There is no way to know actual completing write data by QSerialPort

When data is written by QSerialPort, there is still data in buffer or not complete to write data even QSerialPort::bytesToWrite is 0 after calling QSerialPort::write.
 
Reproducible steps:
 

• Using cross cable.

  /dev/ttyUSB0 /dev/ttyUSB1
  (FT232R 1)    (FT232R 2)
  TXD ----------- RXD
  RXD ----------- TXD
  GND ----------- GND
  

• Create data

  $ base64 qtserialport-everywhere-src-5.12.3.tar.xz > INPUT
  $ wc -c INPUT
  408968 INPUT
  

• Using creadersync and cwritersync which are contained examples for Qt SerialPort.

  $ PATH/TO/examples/serialport/creadersync/creadersync ttyUSB1 9600 | sed -e '1 d' > OUTPUT &
  [1] 00000 00000
  $ cat INPUT | PATH/TO/examples/serialport/cwritersync/cwritersync ttyUSB0 9600
  Data successfully sent to port ttyUSB0
  $
  [1] + 00000 done ./creadersync/creadersync ttyUSB1 | 00000 done sed -e '1 d' > ./OUTPUT
  $
  

   
  Actual: cwritersync is terminated even though it has not completed writing, then read data by creadersync is dropped.

  $ wc -c OUTPUT
  5381 OUTPUT
  

  Expected: Get actual timing to complete writing data.