Node event loop stops while the write operation is in progress

[kimushu]

• SerialPort Version: 6.0.0

• NodeJS Version: 6.9.5 (64-bit)

• Operating System and Hardware Platform: Windows 10 Pro (10.0.15063) x64

  • Serial port: FTDI USB-Serial converter cable TTL-232R-3V3 (FT232RQ chip)

• Have you checked the right version of the api docs?: yes

• Are you having trouble installing and you checked the Installation Special Cases docs? no

• Are you using Electron and have you checked the Electron Docs?: no

Summary of Problem

Event loop stops even if there is an incomplete write operation.
For example, simple open & write program (here) does not work.
I guess the new I/O for windows (#1221) does not preserve any AsyncWorker in write operation.

Steps and Code to Reproduce the Issue

• Event loop stops before message written message if there are no pending callbacks nor timers.

$ cat open_write.js
var SerialPort = require('serialport');
var port = new SerialPort('COM8');

port.write('main screen turn on', function(err) {
  if (err) {
    return console.log('Error on write: ', err.message);
  }
  console.log('message written');
});

// Open errors will be emitted as an error event
port.on('error', function(err) {
  console.log('Error: ', err.message);
});

$ node open_write.js
                                        # <- No output here!
$ echo $?
0                                       # <- node stopped without error...

• If there are some pending callbacks or timers, write operation & callback seem to be ok. However, event loop does not stop even if write operation has been finished

$ cat open_write_with_timer.js
var SerialPort = require('serialport');
var port = new SerialPort('COM8');

port.write('main screen turn on', function(err) {
  if (err) {
    return console.log('Error on write: ', err.message);
  }
  console.log('message written');
});

// Open errors will be emitted as an error event
port.on('error', function(err) {
  console.log('Error: ', err.message);
});

setTimeout(() => { console.log("workaround!"); }, 1000);

$ node open_write_with_timer.js
message written                         # <- Write callback is ok...
workaround!
                                        # However, event loop seems be frozen (Node is still running...)

[reconbot]

So if we have a pending write the event loop still appears empty and node will quit. And possibly we keep something in the loop queue after the write finishes so we never quit.

[kimushu]

Correct. There are two problems.
As for the former one (node quit during write), I have an idea to fix it by adding uv_idle (kimushu@f3a94bf)

[kimushu]

uv_async_t handle is not correctly closed (here). This disturbs finish of event loop.
I have a proposal to solve both problems in this issue. See #1367.

[reconbot]

This is was great, thank you so much. It's always nice when people give good bug reports, and it's even more amazing when they close them too. =)

[kimushu]

It's a pleasure to contribute :-)