Support stdout / stdin for backup and restore

This adds support for passing a single dash character as the filename
to the backup command, resulting in it using stdin.  It also changes
the restore command to default to reading from stdin if no filename is
given.

Fixes: #284

internal/cmd/backup.go

@@ -29,18 +29,30 @@ var backupCmd = &cobra.Command{
 	RunE:  backupCmdFunc,
 }
 
-func backupCmdFunc(cmd *cobra.Command, args []string) error {
-	filename := args[0]
+func createBackupFile(filename string) (*os.File, error) {
+	if filename == "-" {
+		log.Trace().Str("filename", "- (stdout)").Send()
+		return os.Stdout, nil
+	}
 
 	log.Trace().Str("filename", filename).Send()
 
 	if _, err := os.Stat(filename); err == nil {
-		return fmt.Errorf("backup file already exists: %s", filename)
+		return nil, fmt.Errorf("backup file already exists: %s", filename)
 	}
 
 	f, err := os.Create(filename)
 	if err != nil {
-		return fmt.Errorf("unable to create backup file: %w", err)
+		return nil, fmt.Errorf("unable to create backup file: %w", err)
+	}
+
+	return f, nil
+}
+
+func backupCmdFunc(cmd *cobra.Command, args []string) error {
+	f, err := createBackupFile(args[0])
+	if err != nil {
+		return err
 	}
 
 	client, err := client.NewClient(cmd)
@@ -116,8 +128,10 @@ func backupCmdFunc(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("error closing backup encoder: %w", err)
 	}
 
-	if err := f.Sync(); err != nil {
-		return fmt.Errorf("error syncing backup file: %w", err)
+	if f != os.Stdout {
+		if err := f.Sync(); err != nil {
+			return fmt.Errorf("error syncing backup file: %w", err)
+		}
 	}
 
 	if err := f.Close(); err != nil {

internal/cmd/restore.go

@@ -28,31 +28,49 @@ func registerRestoreCmd(rootCmd *cobra.Command) {
 var restoreCmd = &cobra.Command{
 	Use:   "restore <filename>",
 	Short: "Restore a permission system from a file",
-	Args:  cobra.ExactArgs(1),
+	Args:  cobra.MaximumNArgs(1),
 	RunE:  restoreCmdFunc,
 }
 
-func restoreCmdFunc(cmd *cobra.Command, args []string) error {
-	filename := args[0]
+func openRestoreFile(filename string) (*os.File, int64, error) {
+	if filename == "" {
+		log.Trace().Str("filename", "(stdin)").Send()
+		return os.Stdin, -1, nil
+	}
 
 	log.Trace().Str("filename", filename).Send()
 
 	stats, err := os.Stat(filename)
 	if err != nil {
-		return fmt.Errorf("unable to stat restore file: %w", err)
+		return nil, 0, fmt.Errorf("unable to stat restore file: %w", err)
 	}
 
 	f, err := os.Open(filename)
 	if err != nil {
-		return fmt.Errorf("unable to open restore file: %w", err)
+		return nil, 0, fmt.Errorf("unable to open restore file: %w", err)
+	}
+
+	return f, stats.Size(), nil
+}
+
+func restoreCmdFunc(cmd *cobra.Command, args []string) error {
+	filename := "" // Default to stdin.
+
+	if len(args) > 0 {
+		filename = args[0]
+	}
+
+	f, fSize, err := openRestoreFile(filename)
+	if err != nil {
+		return err
 	}
 
 	printZTOnly := cobrautil.MustGetBool(cmd, "print-zedtoken-only")
 
 	var hasProgressbar bool
 	var restoreReader io.Reader = f
 	if isatty.IsTerminal(os.Stderr.Fd()) && !printZTOnly {
-		bar := progressbar.DefaultBytes(stats.Size(), "restoring")
+		bar := progressbar.DefaultBytes(fSize, "restoring")
 		restoreReader = io.TeeReader(f, bar)
 		hasProgressbar = true
 	}