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()

tests/test_sync.js

@@ -0,0 +1,30 @@
+const assert = require("node:assert/strict");
+const sync = require("../static/sync.js");
+
+// A second client advances the shared revision while this client is saving.
+// The in-flight local value must not be replaced by the older server value.
+const local = { "crop-parsnip": true, "fish-eel": false };
+const remoteWhileSaving = {
+  revision: 6,
+  checks: { "crop-parsnip": false, "fish-eel": true }
+};
+const merged = sync.mergeSnapshot(local, new Set(["crop-parsnip"]), 5, remoteWhileSaving);
+assert.equal(merged.applied, true);
+assert.equal(merged.checks["crop-parsnip"], true);
+assert.equal(merged.checks["fish-eel"], true);
+
+// A response already superseded by a later write must never roll the client back.
+const stale = sync.mergeSnapshot(merged.checks, new Set(), 7, remoteWhileSaving);
+assert.equal(stale.applied, false);
+assert.equal(stale.revision, 7);
+
+// The write response is authoritative for its own item even after an interleaved poll.
+const confirmed = sync.confirmWrite(merged.checks, 6, {
+  itemId: "crop-parsnip",
+  checked: true,
+  revision: 7
+});
+assert.equal(confirmed.checks["crop-parsnip"], true);
+assert.equal(confirmed.revision, 7);
+
+console.log("sync race tests passed");