cmd/syncthing: Copy config on upgrade, instead of renaming (fixes #3525)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3560

cmd/syncthing/main.go

@@ -889,19 +889,32 @@ func loadOrCreateConfig() *config.Wrapper {
 }
 
 func archiveAndSaveConfig(cfg *config.Wrapper) error {
-	// To prevent previous config from being cleaned up, quickly touch it too
-	now := time.Now()
-	_ = os.Chtimes(cfg.ConfigPath(), now, now) // May return error on Android etc; no worries
-
+	// Copy the existing config to an archive copy
 	archivePath := cfg.ConfigPath() + fmt.Sprintf(".v%d", cfg.Raw().OriginalVersion)
 	l.Infoln("Archiving a copy of old config file format at:", archivePath)
-	if err := osutil.Rename(cfg.ConfigPath(), archivePath); err != nil {
+	if err := copyFile(cfg.ConfigPath(), archivePath); err != nil {
 		return err
 	}
 
+	// Do a regular atomic config sve
 	return cfg.Save()
 }
 
+func copyFile(src, dst string) error {
+	bs, err := ioutil.ReadFile(src)
+	if err != nil {
+		return err
+	}
+
+	if err := ioutil.WriteFile(dst, bs, 0600); err != nil {
+		// Attempt to clean up
+		os.Remove(dst)
+		return err
+	}
+
+	return nil
+}
+
 func startAuditing(mainService *suture.Supervisor) {
 	auditFile := timestampedLoc(locAuditLog)
 	fd, err := os.OpenFile(auditFile, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)