func (tr *Reader) readHeader() (*Header, *block, error) {
	// Two blocks of zero bytes marks the end of the archive.
	if _, err := io.ReadFull(tr.r, tr.blk[:]); err != nil {
		return nil, nil, err // EOF is okay here; exactly 0 bytes read
	}
	if bytes.Equal(tr.blk[:], zeroBlock[:]) {
		if _, err := io.ReadFull(tr.r, tr.blk[:]); err != nil {
			return nil, nil, err // EOF is okay here; exactly 1 block of zeros read
		}

	if err != nil {
		t.Fatal("opening:", err)
	}
	rc.(*checksumReader).hash = fakeHash32{}
	for i := 0; i < chunks; i++ {
		_, err := io.ReadFull(rc, chunk)
		if err != nil {
			t.Fatal("read:", err)
		}
	}
	if frag := int(size % chunkSize); frag > 0 {
		_, err := io.ReadFull(rc, chunk[:frag])
		if err != nil {
			t.Fatal("read:", err)
		}
	}
	gotEnd, err := io.ReadAll(rc)
	if err != nil {