Parallel runs expose usage of closed fds

[vit-zikmund]

Issue

Running tox in parallel intermittently exposes operation on a closed fd. That results in various intermittent errors like Bad file descriptor or Input/output error.

This is broken on the very current release 4.56.2. Previous releases (e.g. 4.56.1) don't manifest this error.

Environment

Provide at least:

• OS: Fedora Linux 43

Output of uv pip list of the host Python, where tox is installed

Package          Version Editable project location
---------------- ------- ----------------------------------------------------
cachetools       7.1.4
colorama         0.4.6
coverage         7.15.0
distlib          0.4.3
filelock         3.29.7
iniconfig        2.3.0
packaging        26.2
platformdirs     4.10.0
pluggy           1.6.0
pygments         2.20.0
pyproject-api    1.10.1
pytest           9.1.1
pytest-asyncio   1.4.0
pytest-cov       7.1.0
pytest-mock      3.15.1
python-discovery 1.4.3
test-test        0.1.0   /tmp/test-test2
tomli-w          1.2.0
tox              4.56.2
tox-uv-bare      1.33.4
virtualenv       21.6.0

Output of running tox

Output of uv run -- tox -rvv --parallel auto -e 3.10,3.11,3.12,3.13,3.14

3.10: 174 D using system uv from PATH: ~/.local/bin/uv (uv 0.11.6 (x86_64-unknown-linux-gnu)) [tox_uv/_venv.py:248]
3.11: 181 D using system uv from PATH: ~/.local/bin/uv (uv 0.11.6 (x86_64-unknown-linux-gnu)) [tox_uv/_venv.py:248]
3.12: 191 D using system uv from PATH: ~/.local/bin/uv (uv 0.11.6 (x86_64-unknown-linux-gnu)) [tox_uv/_venv.py:248]
3.13: 200 D using system uv from PATH: ~/.local/bin/uv (uv 0.11.6 (x86_64-unknown-linux-gnu)) [tox_uv/_venv.py:248]
3.14: 209 D using system uv from PATH: ~/.local/bin/uv (uv 0.11.6 (x86_64-unknown-linux-gnu)) [tox_uv/_venv.py:248]
current: 216 D using system uv from PATH: ~/.local/bin/uv (uv 0.11.6 (x86_64-unknown-linux-gnu)) [tox_uv/_venv.py:248]
3.10: FAIL ✖ in 0.11 seconds
3.10: 217 W remove tox env folder /tmp/test-test2/.tox/3.10 [tox/tox_env/api.py:348]
3.10: 271 W venv> ~/.local/bin/uv venv -p cpython3.10 --allow-existing '--prompt=test-test2[3.10]' -v --python-preference system /tmp/test-test2/.tox/3.10 [tox/tox_env/api.py:485]
3.10: 331 C exit 101 (0.06 seconds) /tmp/test-test2> ~/.local/bin/uv venv -p cpython3.10 --allow-existing '--prompt=test-test2[3.10]' -v --python-preference system /tmp/test-test2/.tox/3.10 pid=617259 [tox/execute/api.py:308]
3.14: OK ✔ in 1.25 seconds
⠼ [3/5] 3.11 | 3.12 | 3.13DEBUG uv 0.11.6 (x86_64-unknown-linux-gnu)
DEBUG Found project root: `/tmp/test-test2`
DEBUG No workspace root found, using project root
DEBUG Using Python request `cpython@3.10` from explicit request
DEBUG Using request connect timeout of 10s and read timeout of 30s
Exception in thread tox-r-err-617478-20:
Traceback (most recent call last):
  File "~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/lib/python3.14/threading.py", line 1082, in _bootstrap_inner
    self._context.run(self.run)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/lib/python3.14/threading.py", line 1024, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.venv/lib/python3.14/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 32, in _read_stream
