lock: add help message how to recover from invalid locks

cmd/restic/lock.go

@@ -44,6 +44,9 @@ func lockRepository(ctx context.Context, repo restic.Repository, exclusive bool)
 	}
 
 	lock, err := lockFn(ctx, repo)
+	if restic.IsInvalidLock(err) {
+		return nil, ctx, errors.Fatalf("%v\n\nthe `unlock --remove-all` command can be used to remove invalid locks. Make sure that no other restic process is accessing the repository when running the command", err)
+	}
 	if err != nil {
 		return nil, ctx, errors.Fatalf("unable to create lock in backend: %v", err)
 	}

internal/restic/lock.go

@@ -60,6 +60,27 @@ func IsAlreadyLocked(err error) bool {
 	return errors.As(err, &e)
 }
 
+// invalidLockError is returned when NewLock or NewExclusiveLock fail due
+// to an invalid lock.
+type invalidLockError struct {
+	err error
+}
+
+func (e *invalidLockError) Error() string {
+	return fmt.Sprintf("invalid lock file: %v", e.err)
+}
+
+func (e *invalidLockError) Unwrap() error {
+	return e.err
+}
+
+// IsInvalidLock returns true iff err indicates that locking failed due to
+// an invalid lock.
+func IsInvalidLock(err error) bool {
+	var e *invalidLockError
+	return errors.As(err, &e)
+}
+
 // NewLock returns a new, non-exclusive lock for the repository. If an
 // exclusive lock is already held by another process, it returns an error
 // that satisfies IsAlreadyLocked.
@@ -168,6 +189,9 @@ func (l *Lock) checkForOtherLocks(ctx context.Context) error {
 			return err
 		}
 	}
+	if errors.Is(err, ErrInvalidData) {
+		return &invalidLockError{err}
+	}
 	return err
 }