require(reasonBytes.size <= CLOSE_MESSAGE_MAX) {
        "reason.size() > $CLOSE_MESSAGE_MAX: $reason"
      }
    }

    if (failed || enqueuedClose) return false

    // Immediately prevent further frames from being enqueued.
    enqueuedClose = true

    // Enqueue the close frame.
    messageAndCloseQueue.add(Close(code, reasonBytes, cancelAfterCloseMillis))
    runWriter()
    return true
  }