stepwise doesn't stop the suite on first failure with xdist

Stepwise is supposed to behave like -x but this doesn't work with xdist:

You may think of it as a combination of the -x option, which exits the test run after a failing test, and the --lf option from pytest-cache, which only runs failing tests.

graingert@onomastic:~/projects/foo$ cat tests/test_foo.py 
import pytest

def test_1():
    assert True

def test_2():
    assert True

def test_3():
    assert False

def test_4():
    assert True

def test_5():
    assert True

def test_6():
    assert True

Running it the first time, the test suite doesn't stop: (no -x behavior)

graingert@onomastic:~/projects/foo$ pytest -vvv --sw -n 1 tests/
=========================================================================================== test session starts ============================================================================================
platform linux -- Python 3.7.3, pytest-5.0.1, py-1.8.0, pluggy-0.12.0 -- /home/graingert/.virtualenvs/pytest/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/graingert/projects/foo/tests, inifile: pytest.ini
plugins: xdist-1.29.0, forked-1.0.2
[gw0] linux Python 3.7.3 cwd: /home/graingert/projects/foo
[gw0] Python 3.7.3 (default, Apr  3 2019, 05:39:12)  -- [GCC 8.3.0]
gw0 [6]
scheduling tests via LoadScheduling

tests/test_foo.py::test_1 
[gw0] [ 16%] PASSED tests/test_foo.py::test_1 
tests/test_foo.py::test_2 
[gw0] [ 33%] PASSED tests/test_foo.py::test_2 
tests/test_foo.py::test_3 
[gw0] [ 50%] FAILED tests/test_foo.py::test_3 
tests/test_foo.py::test_4 
[gw0] [ 66%] PASSED tests/test_foo.py::test_4 
tests/test_foo.py::test_5 
[gw0] [ 83%] PASSED tests/test_foo.py::test_5 
tests/test_foo.py::test_6 
[gw0] [100%] PASSED tests/test_foo.py::test_6 

================================================================================================= FAILURES =================================================================================================
__________________________________________________________________________________________________ test_3 __________________________________________________________________________________________________
[gw0] linux -- Python 3.7.3 /home/graingert/.virtualenvs/pytest/bin/python3.7

    def test_3():
>       assert False
E       assert False

tests/test_foo.py:10: AssertionError
==================================================================================== 1 failed, 5 passed in 0.23 seconds ====================================================================================

Running it the second time, the test suite starts from the first failing test
but doesn't stop: (no -x behavior)

graingert@onomastic:~/projects/foo$ pytest -vvv --sw -n 1 tests/
=========================================================================================== test session starts ============================================================================================
platform linux -- Python 3.7.3, pytest-5.0.1, py-1.8.0, pluggy-0.12.0 -- /home/graingert/.virtualenvs/pytest/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/graingert/projects/foo/tests, inifile: pytest.ini
plugins: xdist-1.29.0, forked-1.0.2
[gw0] linux Python 3.7.3 cwd: /home/graingert/projects/foo
[gw0] Python 3.7.3 (default, Apr  3 2019, 05:39:12)  -- [GCC 8.3.0]
gw0 [4]
scheduling tests via LoadScheduling

tests/test_foo.py::test_3 
[gw0] [ 25%] FAILED tests/test_foo.py::test_3 
tests/test_foo.py::test_4 
[gw0] [ 50%] PASSED tests/test_foo.py::test_4 
tests/test_foo.py::test_5 
[gw0] [ 75%] PASSED tests/test_foo.py::test_5 
tests/test_foo.py::test_6 
[gw0] [100%] PASSED tests/test_foo.py::test_6 

================================================================================================= FAILURES =================================================================================================
__________________________________________________________________________________________________ test_3 __________________________________________________________________________________________________
[gw0] linux -- Python 3.7.3 /home/graingert/.virtualenvs/pytest/bin/python3.7

    def test_3():
>       assert False
E       assert False

tests/test_foo.py:10: AssertionError
==================================================================================== 1 failed, 3 passed in 0.21 seconds ====================================================================================

Ok lets make the test pass:

graingert@onomastic:~/projects/foo$ cat tests/test_foo.py 
import pytest

def test_1():
    assert True

def test_2():
    assert True

def test_3():
    assert True

def test_4():
    assert True

def test_5():
    assert True

def test_6():
    assert True

ok it starts from the failure and continues the suite, this is correct

graingert@onomastic:~/projects/foo$ pytest -vvv --sw -n 1 tests/
=========================================================================================== test session starts ============================================================================================
platform linux -- Python 3.7.3, pytest-5.0.1, py-1.8.0, pluggy-0.12.0 -- /home/graingert/.virtualenvs/pytest/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/graingert/projects/foo/tests, inifile: pytest.ini
plugins: xdist-1.29.0, forked-1.0.2
[gw0] linux Python 3.7.3 cwd: /home/graingert/projects/foo
[gw0] Python 3.7.3 (default, Apr  3 2019, 05:39:12)  -- [GCC 8.3.0]
gw0 [4]
scheduling tests via LoadScheduling

tests/test_foo.py::test_3 
[gw0] [ 25%] PASSED tests/test_foo.py::test_3 
tests/test_foo.py::test_4 
[gw0] [ 50%] PASSED tests/test_foo.py::test_4 
tests/test_foo.py::test_5 
[gw0] [ 75%] PASSED tests/test_foo.py::test_5 
tests/test_foo.py::test_6 
[gw0] [100%] PASSED tests/test_foo.py::test_6 

========================================================================================= 4 passed in 0.22 seconds =========================================================================================

and when run again the whole suite passes, this is correct too

graingert@onomastic:~/projects/foo$ pytest -vvv --sw -n 1 tests/
=========================================================================================== test session starts ============================================================================================
platform linux -- Python 3.7.3, pytest-5.0.1, py-1.8.0, pluggy-0.12.0 -- /home/graingert/.virtualenvs/pytest/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/graingert/projects/foo/tests, inifile: pytest.ini
plugins: xdist-1.29.0, forked-1.0.2
[gw0] linux Python 3.7.3 cwd: /home/graingert/projects/foo
[gw0] Python 3.7.3 (default, Apr  3 2019, 05:39:12)  -- [GCC 8.3.0]
gw0 [6]
scheduling tests via LoadScheduling

tests/test_foo.py::test_1 
[gw0] [ 16%] PASSED tests/test_foo.py::test_1 
tests/test_foo.py::test_2 
[gw0] [ 33%] PASSED tests/test_foo.py::test_2 
tests/test_foo.py::test_3 
[gw0] [ 50%] PASSED tests/test_foo.py::test_3 
tests/test_foo.py::test_4 
[gw0] [ 66%] PASSED tests/test_foo.py::test_4 
tests/test_foo.py::test_5 
[gw0] [ 83%] PASSED tests/test_foo.py::test_5 
tests/test_foo.py::test_6 
[gw0] [100%] PASSED tests/test_foo.py::test_6 

========================================================================================= 6 passed in 0.21 seconds =========================================================================================

Start by reproducing the reported commands with pytest, --sw, -n 1, and the example tests/test_foo.py suite. Compare the observed run with the expected -x behavior: execution should stop at the first failure while retaining stepwise restart behavior, and the suite should continue normally after that failure passes.