for (int i = 0; i < threadCount; i++) {
      executor.execute(() -> {
        try {
          drainQueue();
        } catch (Throwable e) {
          e.printStackTrace();
        }
      });
    }
    executor.shutdown();
  }

  private void drainQueue() throws Exception {
    for (HttpUrl url; (url = queue.take()) != null; ) {
      if (!fetchedUrls.add(url)) {
        continue;

          return existing;
        }
      }
      drainQueue();
      return created;
    }

    // N.B. Draining the queue is only necessary to ensure that we don't accumulate empty references
    // in the array. We could skip this if we decide we don't care about holding on to Reference
    // objects indefinitely.
    private void drainQueue() {
      Reference<? extends L> ref;