Fix concurrent checklist sync and consistent actions

tests/test_app.py

@@ -1,5 +1,6 @@
 import tempfile
 import unittest
+from concurrent.futures import ThreadPoolExecutor
 from pathlib import Path
 
 import app
@@ -30,6 +31,21 @@ class StoreTests(unittest.TestCase):
         self.assertFalse(snapshot["checks"]["crop-parsnip"])
         self.assertEqual(snapshot["revision"], 2)
 
+    def test_two_clients_keep_distinct_concurrent_changes(self):
+        second_client = app.Store(self.store.path)
+
+        def write(index):
+            store = self.store if index % 2 else second_client
+            store.set_check(f"concurrent-item-{index}", True, f"player-{index % 2}")
+
+        with ThreadPoolExecutor(max_workers=2) as pool:
+            list(pool.map(write, range(20)))
+
+        snapshot = self.store.snapshot()
+        self.assertEqual(snapshot["revision"], 20)
+        self.assertEqual(len(snapshot["checks"]), 20)
+        self.assertTrue(all(snapshot["checks"].values()))
+
 
 if __name__ == "__main__":
     unittest.main()