Add test for xargs import with blocked sysconf

- Add subprocess-based test to expose import-time os.sysconf bug
- Test runs in fresh subprocess to avoid conftest.py import issues
- Test fails with current buggy implementation, will pass after fix

tests/xargs_with_blocked_sysconf_test.py

@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+import subprocess
+import sys
+import textwrap
+
+# flake8: noqa: E501
+
+
+# TODO
+# import module without throwing exception
+# create object without throwing exception
+#  -- explicit _length
+#  -- implicit _length
+
+
+def test_import_module_without_throwing_exception_as_subprocess() -> None:
+    """Import pre_commit.xargs in a subprocess even when os.sysconf is blocked.
+
+    This test runs in a fresh subprocess where we can control os.sysconf
+    BEFORE any imports happen, avoiding the conftest.py chicken/egg problem.
+
+    With the current buggy implementation, this subprocess will crash
+    (returncode != 0) because os.sysconf('SC_ARG_MAX') is called at import
+    time and raises OSError.
+
+    After the fix, the subprocess should succeed (returncode == 0).
+    """
+    script = textwrap.dedent("""
+        import importlib
+        import os
+        from unittest import mock
+
+        def main():
+            # Force a posix-style environment.
+            os.name = 'posix'
+
+            # Block os.sysconf so that any SC_ARG_MAX lookup fails with OSError.
+            with mock.patch.object(os, 'sysconf', side_effect=OSError('blocked')):
+                # This should NOT raise with the fixed implementation.
+                importlib.import_module('pre_commit.xargs')
+
+        if __name__ == '__main__':
+            main()
+    """)
+
+    proc = subprocess.run(
+        [sys.executable, '-c', script],
+        capture_output=True,
+        text=True,
+        cwd='/Users/michael/repos/pre-commit',
+    )
+
+    # Test-first: This test FAILS with current buggy behavior (subprocess crashes).
+    # After fix, subprocess should succeed (returncode == 0).
+    assert proc.returncode == 0, (
+        f'Subprocess failed to import pre_commit.xargs when os.sysconf is blocked. '
+        f'This exposes the bug: os.sysconf is called at import time. '
+        f'stderr: {proc.stderr!r}'
+    )
+