report success if existing data matches new data during update

index.js

@@ -394,12 +394,34 @@ async function postGameHandler(req, res, next) {
   /* Now wait for all the queries to finish. */
   const result = await transactionP;
 
-  if (!result || result.modified !== params.$time) {
+  if (result && result.modified === params.$time) {
+    /* Only signal an update if the record was actually changed. No-ops don't count. */
+    signalGameUpdate(gameId);
+  } else {
+    /*
+     * If the record exists and all the values (other than .modified) match the
+     * request, treat this as a successful no-op. This ensures that repeated
+     * requests for the same update (i.e. retries) do not fail just because the
+     * modified time was changed.
+     */
+    let same = false;
+
+    if (result) {
+      same = true;
+      for (const key in body) {
+        /* Use the value from params since e.g. body.board may be adjusted. */
+        if (key !== 'modified' && result[key] !== params['$' + key]) {
+          same = false;
+          break;
+        }
+      }
+    }
+
+    if (!same) {
       res.status(409).json({ message: 'update failed', modified: (result || {}).modified });
       return;
     }
-
+  }
-  signalGameUpdate(gameId);
 
   res.json({ success: true, modified: result.modified });
 }