repair packs: Add stub feature flag implementation

cmd/restic/cmd_check.go

@@ -349,7 +349,7 @@ func runCheck(ctx context.Context, opts CheckOptions, gopts GlobalOptions, args
 			for _, id := range salvagePacks {
 				strIds = append(strIds, id.String())
 			}
-			Warnf("restic repair packs %v\nrestic repair snapshots --forget\n\n", strings.Join(strIds, " "))
+			Warnf("RESTIC_FEATURES=repair-packs-v1 restic repair packs %v\nrestic repair snapshots --forget\n\n", strings.Join(strIds, " "))
 			Warnf("Corrupted blobs are either caused by hardware problems or bugs in restic. Please open an issue at https://github.com/restic/restic/issues/new/choose for further troubleshooting!\n")
 		}
 	}

cmd/restic/cmd_repair_packs.go

@@ -36,9 +36,14 @@ func init() {
 	cmdRepair.AddCommand(cmdRepairPacks)
 }
 
-// FIXME feature flag
-
 func runRepairPacks(ctx context.Context, gopts GlobalOptions, args []string) error {
+	// FIXME discuss and add proper feature flag mechanism
+	flag, _ := os.LookupEnv("RESTIC_FEATURES")
+	if flag != "repair-packs-v1" {
+		return errors.Fatal("This command is experimental and may change/be removed without notice between restic versions. " +
+			"Set the environment variable 'RESTIC_FEATURES=repair-packs-v1' to enable it.")
+	}
+
 	ids := restic.NewIDSet()
 	for _, arg := range args {
 		id, err := restic.ParseID(arg)