwriting it, in case writing it fails.
-	if err := ioutil.WriteFile(filepath.Join(dldir, ".lastdone"), []byte(location), 0600); err != nil {
+	oldPath := filepath.Join(dldir, ".lastdone")
+	newPath := oldPath + ".bak"
+	if err := os.Rename(oldPath, newPath); err != nil {
+		if !os.IsNotExist(err) {
+			return err
+		}
+	}
+	if err := ioutil.WriteFile(oldPath, []byte(location), 0600); err != nil {
+		// restore from backup
+		if err := os.Rename(newPath, oldPath); err != nil {
+			if !os.IsNotExist(err) {
+				return err
+			}
+		}
 		return err
 	}
 	return nil
@@ -519,6 +531,9 @@ func (s *Session) download(ctx context.Context, location string) (string, error)
 			if v.Name() == ".lastdone" {
 				continue
 			}
+			if v.Name() == ".lastdone.bak" {
+				continue
+			}
 			fileEntries = append(fileEntries, v)
 		}
 		if len(fileEntries) < 1 {