ResourceWarning from some tests in python<=3.9

[asottile]

a bit of a self-tests issue -- I noticed this while implementing a test bisector

I haven't quite figured out the "why" for this yet either -- for me it only reproduces in python<=3.9 and not in python3.10+

$ ./venv/bin/pytest testing/test_recwarn.py::TestWarns::test_can_capture_previously_warned 'testing/test_terminal.py::test_summary_stats[red-exp_line0-stats_arg0]'
============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.1.0.dev230+g3b549642e, pluggy-1.0.0
rootdir: /tmp/pytest, configfile: pyproject.toml
plugins: hypothesis-6.37.2
collected 2 items                                                              

testing/test_recwarn.py .                                                [ 50%]
testing/test_terminal.py .                                               [100%]

============================== 2 passed in 0.15s ===============================
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='r' encoding='UTF-8'>
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name="<_io.FileIO name=14 mode='rb+' closefd=True>" mode='r+' encoding='utf-8'>
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name="<_io.FileIO name=16 mode='rb+' closefd=True>" mode='r+' encoding='utf-8'>

running either of those tests separately does not seem to cause the issue -- but running them together leads to these 3 leaked files

pytest/testing/test_recwarn.py

Lines 395 to 404 in 9af3e23

	@pytest.mark.filterwarnings("ignore")
	def test_can_capture_previously_warned(self) -> None:
	def f() -> int:
	warnings.warn(UserWarning("ohai"))
	return 10
	assert f() == 10
	assert pytest.warns(UserWarning, f) == 10
	assert pytest.warns(UserWarning, f) == 10
	assert pytest.warns(UserWarning, f) != "10" # type: ignore[comparison-overlap]

pytest/testing/test_terminal.py

Lines 1892 to 1915 in 9af3e23

	def test_summary_stats(
	tr: TerminalReporter,
	exp_line: List[Tuple[str, Dict[str, bool]]],
	exp_color: str,
	stats_arg: Dict[str, List[object]],
	) -> None:
	tr.stats = stats_arg
	# Fake "_is_last_item" to be True.
	class fake_session:
	testscollected = 0
	tr._session = fake_session # type: ignore[assignment]
	assert tr._is_last_item
	# Reset cache.
	tr._main_color = None
	print("Based on stats: %s" % stats_arg)
	print(f'Expect summary: "{exp_line}"; with color "{exp_color}"')
	(line, color) = tr.build_summary_stats_line()
	print(f'Actually got: "{line}"; with color "{color}"')
	assert line == exp_line
	assert color == exp_color

[nicoddemus]

Thanks for looking into this!

I can reproduce the same warnings on Windows.

Using python -Xdev -Xtracemalloc=20 -m pytest... does not show any additional information.

Passing -s the warnings go away, suggesting it is related to capture.py. Also with --capture=sys the warnings don't appear, suggesting further it is related to FDCapture.