code: @CanIgnoreReturnValue
    ChannelFuture enqueue(QueuedCommand command, boolean flush) {
      // Detect erroneous code that tries to reuse command objects.
      Preconditions.checkArgument(command.promise() == null, "promise must not be set on command");
  
      ChannelPromise promise = channel.newPromise();
      command.promise(promise);
      queue.add(command);
      if (flush) {
        scheduleFlush();
      }
      return promise;
    }
docs: Enqueue a write command on the channel.
  
  @param command a write to be executed on the channel.
  @param flush true if a flush of the write should be schedule, false if a later call to
  enqueue will schedule the flush.