monitor: ensure db connection before querying problems

judge/bridge/monitor.py

@@ -5,6 +5,8 @@
 import time
 from pathlib import Path
 
+from django import db
+
 from judge.models import Problem
 
 try:
@@ -25,6 +27,10 @@
 logger = logging.getLogger('judge.monitor')
 
 
+def _ensure_connection():
+    db.connection.close_if_unusable_or_obsolete()
+
+
 def find_glob_root(g: str) -> Path:
     """
     Given a glob, find a directory that contains all its possible patterns
@@ -81,6 +87,7 @@ def update_supported_problems(self):
                     problems.append(problem)
         problems = set(problems)
 
+        _ensure_connection()
         problem_ids = list(Problem.objects.filter(code__in=list(problems)).values_list('id', flat=True))
         self.judges.update_problems_all(problems, problem_ids)