Cache provisional weekday timetables

cache.py

@@ -1,6 +1,7 @@
 import json
 import os
 import time
+import uuid
 
 from config.default import CACHE_DIR  # overridden by app config after import
 
@@ -13,15 +14,21 @@ def _cache_path(key: str) -> str:
 def get_cached(key: str, ttl: int | None = None):
     """Return cached data, or None if missing or older than ttl seconds."""
     path = _cache_path(key)
-    if not os.path.exists(path):
+    try:
+        if not os.path.exists(path):
+            return None
+        if ttl is not None and time.time() - os.path.getmtime(path) > ttl:
+            return None
+        with open(path) as f:
+            return json.load(f)
+    except (OSError, json.JSONDecodeError):
         return None
-    if ttl is not None and time.time() - os.path.getmtime(path) > ttl:
-        return None
-    with open(path) as f:
-        return json.load(f)
 
 
 def set_cached(key: str, data) -> None:
     os.makedirs(CACHE_DIR, exist_ok=True)
-    with open(_cache_path(key), 'w') as f:
+    path = _cache_path(key)
+    tmp_path = f"{path}.{os.getpid()}.{uuid.uuid4().hex}.tmp"
+    with open(tmp_path, 'w') as f:
         json.dump(data, f, indent=2)
+    os.replace(tmp_path, path)