Fix concurrent checklist sync and consistent actions

app.py

@@ -9,6 +9,7 @@ import logging
 import mimetypes
 import re
 import sqlite3
+from contextlib import closing
 from datetime import datetime, timezone
 from http import HTTPStatus
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
@@ -38,7 +39,7 @@ class Store:
         return connection
 
     def initialize(self) -> None:
-        with self.connect() as db:
+        with closing(self.connect()) as db, db:
             db.executescript(
                 """
                 CREATE TABLE IF NOT EXISTS checks (
@@ -56,7 +57,7 @@ class Store:
             )
 
     def snapshot(self) -> dict:
-        with self.connect() as db:
+        with closing(self.connect()) as db, db:
             revision = int(
                 db.execute("SELECT value FROM metadata WHERE key = 'revision'").fetchone()[0]
             )
@@ -75,7 +76,7 @@ class Store:
 
     def set_check(self, item_id: str, checked: bool, user: str) -> dict:
         changed_at = utc_now()
-        with self.connect() as db:
+        with closing(self.connect()) as db, db:
             db.execute("BEGIN IMMEDIATE")
             db.execute(
                 """
@@ -206,7 +207,10 @@ class Handler(BaseHTTPRequestHandler):
         }
         self.send_header("Content-Type", f"{content_type}; charset=utf-8" if textual else content_type)
         self.send_header("Content-Length", str(len(body)))
-        self.send_header("Cache-Control", "no-cache" if target.name == "index.html" else "public, max-age=3600")
+        # Source filenames are intentionally stable. Always revalidate them so
+        # two household clients cannot run different interaction code for an
+        # hour after a deployment.
+        self.send_header("Cache-Control", "no-cache" if textual else "public, max-age=3600")
         self.end_headers()
         self.wfile.write(body)