code: public ApiFuture<?> mutate(final Mutation mutation) {
      closedReadLock.lock();
      try {
        if (closed) {
          throw new IllegalStateException("Cannot mutate when the BufferedMutator is closed.");
        }
        return offer(mutation);
      } finally {
        closedReadLock.unlock();
      }
    }
docs: Being a Mutation. This method will block if either of the following are true:
  1) There are more than {@code maxInflightRpcs} RPCs in flight
  2) There are more than {@link #getWriteBufferSize()} bytes pending
  @param mutation a {@link Mutation} object.
  @return a {@link ApiFuture} object.