fix: keep the modules cache when the downloaded JSON is malformed

lean/models/__init__.py

@@ -31,9 +31,17 @@ def _download_modules_json() -> None:
     res = get(url, timeout=5)
     if res.ok:
         from json import dump
+        # Decode the response and write a temporary file first, so a malformed response
+        # or a failed write leaves the existing file untouched.
+        data = res.json()
         file_path.parent.mkdir(parents=True, exist_ok=True)
-        with open(file_path, 'w', encoding='utf-8') as f:
-            dump(res.json(), f, ensure_ascii=False, indent=4)
+        temporary_file_path = file_path.with_name(file_path.name + ".tmp")
+        try:
+            with open(temporary_file_path, 'w', encoding='utf-8') as f:
+                dump(data, f, ensure_ascii=False, indent=4)
+            temporary_file_path.replace(file_path)
+        finally:
+            temporary_file_path.unlink(missing_ok=True)
     else:
         res.raise_for_status()
 

tests/models/test_download_modules_json.py

@@ -0,0 +1,44 @@
+import json
+from pathlib import Path
+
+import pytest
+from requests.exceptions import JSONDecodeError
+from responses import RequestsMock
+
+import lean.models as models
+
+
+def _write_cache(path: Path) -> str:
+    content = json.dumps({"modules": {"SavedModule": {"version": "1.0"}}})
+    path.write_text(content, encoding="utf-8")
+    return content
+
+
+def test_download_modules_json_keeps_cache_when_response_is_not_json(requests_mock: RequestsMock,
+                                                                      monkeypatch: pytest.MonkeyPatch) -> None:
+    cache = Path.cwd() / "cache" / "modules.json"
+    cache.parent.mkdir()
+    content = _write_cache(cache)
+    monkeypatch.setattr(models, "file_path", cache)
+    requests_mock.add(requests_mock.GET, models.url, body="not json", status=200)
+
+    with pytest.raises(JSONDecodeError):
+        models._download_modules_json()
+
+    assert cache.read_text(encoding="utf-8") == content
+    assert [p.name for p in cache.parent.iterdir()] == ["modules.json"]
+
+
+def test_download_modules_json_replaces_cache_with_valid_response(requests_mock: RequestsMock,
+                                                                  monkeypatch: pytest.MonkeyPatch) -> None:
+    cache = Path.cwd() / "cache" / "modules.json"
+    cache.parent.mkdir()
+    _write_cache(cache)
+    monkeypatch.setattr(models, "file_path", cache)
+    new_modules = {"modules": {"NewModule": {"version": "2.0"}}}
+    requests_mock.add(requests_mock.GET, models.url, json=new_modules, status=200)
+
+    models._download_modules_json()
+
+    assert json.loads(cache.read_text(encoding="utf-8")) == new_modules
+    assert [p.name for p in cache.parent.iterdir()] == ["modules.json"]