private void callSpec(Closure<?> dirSpec, Path path) {
    Class<?> newContextClass = contextClass;
    Object owner = dirSpec.getOwner();
    if (owner instanceof Class) {
      newContextClass = (Class<?>)owner;
    } else if (owner != null) {
      newContextClass = owner.getClass();
    }
    FileSystemFixture fileSystemFixture = new FileSystemFixture(path, newContextClass);
    dirSpec.setResolveStrategy(Closure.DELEGATE_FIRST);
    dirSpec.setDelegate(fileSystemFixture);
    dirSpec.call();
  }
Basically the same as {@link DirectoryFixture#dir(String, Closure)}, but with the current root as base.
spock-core/src/main/java/spock/util/io/FileSystemFixture.java