Ignore error for Chmod() on FS that don't support it

See #1079

src/restic/backend/local/local_unix.go

@@ -5,15 +5,9 @@ package local
 import (
 	"os"
 	"restic/fs"
-	"syscall"
 )
 
 // set file to readonly
 func setNewFileMode(f string, fi os.FileInfo) error {
-	err := fs.Chmod(f, fi.Mode()&os.FileMode(^uint32(0222)))
-	// ignore the error if the FS does not support setting this mode (e.g. CIFS with gvfs on Linux)
-	if perr, ok := err.(*os.PathError); ok && perr.Err == syscall.ENOTSUP {
-		err = nil
-	}
-	return err
+	return fs.Chmod(f, fi.Mode()&os.FileMode(^uint32(0222)))
 }

src/restic/fs/file.go

@@ -19,11 +19,6 @@ type File interface {
 	Stat() (os.FileInfo, error)
 }
 
-// Chmod changes the mode of the named file to mode.
-func Chmod(name string, mode os.FileMode) error {
-	return os.Chmod(fixpath(name), mode)
-}
-
 // Mkdir creates a new directory with the specified name and permission bits.
 // If there is an error, it will be of type *PathError.
 func Mkdir(name string, perm os.FileMode) error {

src/restic/fs/file_unix.go

@@ -5,6 +5,7 @@ package fs
 import (
 	"io/ioutil"
 	"os"
+	"syscall"
 )
 
 // fixpath returns an absolute path on windows, so restic can open long file
@@ -35,3 +36,23 @@ func TempFile(dir, prefix string) (f *os.File, err error) {
 
 	return f, nil
 }
+
+// isNotSuported returns true if the error is caused by an unsupported file system feature.
+func isNotSupported(err error) bool {
+	if perr, ok := err.(*os.PathError); ok && perr.Err == syscall.ENOTSUP {
+		return true
+	}
+	return false
+}
+
+// Chmod changes the mode of the named file to mode.
+func Chmod(name string, mode os.FileMode) error {
+	err := os.Chmod(fixpath(name), mode)
+
+	// ignore the error if the FS does not support setting this mode (e.g. CIFS with gvfs on Linux)
+	if err != nil && isNotSupported(err) {
+		return nil
+	}
+
+	return err
+}

src/restic/fs/file_windows.go

@@ -91,3 +91,8 @@ func MkdirAll(path string, perm os.FileMode) error {
 func TempFile(dir, prefix string) (f *os.File, err error) {
 	return ioutil.TempFile(dir, prefix)
 }
+
+// Chmod changes the mode of the named file to mode.
+func Chmod(name string, mode os.FileMode) error {
+	return os.Chmod(fixpath(name), mode)
+}