private @Nullable WatchKey check(@Nullable WatchKey key) {
    if (key == poison) {
      // ensure other blocking threads get the poison
      queue.offer(poison);
      throw new ClosedWatchServiceException();
    }
    return key;
  }
Returns the given key, throwing an exception if it's the poison.
jimfs/src/main/java/com/google/common/jimfs/AbstractWatchService.java