    self._read_until_eof(selector)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/tmp/test-test2/.venv/lib/python3.14/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 39, in _read_until_eof
    ready = selector.select(timeout=TIMEOUT_FOR_INTERRUPT)
  File "~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/lib/python3.14/selectors.py", line 452, in select
    fd_event_list = self._selector.poll(timeout, max_ev)
OSError: [Errno 9] Bad file descriptor
3.13: FAIL ✖ in 1.46 seconds
DEBUG uv 0.11.6 (x86_64-unknown-linux-gnu)
DEBUG Found project root: `/tmp/test-test2`
DEBUG No workspace root found, using project root
DEBUG Using Python request `cpython@3.13` from explicit request
DEBUG Using request connect timeout of 10s and read timeout of 30s
DEBUG Searching for CPython 3.13 in search path or managed installations
DEBUG Found `cpython-3.14.2-linux-x86_64-gnu` at `/tmp/test-test2/.venv/bin/python3` (first executable in the search path)
DEBUG Ignoring Python interpreter at `/tmp/test-test2/.venv/bin/python3`: system interpreter required
DEBUG Found `cpython-3.14.2-linux-x86_64-gnu` at `/tmp/test-test2/.venv/bin/python` (search path)
DEBUG Ignoring Python interpreter at `/tmp/test-test2/.venv/bin/python`: system interpreter required
DEBUG Found `cpython-3.14.6-linux-x86_64-gnu` at `/usr/bin/python3` (search path)
DEBUG Skipping interpreter at `/usr/bin/python3` from search path: does not satisfy request `3.13`
DEBUG Found `cpython-3.14.6-linux-x86_64-gnu` at `/usr/bin/python` (search path)
DEBUG Skipping interpreter at `/usr/bin/python` from search path: does not satisfy request `3.13`
DEBUG Searching for managed installations at `~/.local/share/uv/python`
DEBUG Skipping managed installation `cpython-3.14.2-linux-x86_64-gnu`: does not satisfy `3.13`
DEBUG Found managed installation `cpython-3.13.13-linux-x86_64-gnu`
DEBUG Found `cpython-3.13.13-linux-x86_64-gnu` at `~/.local/share/uv/python/cpython-3.13-linux-x86_64-gnu/bin/python3.13` (managed installations)
DEBUG Skipping managed installation cpython-3.13.13-linux-x86_64-gnu: system installation preferred
DEBUG Allowing managed installation cpython-3.13.13-linux-x86_64-gnu: no system installations
Using CPython 3.13.13
Creating virtual environment at: .tox/3.13
DEBUG Assessing Python executable as base candidate: ~/.local/share/uv/python/cpython-3.13-linux-x86_64-gnu/bin/python3.13
DEBUG Using base executable for virtual environment: ~/.local/share/uv/python/cpython-3.13-linux-x86_64-gnu/bin/python3.13
DEBUG Allowing existing directory due to `--allow-existing`
DEBUG Using symlink directory ~/.local/share/uv/python/cpython-3.13-linux-x86_64-gnu instead of base Python path: ~/.local/share/uv/python/cpython-3.13-linux-x86_64-gnu/bin/python3.13
DEBUG Detected parent process ID: 617205
DEBUG Parent process executable: ~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/bin/python3.14
DEBUG Parent process comm: tox
DEBUG Could not determine shell from parent process
DEBUG uv 0.11.6 (x86_64-unknown-linux-gnu)
DEBUG Found project root: `/tmp/test-test2`
DEBUG No workspace root found, using project root
DEBUG Using Python request `cpython@3.13` from explicit request
DEBUG Checking for Python environment at: `.tox/3.13`
DEBUG The project environment's Python version satisfies the request: `CPython 3.13`
