Run library, checks and downloads in background threads with parallel fetching

tests/test_fileutil.py

@@ -0,0 +1,48 @@
+"""Tests for atomic file writes."""
+
+from pathlib import Path
+
+import pytest
+
+from src.fileutil import atomic_write_text
+
+
+def test_writes_content(tmp_path: Path) -> None:
+    target = tmp_path / "out.json"
+    atomic_write_text(target, '{"a": 1}')
+    assert target.read_text(encoding="utf-8") == '{"a": 1}'
+
+
+def test_creates_parent_dirs(tmp_path: Path) -> None:
+    target = tmp_path / "nested" / "deep" / "out.txt"
+    atomic_write_text(target, "hi")
+    assert target.read_text(encoding="utf-8") == "hi"
+
+
+def test_overwrites_existing(tmp_path: Path) -> None:
+    target = tmp_path / "out.txt"
+    target.write_text("old", encoding="utf-8")
+    atomic_write_text(target, "new")
+    assert target.read_text(encoding="utf-8") == "new"
+
+
+def test_leaves_no_temp_files_on_success(tmp_path: Path) -> None:
+    target = tmp_path / "out.txt"
+    atomic_write_text(target, "data")
+    assert [p.name for p in tmp_path.iterdir()] == ["out.txt"]
+
+
+def test_original_intact_when_write_fails(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    target = tmp_path / "out.txt"
+    target.write_text("original", encoding="utf-8")
+
+    def boom(*args: object, **kwargs: object) -> None:
+        raise OSError("disk full")
+
+    monkeypatch.setattr("os.replace", boom)
+    with pytest.raises(OSError):
+        atomic_write_text(target, "new")
+
+    # Original file is untouched and no temp file is left behind.
+    assert target.read_text(encoding="utf-8") == "original"
+    assert sorted(p.name for p in tmp_path.iterdir()) == ["out.txt"]