fix: roll back rejected general settings save

Issue: #439
User-Visible: yes

test/serialized-write-queue.test.mjs

@@ -1,7 +1,11 @@
 import assert from 'node:assert/strict';
 import test from 'node:test';
 
-import { enqueueSerializedWrite } from '../test-build/serialized-write-queue.js';
+import {
+  enqueueSerializedWrite,
+  optimisticAttempt,
+  rollbackOptimistic,
+} from '../test-build/serialized-write-queue.js';
 
 function deferred() {
   let resolve;
@@ -51,3 +55,48 @@ test('a failed write does not poison the next queued edit (#224)', async () => {
   await recovered;
   assert.deepEqual(seen, ['failed', 'recovered']);
 });
+
+test('a rejected optimistic write restores only its own candidate (#439)', () => {
+  const fingerprint = (value) => JSON.stringify(value);
+  const previous = { settings: { enabled: true } };
+  const attempted = { settings: { enabled: false } };
+  const attempt = optimisticAttempt(previous, attempted, 'server-fingerprint', 7, fingerprint);
+  let updates = 0;
+  const host = {
+    _serverCfg: attempted,
+    _cfgRev: 7,
+    _cfgContentFingerprint: fingerprint(attempted),
+    requestUpdate: () => { updates += 1; },
+  };
+
+  assert.equal(rollbackOptimistic(host, attempt, fingerprint), true);
+  assert.deepEqual(host._serverCfg, previous);
+  assert.notEqual(host._serverCfg, previous, 'the rollback snapshot is isolated from later mutations');
+  assert.equal(host._cfgContentFingerprint, 'server-fingerprint');
+  assert.equal(updates, 1);
+});
+
+test('a conflict reload or newer mutation wins over a rejected candidate (#439)', () => {
+  const fingerprint = (value) => JSON.stringify(value);
+  const previous = { settings: { value: 'server-before' } };
+  const attempted = { settings: { value: 'draft' } };
+  const attempt = optimisticAttempt(previous, attempted, 'before', 3, fingerprint);
+  const authoritative = { settings: { value: 'server-after' } };
+  let updates = 0;
+  const host = {
+    _serverCfg: authoritative,
+    _cfgRev: 4,
+    _cfgContentFingerprint: fingerprint(authoritative),
+    requestUpdate: () => { updates += 1; },
+  };
+
+  assert.equal(rollbackOptimistic(host, attempt, fingerprint), false);
+  assert.equal(host._serverCfg, authoritative);
+  assert.equal(updates, 0);
+
+  host._cfgRev = 3;
+  host._serverCfg = { settings: { value: 'newer-local-edit' } };
+  assert.equal(rollbackOptimistic(host, attempt, fingerprint), false);
+  assert.equal(host._serverCfg.settings.value, 'newer-local-edit');
+  assert.equal(updates, 0);
+});