Combining pytest-cov and pytest-xdist leads to internal errors on setuptools

[ngoldbaum]

Summary

I noticed that coverage configuration in setuptools doesn't record imports as covered, because it doesn't pass --cov=setuptools to pytest. When I make that change, I get an internal error from pytest with a traceback in pytest-cov:

goldbaum at Nathans-MBP in ~/Documents/setuptools on cast-more-itertools!
± tox -e diffcov
diffcov: install_deps> python -I -m pip install diff-cover
.pkg: _optional_hooks> python /Users/goldbaum/.pyenv/versions/3.14.3/lib/python3.14/site-packages/pyproject_api/_backend.py True setuptools.build_meta
.pkg: get_requires_for_build_editable> python /Users/goldbaum/.pyenv/versions/3.14.3/lib/python3.14/site-packages/pyproject_api/_backend.py True setuptools.build_meta
.pkg: build_editable> python /Users/goldbaum/.pyenv/versions/3.14.3/lib/python3.14/site-packages/pyproject_api/_backend.py True setuptools.build_meta
diffcov: install_package_deps> python -I -m pip install 'build[virtualenv]>=1.0.3' 'filelock>=3.4.0' 'importlib_metadata>=6; python_version < "3.10"' 'importlib_metadata>=7.0.2; python_version < "3.10"' 'ini2toml[lite]>=0.14' 'jaraco.develop>=7.21; python_version >= "3.9" and sys_platform != "cygwin"' 'jaraco.develop>=7.21; sys_platform != "cygwin"' 'jaraco.envs>=2.2' 'jaraco.functools>=4' 'jaraco.path>=3.7.2' 'jaraco.test>=5.5' 'jaraco.text>=3.7' more_itertools 'more_itertools>=8.8' 'mypy==1.18.*' 'packaging>=24.2' 'pip>=19.1' 'pyproject-hooks!=1.1' 'pytest!=8.1.*,>=6' 'pytest-checkdocs>=2.4' pytest-cov 'pytest-enabler>=2.2' 'pytest-home>=0.5' pytest-mypy 'pytest-perf; sys_platform != "cygwin"' 'pytest-ruff>=0.2.1; sys_platform != "cygwin"' pytest-subprocess pytest-timeout 'pytest-xdist>=3' 'ruff>=0.13.0; sys_platform != "cygwin"' 'tomli-w>=1.0.0' 'tomli>=2.0.1; python_version < "3.11"' 'virtualenv>=13.0.0' 'wheel>=0.43.0' 'wheel>=0.44.0'
diffcov: install_package> python -I -m pip install --force-reinstall --no-deps /Users/goldbaum/Documents/setuptools/.tox/.tmp/package/1/setuptools-82.0.1-0.editable-py3-none-any.whl
diffcov: commands[0]> pytest --cov=setuptools --cov-report xml
====================================================================================================== test session starts ======================================================================================================
platform darwin -- Python 3.14.3, pytest-9.0.2, pluggy-1.6.0
cachedir: .tox/diffcov/.pytest_cache
rootdir: /Users/goldbaum/Documents/setuptools
configfile: pytest.ini
plugins: mypy-1.0.1, ruff-0.5, cov-7.1.0, xdist-3.8.0, timeout-2.4.0, jaraco.vcs-2.4.1, checkdocs-2.14.0, typeguard-4.5.1, jaraco.mongodb-12.4.0, home-0.6.0, perf-0.15.0, enabler-3.4.0, subprocess-1.5.4, jaraco.test-5.6.0
initialized: 1/11 workersINTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/_pytest/main.py", line 316, in wrap_session
INTERNALERROR>     config.hook.pytest_sessionstart(session=session)
INTERNALERROR>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_hooks.py", line 512, in __call__
INTERNALERROR>     return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
INTERNALERROR>            ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_manager.py", line 120, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
INTERNALERROR>            ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_callers.py", line 167, in _multicall
INTERNALERROR>     raise exception
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_callers.py", line 139, in _multicall
INTERNALERROR>     teardown.throw(exception)
INTERNALERROR>     ~~~~~~~~~~~~~~^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/_pytest/logging.py", line 780, in pytest_sessionstart
INTERNALERROR>     return (yield)
INTERNALERROR>             ^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_callers.py", line 121, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/xdist/dsession.py", line 90, in pytest_sessionstart
INTERNALERROR>     nodes = self.nodemanager.setup_nodes(putevent=self.queue.put)
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/xdist/workermanage.py", line 97, in setup_nodes
INTERNALERROR>     return [self.setup_node(spec, putevent) for spec in self.specs]
INTERNALERROR>             ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/xdist/workermanage.py", line 112, in setup_node
INTERNALERROR>     node.setup()
INTERNALERROR>     ~~~~~~~~~~^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/xdist/workermanage.py", line 342, in setup
INTERNALERROR>     self.config.hook.pytest_configure_node(node=self)
INTERNALERROR>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_hooks.py", line 512, in __call__
INTERNALERROR>     return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
INTERNALERROR>            ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_manager.py", line 120, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
INTERNALERROR>            ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_callers.py", line 167, in _multicall
INTERNALERROR>     raise exception
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pluggy/_callers.py", line 121, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/Users/goldbaum/Documents/setuptools/.tox/diffcov/lib/python3.14/site-packages/pytest_cov/plugin.py", line 307, in pytest_configure_node
INTERNALERROR>     self.cov_controller.configure_node(node)
INTERNALERROR>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR> AttributeError: 'Central' object has no attribute 'configure_node'
diffcov: exit 3 (2.88 seconds) /Users/goldbaum/Documents/setuptools> pytest --cov=setuptools --cov-report xml pid=32072
  diffcov: FAIL code 3 (16.43=setup[13.55]+cmd[2.88] seconds)
  evaluation failed :( (16.49 seconds)

The tests run correctly if pytest-xdist is disabled. Seems possibly related to #263.

Expected vs actual result

I expected the tests to run without triggering an exception in pytest-cov.

Reproducer

Apply the following diff to a git clone of setuptools:

diff --git a/tox.ini b/tox.ini
index 6562ac777..1257c788a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -43,7 +43,7 @@ deps =
        {[testenv]deps}
        diff-cover
 commands =
-       pytest {posargs} --cov-report xml
+       pytest {posargs} --cov=setuptools --cov-report xml
        diff-cover coverage.xml --compare-branch=origin/main --html-report diffcov.html
        diff-cover coverage.xml --compare-branch=origin/main --fail-under=100

And then run tox -e diffcov

Versions

See tox output.

Config

See setuptools repository.

Code

See setuptools repository.