def METHOD_NAME(server, monkeypatch):
    import threading
    import time
    import fsspec
    head = {"head_ok": "true", "head_give_length": "true"}
    urla = server + "/index/realfile"
    h = fsspec.filesystem("http", headers=head)
    thread_ids = {threading.current_thread().ident}
    f = h.open(urla, block_size=5, cache_type="background")
    orig = f.cache._fetch_block
    def wrapped(*a, **kw):
        thread_ids.add(threading.current_thread().ident)
        return orig(*a, **kw)
    f.cache._fetch_block = wrapped
    assert len(thread_ids) == 1
    f.read(1)
    time.sleep(0.1)  # second block is loading
    assert len(thread_ids) == 2