DEBUG Using request connect timeout of 10s and read timeout of 30s
DEBUG Computed cache info: Timestamp(SystemTime { tv_sec: 1783518335, tv_nsec: 142410223 }), None, None, {}, {"src": Some(Timestamp(Timestamp(SystemTime { tv_sec: 1783518057, tv_nsec: 895063180 })))}. Most recently modified: pyproject.toml
DEBUG Directory source requirement already cached: test-test==0.1.0 (from file:///tmp/test-test2)
DEBUG Registry requirement already cached: pytest==9.1.1
DEBUG Registry requirement already cached: pytest-asyncio==1.4.0
DEBUG Registry requirement already cached: pytest-cov==7.1.0
DEBUG Registry requirement already cached: pytest-mock==3.15.1
DEBUG Registry requirement already cached: iniconfig==2.3.0
DEBUG Registry requirement already cached: packaging==26.2
DEBUG Registry requirement already cached: pluggy==1.6.0
DEBUG Registry requirement already cached: pygments==2.20.0
DEBUG Identified uncached distribution: coverage==7.15.0
DEBUG No cache entry for: https://files.pythonhosted.org/packages/a1/fa/b70062750686bd7da454da27927622f48bbac6990ac7a4c4a4653e7b0036/coverage-7.15.0-cp313-cp313-manylinux1_x86_64.manylinux_2_28_x86_64.manylinux_2_5_x86_64.whl
DEBUG Sending fresh GET request for: https://files.pythonhosted.org/packages/a1/fa/b70062750686bd7da454da27927622f48bbac6990ac7a4c4a4653e7b0036/coverage-7.15.0-cp313-cp313-manylinux1_x86_64.manylinux_2_28_x86_64.manylinux_2_5_x86_64.whl
Prepared 1 package in 299ms
Installed 10 packages in 10ms
 + coverage==7.15.0
 + iniconfig==2.3.0
 + packaging==26.2
 + pluggy==1.6.0
 + pygments==2.20.0
 + pytest==9.1.1
 + pytest-asyncio==1.4.0
 + pytest-cov==7.1.0
 + pytest-mock==3.15.1
 + test-test==0.1.0 (from file:///tmp/test-test2)
Traceback (most recent call last):
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/main.py", line 328, in wrap_session
    config.hook.pytest_sessionstart(session=session)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_hooks.py", line 512, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_manager.py", line 120, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 167, in _multicall
    raise exception
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 139, in _multicall
    teardown.throw(exception)
    ~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/logging.py", line 795, in pytest_sessionstart
    return (yield)
            ^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 121, in _multicall
    res = hook_impl.function(*args)
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/terminal.py", line 852, in pytest_sessionstart
    self.write_sep("=", "test session starts", bold=True)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/terminal.py", line 566, in write_sep
    self._tw.sep(sep, title, fullwidth, **markup)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 152, in sep
    self.line(line, **markup)
    ~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 184, in line
    self.write("\n")
    ~~~~~~~~~~^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 164, in write
    self.write_raw(msg, flush=flush)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 168, in write_raw
    self._file.write(msg)
    ~~~~~~~~~~~~~~~~^^^^^
OSError: [Errno 5] Input/output error

