test: narrow the control-character skip and find Windows by SYSTEMROOT

Review feedback, both valid.

The control-character fixture caught every OSError, so an unwritable
temp directory or a full disk would have reported these security tests
as skipped instead of failing. It now skips only for errors that mean
the name itself is unrepresentable (EINVAL, EILSEQ, ENAMETOOLONG) and
re-raises everything else.

The mount-policy test derived the protected directory from the
checkout's drive, so a repo cloned to D: would look for D:\Windows,
miss, and skip the branch the test exists to cover. Ask Windows where
it is installed via SYSTEMROOT instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/conftest.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import errno
 import os
 from typing import TYPE_CHECKING
 
@@ -85,6 +86,13 @@ def _isolate_git_config(
     monkeypatch.setenv("GIT_COMMITTER_EMAIL", "tests@example.com")
 
 
+# Errors that mean "this name is not representable here", as opposed to a
+# filesystem that is full, read-only, or otherwise broken.
+_REJECTED_NAME_ERRNOS = frozenset(
+    {errno.EINVAL, errno.EILSEQ, errno.ENAMETOOLONG},
+)
+
+
 @pytest.fixture
 def write_control_character_file() -> Callable[[Path, str, str], Path]:
     """Create a file whose *name* carries terminal control characters, or skip.
@@ -103,6 +111,11 @@ def write_control_character_file() -> Callable[[Path, str, str], Path]:
         try:
             path.write_text(content, encoding="utf-8")
         except OSError as exc:  # pragma: no cover - platform dependent
+            # Only "the name itself is unacceptable" is a reason to skip. A
+            # full disk or an unwritable temp directory must still fail these
+            # security tests rather than quietly reporting them as skipped.
+            if exc.errno not in _REJECTED_NAME_ERRNOS:
+                raise
             pytest.skip(f"this filesystem rejects control characters in filenames: {exc}")
         return path
 

tests/test_local_sources.py

@@ -182,9 +182,14 @@ def test_infer_target_type_applies_the_mount_policy() -> None:
     # so pick a protected system directory this platform actually has. The
     # policy itself already knows about both families (`_FORBIDDEN_MOUNT_TREES`
     # and `_FORBIDDEN_WINDOWS_TREE_NAMES`); only the fixture was Unix-only.
-    candidates = (
-        [Path(Path.cwd().anchor) / "Windows"] if os.name == "nt" else [Path("/etc"), Path("/usr")]
-    )
+    if os.name == "nt":
+        # Ask Windows where it is installed. Deriving the drive from the
+        # checkout would look for D:\Windows on a repo cloned to D:, miss, and
+        # skip the very branch this test exists to cover.
+        system_root = os.environ.get("SYSTEMROOT") or os.environ.get("WINDIR")
+        candidates = [Path(system_root)] if system_root else []
+    else:
+        candidates = [Path("/etc"), Path("/usr")]
     system_dir = next((p for p in candidates if p.is_dir()), None)
     if system_dir is None:
         pytest.skip("no protected system directory on this platform")