* @return the value returned by the factory
     */
    public abstract <V> V guardByKey(T key, Supplier<V> supplier);

    private static class AdaptiveProducerGuard<T> extends ProducerGuard<T> {
        private final Set<T> producing = new HashSet<>();

        @Override
        public <V> V guardByKey(T key, Supplier<V> supplier) {
            synchronized (producing) {
                while (!producing.add(key)) {

                    case Directory:
                        return Optional.empty();
                    case RegularFile:
                        return Optional.of(producingSnapshots.guardByKey(location,
                            () -> virtualFileSystem.findSnapshot(location)
                                .orElseGet(() -> {

            throw UncheckedException.throwAsUncheckedException(e);
        }
    }

    @Override
    public V get(final K key, final Function<? super K, ? extends V> producer) {
        return producerGuard.guardByKey(key, () -> {
            if (!entries.containsKey(key)) {
                put(key, producer.apply(key));
            }
            return TestInMemoryIndexedCache.this.getIfPresent(key);
        });
    }