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/test-test2/.tox/3.13/bin/pytest", line 10, in <module>
    sys.exit(_console_main())
             ~~~~~~~~~~~~~^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/config/__init__.py", line 253, in _console_main
    code = _main(prog=_get_prog_name(sys.argv))
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/config/__init__.py", line 229, in _main
    ret: ExitCode | int = config.hook.pytest_cmdline_main(config=config)
                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_hooks.py", line 512, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_manager.py", line 120, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 167, in _multicall
    raise exception
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 121, in _multicall
    res = hook_impl.function(*args)
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/main.py", line 377, in pytest_cmdline_main
    return wrap_session(config, _main)
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/main.py", line 350, in wrap_session
    config.notify_exception(excinfo, config.option)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/config/__init__.py", line 1267, in notify_exception
    res = self.hook.pytest_internalerror(excrepr=excrepr, excinfo=excinfo)
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_hooks.py", line 512, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_manager.py", line 120, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 167, in _multicall
    raise exception
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/pluggy/_callers.py", line 121, in _multicall
    res = hook_impl.function(*args)
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/terminal.py", line 582, in pytest_internalerror
    self.write_line("INTERNALERROR> " + line)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/terminal.py", line 538, in write_line
    self._tw.line(line, **markup)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 184, in line
    self.write("\n")
    ~~~~~~~~~~^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 164, in write
    self.write_raw(msg, flush=flush)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.tox/3.13/lib/python3.13/site-packages/_pytest/_io/terminalwriter.py", line 168, in write_raw
    self._file.write(msg)
    ~~~~~~~~~~~~~~~~^^^^^
OSError: [Errno 5] Input/output error
Exception ignored on flushing sys.stdout:
OSError: [Errno 5] Input/output error
Exception in thread tox-r-out-617498-5:
Traceback (most recent call last):
  File "~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/lib/python3.14/threading.py", line 1082, in _bootstrap_inner
    self._context.run(self.run)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/lib/python3.14/threading.py", line 1024, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/test-test2/.venv/lib/python3.14/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 32, in _read_stream
    self._read_until_eof(selector)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/tmp/test-test2/.venv/lib/python3.14/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 39, in _read_until_eof
    ready = selector.select(timeout=TIMEOUT_FOR_INTERRUPT)
  File "~/.local/share/uv/python/cpython-3.14.2-linux-x86_64-gnu/lib/python3.14/selectors.py", line 452, in select
    fd_event_list = self._selector.poll(timeout, max_ev)
OSError: [Errno 9] Bad file descriptor
3.13: 219 W remove tox env folder /tmp/test-test2/.tox/3.13 [tox/tox_env/api.py:348]
3.13: 263 W venv> ~/.local/bin/uv venv -p cpython3.13 --allow-existing '--prompt=test-test2[3.13]' -v --python-preference system /tmp/test-test2/.tox/3.13 [tox/tox_env/api.py:485]
3.13: 325 I exit 0 (0.06 seconds) /tmp/test-test2> ~/.local/bin/uv venv -p cpython3.13 --allow-existing '--prompt=test-test2[3.13]' -v --python-preference system /tmp/test-test2/.tox/3.13 pid=617239 [tox/execute/api.py:308]
3.13: 326 W uv-sync> ~/.local/bin/uv sync --python-preference system -v --link-mode symlink --frozen --no-default-groups --group unit-test --all-extras --color never --no-progress -p cpython3.13 [tox/tox_env/api.py:485]
3.13: 678 I exit 0 (0.35 seconds) /tmp/test-test2> ~/.local/bin/uv sync --python-preference system -v --link-mode symlink --frozen --no-default-groups --group unit-test --all-extras --color never --no-progress -p cpython3.13 pid=617301 [tox/execute/api.py:308]
3.13: 679 W commands[0]> pytest -v -p no:cacheprovider --cov=. --cov-config=pyproject.toml --cov-report=term-missing:skip-covered --junit-prefix 3.13 --color=no tests [tox/tox_env/api.py:485]
3.13: 1683 C exit 120 (1.00 seconds) /tmp/test-test2> pytest -v -p no:cacheprovider --cov=. --cov-config=pyproject.toml --cov-report=term-missing:skip-covered --junit-prefix 3.13 --color=no tests pid=617483 [tox/execute/api.py:308]
3.12: OK ✔ in 1.54 seconds
  3.10: FAIL code 101 (0.11 seconds)
  3.11: OK (1.55=setup[0.48]+cmd[1.07] seconds)
  3.12: OK (1.54=setup[0.44]+cmd[1.11] seconds)
  3.13: FAIL code 120 (1.46=setup[0.46]+cmd[1.00] seconds)
  3.14: OK (1.25=setup[0.21]+cmd[1.04] seconds)
  evaluation failed :( (1.63 seconds)

Minimal example

Use the attached recreate project and inside it run

uv run -- tox -rvv --parallel auto  -e 3.10,3.11,3.12,3.13,3.14

# running the tests one at a time works ok
uv run -- tox -rvv --parallel 1  -e 3.10,3.11,3.12,3.13,3.14