fix(backend): use asyncio.to_thread for stats IO, guard malformed JSON, isolate test side-effects

main.py

@@ -1,3 +1,4 @@
+import asyncio
 import json
 import threading
 import uuid
@@ -21,8 +22,11 @@ _stats_lock = threading.Lock()
 def _read_stats() -> dict:
     if not STATS_FILE.exists():
         return {"total_visits": 0, "total_downloads": 0}
-    with open(STATS_FILE, "r") as f:
-        return json.load(f)
+    try:
+        with open(STATS_FILE, "r") as f:
+            return json.load(f)
+    except (json.JSONDecodeError, OSError):
+        return {"total_visits": 0, "total_downloads": 0}
 
 
 def _write_stats(stats: dict) -> None:
@@ -70,15 +74,19 @@ async def health():
     return {"status": "ok"}
 
 
-@app.get("/stats")
-async def get_stats():
+def _read_stats_locked() -> dict:
     with _stats_lock:
         return _read_stats()
 
 
+@app.get("/stats")
+async def get_stats():
+    return await asyncio.to_thread(_read_stats_locked)
+
+
 @app.post("/stats/visit", status_code=204)
 async def record_visit():
-    _increment_stat("total_visits")
+    await asyncio.to_thread(_increment_stat, "total_visits")
 
 
 @app.get("/status/{job_id}", response_model=JobStatus)
@@ -135,7 +143,7 @@ async def download(job_id: str):
     if not output_file.exists():
         raise HTTPException(status_code=404, detail="Output file not found")
 
-    _increment_stat("total_downloads")
+    await asyncio.to_thread(_increment_stat, "total_downloads")
 
     return FileResponse(
         path=output_file,

tests/test_main.py

@@ -97,6 +97,7 @@ async def test_download_done_job(client, tmp_path, monkeypatch):
     fake_status = JobStatus(job_id=job_id, state=JobState.done, progress=100)
     monkeypatch.setitem(processor._jobs, job_id, fake_status)
     monkeypatch.setattr(main, "OUTPUT_DIR", tmp_path)
+    monkeypatch.setattr(main, "STATS_FILE", tmp_path / "stats.json")
 
     response = await client.get(f"/download/{job_id}")
     assert response.status_code == 200