// Setup test environment.
	if err = xlStorage.MakeVol(t.Context(), "success-vol"); err != nil {
		t.Fatalf("Unable to create volume, %s", err)
	}

	// TestXLStorage failure cases.
	vol := slashpath.Join(path, "nonempty-vol")
	if err = os.Mkdir(vol, 0o777); err != nil {
		t.Fatalf("Unable to create directory, %s", err)
	}
	if err = os.WriteFile(slashpath.Join(vol, "test-file"), []byte{}, os.ModePerm); err != nil {

    ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> metaIterator) {
      iterator = emptyIterator();
      topMetaIterator = checkNotNull(metaIterator);
    }

    // Returns a nonempty meta-iterator or, if all meta-iterators are empty, null.
    private @Nullable Iterator<? extends Iterator<? extends T>> getTopMetaIterator() {
      while (topMetaIterator == null || !topMetaIterator.hasNext()) {

    ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> metaIterator) {
      iterator = emptyIterator();
      topMetaIterator = checkNotNull(metaIterator);
    }

    // Returns a nonempty meta-iterator or, if all meta-iterators are empty, null.
    private @Nullable Iterator<? extends Iterator<? extends T>> getTopMetaIterator() {
      while (topMetaIterator == null || !topMetaIterator.hasNext()) {