Fix cache multithreading issues on startup?

core/cache_utils.py

@@ -1,3 +1,4 @@
+import threading
 from datetime import timedelta
 
 from requests_cache import CachedSession
@@ -5,6 +6,19 @@ from requests_cache import CachedSession
 # Cache for "semi-static" data such as the locations of parks, CSVs of reference lists, etc.
 # This has an expiry time of 30 days, so will re-request from the source after that amount
 # of time has passed. This is used throughout Spothole to cache data that does not change
-# rapidly.
-SEMI_STATIC_URL_DATA_CACHE = CachedSession("cache/semi_static_url_data_cache",
-                                           expire_after=timedelta(days=30))
+# rapidly. The ThreadSafeSession construct here protects it against some multithreading
+# contention weirdness we sometimes used to see on startup where the cache was hammered
+# pretty hard.
+_session = CachedSession("cache/semi_static_url_data_cache", expire_after=timedelta(days=30))
+_lock = threading.Lock()
+
+
+class _ThreadSafeSession:
+    """Wraps CachedSession with a lock to prevent concurrent SQLite access across threads."""
+
+    def get(self, *args, **kwargs):
+        with _lock:
+            return _session.get(*args, **kwargs)
+
+
+SEMI_STATIC_URL_DATA_CACHE = _ThreadSafeSession()