* @see Project#isRootProject()
     */
    @Nonnull
    Path getRootDirectory();

    /**
     * Returns a proto session builder of this instance.
     */
    @Nonnull
    default Builder toBuilder() {
        try {
            return new Builder(
                    getUserProperties(), getSystemProperties(), getStartTime(), getTopDirectory(), getRootDirectory());
        } catch (IllegalStateException e) {

        try {
            final Blob blob = storage.get(BlobId.of(bucket, objectName));
            if (blob != null) {
                // GCS uses metadata instead of tags
                blob.toBuilder().setMetadata(tags).build().update();
            } else {
                throw new StorageException("Object not found: " + objectName);
            }
        } catch (final StorageException e) {
            throw e;

                .lookup(PropertyContributorsHolder.class)
                .getPropertyContributors()
                .values()) {
            protoSession = protoSession.toBuilder()
                    .withUserProperties(propertyContributor.contribute(protoSession))
                    .build();
        }
        context.protoSession = protoSession;
    }