test: lock atomic evidence replacement

tests/test_file_safety.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from headerproof.file_safety import atomic_write_text
+
+
+def test_atomic_write_text_creates_parents_and_replaces_destination(tmp_path: Path) -> None:
+    destination = tmp_path / "nested" / "evidence" / "metadata.json"
+
+    atomic_write_text(destination, "first\n")
+    atomic_write_text(destination, "second complete\n")
+
+    assert destination.read_text(encoding="utf-8") == "second complete\n"
+    assert list(destination.parent.glob("*.tmp")) == []
+
+
+def test_atomic_write_text_tolerates_fsync_oserror(tmp_path: Path, monkeypatch) -> None:
+    destination = tmp_path / "summary.md"
+    destination.write_text("previous\n", encoding="utf-8")
+
+    def fail_fsync(_fd: int) -> None:
+        raise OSError("fsync unavailable")
+
+    monkeypatch.setattr("headerproof.file_safety.os.fsync", fail_fsync)
+
+    atomic_write_text(destination, "replaced\n")
+
+    assert destination.read_text(encoding="utf-8") == "replaced\n"
+    assert not (tmp_path / "summary.md.tmp").exists()