Test collection order inconsistent when parameter options come from sets

Test collection fails for me when using pytest-xdist:

$ pytest -n10 
====================================================== test session starts ======================================================
platform linux -- Python 3.8.4, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: /home/mth/bb/softfab, configfile: pyproject.toml
plugins: xdist-1.32.0, cov-2.10.1
gw0 [201] / gw1 [201] / gw2 [201] / gw3 [201] / gw4 [201] / gw5 [201] / gw6 [201] / gw7 [201] / gw8 [201] / gw9 [201]
collecting 0 items / 1 error                                                                                                    
============================================================ ERRORS =============================================================
_____________________________________________________ ERROR collecting gw0 ______________________________________________________
Different tests were collected between gw2 and gw0. The difference is:
--- gw2

+++ gw0

@@ -1,5 +1,5 @@

+tests/unit/test_commandline.py::test_create_user[user]
 tests/unit/test_commandline.py::test_create_user[guest]
-tests/unit/test_commandline.py::test_create_user[user]
 tests/unit/test_commandline.py::test_create_user[operator]
 tests/unit/test_commandline.py::test_create_user[None]
 tests/unit/test_commandline.py::test_remove_user

Similar diffs occur on the other workers as well.

There are more parameterized tests, but only test_create_user triggers this problem. The reason is that the four test cases after parameter substitution are not always in the same order because the parameter options originate from a set:

roleNames = {'guest', 'user', 'operator'}

@mark.parametrize('role', list(roleNames) + [None])
def test_create_user(cli, role):
    ...

If I change list(roleNames) to sorted(roleNames) or export PYTHONHASHSEED on the command line, the order of the options will always be the same and the problem goes away.

A possible solution would be to set PYTHONHASHSEED to the same value for all workers.

Start by reproducing with pytest -n10 and inspect collection behavior around tests/unit/test_commandline.py::test_create_user, whose role parameters come from a set. Done means workers collect the same test order without requiring users to sort parameters or set PYTHONHASHSEED.