Required options break child processes

$ cat test_stuff.py
def test_stuff():
    pass

$ cat conftest.py
import pytest
@pytest.hookimpl
def pytest_addoption(parser):
    parser.addoption("--my-option", required=True)

$ py.test --my-option=abcd
============================= test session starts ==============================
platform linux -- Python 3.6.5, pytest-4.0.1, py-1.7.0, pluggy-0.8.0
rootdir: /tmp/test, inifile:
plugins: xdist-1.24.1, forked-0.2
collected 1 item                                                               

test_stuff.py .                                                          [100%]

=========================== 1 passed in 0.01 seconds ===========================

$ py.test --my-option=abcd -n2
============================= test session starts ==============================
platform linux -- Python 3.6.5, pytest-4.0.1, py-1.7.0, pluggy-0.8.0
rootdir: /tmp/test, inifile:
plugins: xdist-1.24.1, forked-0.2
gw0 C / gw1 Cusage: -c [options] [file_or_dir] [file_or_dir] [...]
-c: error: the following arguments are required: --my-option
[gw0] node down: Traceback (most recent call last):
  File "/tmp/venv/lib64/python3.6/site-packages/execnet/gateway_base.py", line 1072, in executetask
    do_exec(co, loc)  # noqa
  File "<string>", line 1, in do_exec
  File "<remote exec>", line 263, in <module>
  File "<remote exec>", line 239, in remote_initconfig
  File "/tmp/venv/lib64/python3.6/site-packages/_pytest/config/__init__.py", line 704, in fromdictargs
    config.parse(args, addopts=False)
  File "/tmp/venv/lib64/python3.6/site-packages/_pytest/config/__init__.py", line 851, in parse
    args, self.option, namespace=self.option
  File "/tmp/venv/lib64/python3.6/site-packages/_pytest/config/argparsing.py", line 100, in parse_setoption
    parsedoption = self.parse(args, namespace=namespace)
  File "/tmp/venv/lib64/python3.6/site-packages/_pytest/config/argparsing.py", line 80, in parse
    return self.optparser.parse_args(args, namespace=namespace)
  File "/tmp/venv/lib64/python3.6/site-packages/_pytest/config/argparsing.py", line 348, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib64/python3.6/argparse.py", line 1762, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib64/python3.6/argparse.py", line 1997, in _parse_known_args
    ', '.join(required_actions))
  File "/tmp/venv/lib64/python3.6/site-packages/_pytest/config/argparsing.py", line 344, in error
    self.exit(EXIT_USAGEERROR, _("%(prog)s: error: %(message)s\n") % args)
  File "/usr/lib64/python3.6/argparse.py", line 2376, in exit
    _sys.exit(status)
SystemExit: 4

Replacing crashed worker gw0

The crash and replacement of worker repeats ad infinitum until the process is killed with Ctrl+C.

Start by reproducing the commands using conftest.py, test_stuff.py, and pytest_addoption with -n2. Trace worker configuration initialization around the _pytest/config/init.py entries shown in the traceback and determine how required options reach child processes. Done means required options no longer crash and endlessly replace workers, while the single-process and distributed examples still pass.