fix: handle OSError in pos_args path existence check (#3847)

When positional arguments exceed the filesystem's filename length limit
(typically 255 characters), `Path.exists()` raises `OSError: [Errno 36]
File name too long` instead of returning `False`.

This causes `tox` to crash with an internal error when **long** `-k`
expressions are passed via `{posargs}`, e.g.:

    tox -e py3 -- -k "test_foo or test_bar or test_baz or ..."

The traceback is:

    File ".../tox/config/main.py", line 58, in pos_args
        if path_arg.exists() and not path_arg.is_absolute():
OSError: [Errno 36] File name too long: 'test_foo or test_bar or ...'

This is because `pos_args()` checks each argument with
`path_arg.exists()` to determine whether to rewrite relative paths for
`changedir`. An argument that is too long for the filesystem clearly
cannot be a path, so the fix catches `OSError` and treats such arguments
as non-paths.

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- [ ] ran the linter to address style issues (`tox -e fix`) [XXX]
- [x] wrote descriptive pull request text
- [x] ensured there are test(s) validating the fix
- [x] added news fragment in `docs/changelog` folder -- 
- [ ] updated/extended the documentation

<details><summary>[XXX]: tried but failed to install sphinx... might try
again later</summary>

```
[INFO] Using pre-commit with uv 0.10.7 via pre-commit-uv 4.2.1
An unexpected error has occurred: CalledProcessError: command: ('/home/yoh/proj/misc/tox/.tox/fix/bin/uv', '--project', '/home/yoh/proj/misc/tox', 'pip', 'install', '.', 'sphinx>=9.1')
return code: 1
stdout: (none)
stderr:
    Using Python 3.11.12 environment at: py_env-python3
      × No solution found when resolving dependencies:
      ╰─▶ Because the current Python version (3.11.12) does not satisfy
          Python>=3.12 and sphinx==9.1.0 depends on Python>=3.12, we can conclude
          that sphinx==9.1.0 cannot be used.
          And because only sphinx<=9.1.0 is available and you require sphinx>=9.1,
          we can conclude that your requirements are unsatisfiable.
Check the log at /home/yoh/.cache/pre-commit/pre-commit.log
fix: exit 3 (24.65 seconds) /home/yoh/proj/misc/tox> pre-commit run --all-files --show-diff-on-failure pid=4127791
  fix: FAIL code 3 (25.28=setup[0.62]+cmd[24.65] seconds)
  evaluation failed :( (25.34 seconds)

```
</details>

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

docs/changelog/3847.bugfix.rst

@@ -0,0 +1,3 @@
+Handle ``OSError`` when checking if positional arguments are existing paths in ``pos_args()`` — long arguments (e.g.,
+pytest ``-k`` expressions exceeding the 255-character filesystem name limit) no longer crash with ``OSError: [Errno 36]
+File name too long`` - by :user:`yarikoptic`.

src/tox/config/main.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import logging
 import os
 from collections import OrderedDict, defaultdict
 from itertools import chain, tee
@@ -62,7 +63,12 @@ def pos_args(self, to_path: Path | None) -> tuple[str, ...] | None:
             to_path_str = os.path.abspath(str(to_path))  # noqa: PTH100
             for arg in self._pos_args:
                 path_arg = Path(arg)
-                if path_arg.exists() and not path_arg.is_absolute():
+                try:
+                    is_existing_path = path_arg.exists()
+                except OSError:
+                    logging.debug("could not check if %r is an existing path", arg)
+                    is_existing_path = False
+                if is_existing_path and not path_arg.is_absolute():
                     # we use os.path to unroll .. in path without resolve
                     path_arg_str = os.path.abspath(str(path_arg))  # noqa: PTH100
                     try:

tests/config/test_main.py

@@ -235,6 +235,14 @@ def test_args_are_paths_when_from_child_dir(tox_project: ToxProjectCreator) -> N
     assert result.out == f"[testenv:py]\ncommands = magic.py {project.path} tox.ini . ..\n"
 
 
+def test_args_are_paths_long_arg(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({"tox.ini": "[testenv]\npackage=skip\ncommands={posargs}", "w": {"a.txt": "a"}})
+    long_arg = "a" * 300  # exceeds filesystem filename length limit (typically 255)
+    result = project.run("c", "-e", "py", "-k", "commands", "--", long_arg, from_cwd=project.path / "w")
+    result.assert_success()
+    assert result.out == f"[testenv:py]\ncommands = {long_arg}\n"
+
+
 def test_args_are_paths_when_with_change_dir(tox_project: ToxProjectCreator) -> None:
     project = tox_project({"tox.ini": "[testenv]\npackage=skip\ncommands={posargs}\nchange_dir=w", "w": {"a.txt": "a"}})
     args = "magic.py", str(project.path), "tox.ini", f"w{os.sep}a.txt", "w", "."