added cache class for proper thread safety

server/app.py

@@ -6,8 +6,10 @@ from src.parser import (
     get_last_n_days,
     get_stats,
 )
+from src.cache import Cache
 
 app = Flask(__name__)
+sessions_cache = Cache()
 
 REFETCH_MINUTES = os.environ.get("REFETCH_INTERVAL_MINUTES", "5")
 if not REFETCH_MINUTES.isdigit():
@@ -17,7 +19,7 @@ REFETCH_SECONDS = int(REFETCH_MINUTES) * 60
 
 # initial data fetch (required to work)
 try:
-    sessions = get_this_semesters_sessions()
+    sessions_cache.set(get_this_semesters_sessions())
 except Exception:
     print("Failed to fetch from calDAV server.")
     sys.exit(1)
@@ -25,35 +27,34 @@ except Exception:
 # web ui
 @app.route("/")
 def index():
-    current = list(sessions)
-    labels = [s["subject"] for s in current]
-    values = [s["minutes"] for s in current]
-    return render_template("index.html", labels=labels, values=values, sessions=current)
+    sessions = sessions_cache.get()
+    labels = [s["subject"] for s in sessions]
+    values = [s["minutes"] for s in sessions]
+    return render_template("index.html", labels=labels, values=values, sessions=sessions)
 
 # internal api
 @app.route("/api/dashboard")
 def api_dashboard():
-    current = list(sessions)
-    totals = calculate_totals(current)
+    sessions = sessions_cache.get()
+    totals = calculate_totals(sessions)
     return jsonify({
-        "sessions": current,
+        "sessions": sessions,
         "subjectTotals": totals,
-        "dailyMinutes": get_last_n_days(50, current),
-        "stats": get_stats(current, totals),
+        "dailyMinutes": get_last_n_days(50, sessions),
+        "stats": get_stats(sessions, totals),
     })
 
 # external api
 @app.route("/api/sessions")
 def api_sessions():
-    return jsonify(list(sessions))
+    return jsonify(sessions_cache.get())
 
 # background daemon
 def fetch_job():
-    global sessions
     while True:
         time.sleep(REFETCH_SECONDS)
         try:
-            sessions = get_this_semesters_sessions()
+            sessions_cache.set(get_this_semesters_sessions())
             print("Successfully fetched data from calDAV")
         except Exception:
             print("Failed to refresh from calDAV server, keeping old data.")

server/src/cache.py

@@ -0,0 +1,14 @@
+import threading
+
+class Cache:
+    def __init__(self):
+        self._lock = threading.Lock()
+        self._sessions = []
+
+    def get(self) -> list:
+        with self._lock:
+            return list(self._sessions)
+
+    def set(self, sessions: list):
+        with self._lock:
+            self._sessions = sessions