Add atomic write utility and tests

src/tests/test_atomic_write.py

@@ -0,0 +1,30 @@
+import json
+from multiprocessing import Process
+from pathlib import Path
+
+from utils.atomic_write import atomic_write
+
+
+def _writer(path: Path, content: dict, loops: int) -> None:
+    for _ in range(loops):
+        atomic_write(path, lambda f: json.dump(content, f), mode="w")
+
+
+def test_atomic_write_concurrent(tmp_path: Path) -> None:
+    """Concurrent writers should not leave partial files."""
+
+    file_path = tmp_path / "data.json"
+    contents = [{"proc": i} for i in range(5)]
+
+    procs = [
+        Process(target=_writer, args=(file_path, content, 50)) for content in contents
+    ]
+
+    for p in procs:
+        p.start()
+    for p in procs:
+        p.join()
+
+    final_text = file_path.read_text()
+    final_obj = json.loads(final_text)
+    assert final_obj in contents