Detaching sometimes throws ValueError: I/O operation on closed file.

[karthiknadig]

Environment data

• PTVSD version: master
• OS and version: linux
• Python version (& distribution if applicable, e.g. Anaconda): 3.5
• Using VS Code or Visual Studio: VSC

Actual behavior

I see this error on detach.

Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/ptvsd/_vendored/pydevd/pydevd.py", line 1905, in stoptrace
    debugger.exiting()
  File "/usr/local/lib/python3.5/dist-packages/ptvsd/_vendored/pydevd/pydevd.py", line 1590, in exiting
    sys.stdout.flush()
  File "/usr/local/lib/python3.5/dist-packages/ptvsd/_vendored/pydevd/_pydevd_bundle/pydevd_io.py", line 37, in flush
    r.flush()
ValueError: I/O operation on closed file.

Expected behavior

Should not throw error. Should continue user process.

Steps to reproduce:

Launch the following script:

import multiprocessing
multiprocessing.set_start_method('spawn', True)
import concurrent.futures

FIBS = [28, 10]

def fib(n):
    print(n)
    if n < 2:
        return 1
    return fib(n - 1) + fib(n - 2)


def main():
    with concurrent.futures.ProcessPoolExecutor() as executor:
        executor.map(fib, FIBS)


if __name__ == '__main__':
    main()

Using the following command:
python3 -m ptvsd --host 0.0.0.0 --port 5678 --wait --multiprocess ./mymain.py

Attach to this from VSC, with breakpoint set on the print line.

        {
            "name": "Attach",
            "type": "python",
            "request": "attach",
            "port": 5678,
            "host": "localhost",
            "pathMappings": [
                {
                    "localRoot": "${workspaceFolder}",
                    "remoteRoot": "."
                }
            ],
            "debugStdLib": true,
        },

[int19h]

On Windows, the repro steps trigger a different exception, when detaching from the root process:

Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "C:\Python\3.7-64\lib\concurrent\futures\process.py", line 101, in _python_exit
    thread_wakeup.wakeup()
  File "C:\Python\3.7-64\lib\concurrent\futures\process.py", line 89, in wakeup
    self._writer.send_bytes(b"")
  File "C:\Python\3.7-64\lib\multiprocessing\connection.py", line 183, in send_bytes
    self._check_closed()
  File "C:\Python\3.7-64\lib\multiprocessing\connection.py", line 136, in _check_closed
    raise OSError("handle is closed")
OSError: handle is closed

[int19h]

OSError is an unrelated issue, and happens because we kill child processes when detaching from parent. ProcessPoolExecutor expects them to still be there when running its cleanup code via atexit, and doesn't try to guard against the possibility of failure. Not sure what we can do here.